Python

References

Books

O'Reilly's Python in a Nutshell
The Python language reference

Links

Python 3

Python 3.8.0 documentation

Including Language Reference.

==> The Python 3 Standard Library <==
The Python Tutorial
Built-in Types

Python 2.7

Python 2.7.6 docs
==> The Python 2 Standard Library <==
Python Quick Reference 2.7 — Extremelly complete

Other versions of Python are available [1]

Variants and distributions

ipython
Jupyter — The Jupyter Notebook is a web application that allows you to create and share documents that contain live code, equations, visualizations and explanatory text.
Anaconda

PEP

Coding style

References:

Miscellaneous

[2]

Nice example of generating / testing regex in Python (with nice / small test framework)

Guide to Python Debugging

Some tips on debugging in Python, mostly focussing on using a logger (instead of printf). See also HN page for many interesting comments.

Libraries

seaborn is a powerful python toolkit to visualize statistical data.
plumbum, a library to mimic bash-like commands, to ease rewrite bash scripts in Python, including pipes.
tqdm - progress bar, a library to make easily progress bar out of loops, iterable
pwntools, a CTF framework and exploit development library.

In particular, check pwntools tubes, a library for talking to sockets, processes, ssh connections. Useful for automation (see this CTF writeup for an example).

Profiler

py-spy a sampling profiler for Python

# As simple as
py-spy --pid 12345                         # Display activity of given pid in real-time!

Tutorials

w3schools.com Python tutorial <==

Very clear, terse, covering many topics (strings, regex)

Wikibook on Python I/O
Packaging projects — How to create package / module that can be installed with pip.

Shell

In a command shell, use pydoc to get help:

pydoc repr               # Get help on 'repr' command

Same can be achieved in python interpreter:

help()                 # Interactive help
help('repr')           # Same as typing 'repr' in interactive help
help(repr)             # Help on repr builtin

Testing

pytest: helps you write better programs (see this tutorial).
Hypothesis - Hypothesis is an advanced testing library for Python.
Quickcheck a testing library.

Examples

ProgramCreek, search into existing code for example of use

Install

Virtual Environments

A Virtual Environment is a tool to keep the dependencies required by different projects in separate places, by creating virtual Python environments for them.

References

Guide to Python — Virtual Environments
Is it possible to install another version of Python to Virtualenv? (stackoverflow.com)

Virtualenv

# Install
sudo apt install virtualenv python3-virtualenv

# Create a new environment
virtualenv -p python3 venv         # To use python3. venv is recommended default to add to .gitignore, etc.
source venv/bin/activate           # Enter environment. From now on, packages will only be installed locally
# Do stuff - pip3 install ...
deactivate                         # Exit environment

Update Python

❗	It is not recommended to update the system Python

Some links:

Install pip and setuptools

To install setuptools, the easiest is to use pip, which comes pre-installed in later versions of Python:

pip install -U setuptools

To bootstrap the setuptools on an naked installation:

cd /path/to/your/python
wget https://bootstrap.pypa.io/ez_setup.py -O - | ./python
wget https://bootstrap.pypa.io/ez_setup.py -O - | sudo ./python       # System-wide
wget https://bootstrap.pypa.io/ez_setup.py -O - | ./python - --user   # User-local path

See Install pip setuptools and wheels for more information.

Install module online

Python comes with a wide range of libraries, called modules. There are several ways to install these modules.

Using the distribution

For instance, in Debian:

apt-cache search --names-only python-       # View available modules
sudo apt-get install python-pyscard         # Install the pyscard module

Using pip

pip is the new way to install modules. It uses the wheel format.

sudo pip install Pygments

This is equivalent to:

sudo python -m pip install Pygments

This last form can be used to explicit which python runtime must be used:

sudo /path/to/your/python -m pip install Pygments

Use --user to install for user only:

pip install --user Pygments

Use --target SITE to specify manually the target SITE:

pip install --target SITE Pygments

See tip below on how to obtain the default site.

Using easy_install

easy_install is the old way to install modules. It uses the egg format.

sudo easy_install Pygments

Using the source

Download and uppack the package

wget http://sourceforge.net/projects/pyscard/files/pyscard/pyscard%201.6.12/pyscard-1.6.12.tar.gz#md5=908d2530972ea91eb4bb66987e0e1e98
tar -xvzf pyscard-1.6.12.tar.gz
cd pyscard-1.6.12

To install globally (in /usr/local/lib/python2.7/dist-packages or similar):

sudo ./setup.py install

To install locally (in ~/.local/lib/python2.7/site-packages, use --user:

sudo ./setup.py install --user

One can also use pip to install from source:

sudo pip install .       # Global install
pip install --user .     # Local install

Install modules offline

To install a Python module on a machine that has no connection to Internet [3]:

On a machine with internet connection

# For instance, to install package neovim
mkdir tmp && cd tmp
pip download neovim

On the offline machine, which has access to tmp/:

# For instance, to install package neovim
cd tmp
pip install --no-index --find-links ./ neovim

If you don't have pip on the offline machine, and you can't use an OS package, install directly from source:

Download pip archive from https://pypi.org/project/pip/#files
Copy to offline machine, then

python setup.py install

Install python2 pip on Debian Bullseye

Download python-pip and python-pip-whl from Buster.
Install both packages, this will uninstall python-pip-whl 20.1 and python3-pip

sudo apt install ./python-pip_18.1-5_all.deb ./python-pip-whl_18.1-5_all.deb

Upgrade pip:

sudo python2 -m pip install -U pip

Install back Bullseye python-pip3

sudo apt install python-pip-whl python3-pip

Confirm some packages that were installed with python2 pip:

sudo apt install libpython-all-dev python-all python-all-dev python-pkg-resources python-setuptools

Interactive mode

Python can be run interactively, which is a very powerful way to develop new applications.

Python

To import an existing module, use import as usual:

import mymod             # Import module in current session
from mymod import *      # Idem, but remove mymod. prefix to symbols

iPython / Jupyter

To import an existing module, use import as above or command run:

import mymod       # import file 'mymod.py'
run mymod

Reloading modules automatically

When working on a module, iPython can reload that module automatically [4]:

%load_ext autoreload
%autoreload 2           # Module will be reloaded at each carriage return
import mymod
# ...or...
load_ext autoreload
autoreload 2           # Module will be reloaded at each carriage return
import mymod

%autoreload?           # for help

Python variants

iPy

Use iPy (ipython) to get an interactive shell with auto-completion, instant help...

%magic                    # Get help on %magic commands (%run,...)
?run                      # Get help on %run magic
%run script.py            # Run given script
%run -i script.py         # ... with inspect mode on
%run -i -e script.py      # ... ... and ignore sys.exit() call
!cmd                      # Run shell command 'cmd', for instance ...
!ls                       # ... List file in current directory

Pypy

PyPy is a fast, compliant alternative implementation of the Python language, which usually runs python programs faster thanks to its Just-in-Time compiler.

Install: On Lucid 64-bit, the easiest is to download the dedicated tarball:

wget https://bitbucket.org/pypy/pypy/downloads/pypy-2.2.1-linux64.tar.bz2
tar -cvjf pypy-2.2.1-linux64.tar.bz2

Install virtualenv, then install pypy as virtual environment my-pypy-env

sudo apt-get install python-virtualenv
virtualenv -p pypy-2.2.1-linux64/bin/pypy my-pypy-env

Modules must be installed separatedly for this virtual environment. For instance

./my-pypy-env/bin/pip install libnum

Run: Run python programs using python or pypy

./my-pypy-env/bin/pypy

Python 3 Reference

Source: Python reference, w3schools python tutorial and O'Reilly Python in a Nutshell

Keywords

False      await      else       import     pass
None       break      except     in         raise
True       class      finally    is         return
and        continue   for        lambda     try
as         def        from       nonlocal   while
assert     del        global     not        with
async      elif       if         or         yield

In addition, the following have special meaning:

_*. Also _ is last evaluation result in interactive mode.
__*__ system-defined names.
__* class-private names (rewritten as mangled form by the compiler).

Literals

See Literals in Python reference and Python in a Nutshell.

42         # Integer literal
3.14       # Floating-point literal
3.14e-10   # Floating-point literal
1.0j       # Imaginary literal

[42, 3.14, 'hello']    # List
[]                     # Empty list
100, 200, 300          # Tuple
()                     # Empty tuple
{'x':42, 'y':3.14}     # Dictionary
{}                     # Empty dictionary
{1, 2, 4, 8, 'string'} # Set
# There is no literal to denote an empty set; use set() instead

string literals (str objects)

"hello"
'hello'
"""Good
night"""               # Triple-quoted string literal
r"\b\x"                # raw -- ignore escape sequences
R"\b\x"                # raw -- ignore escape sequences
f"name is {name!r}"    # formatted string literals

multiline string literals (str objects)

"multiline\nstring"    # simple quote with embedded \n
"""multi-line
string"""              # triple quote, preserve newlines, but not indent friendly
("multi-line\n"
"string")              # Using bracket, recommended by PEP. indent friendly.
"multi-line\n" \
"string"               # Using backslash. indent friendly.

bytes literals (bytes objects)

b"abc\x81\x82"
B"abc\x81\x82"
rb"abc\x81\x82"        # raw -- ignore escape sequences
RB"abc\x81\x82"        # raw -- ignore escape sequences

formatted string literals (3.6)

name="Fred"
f'His name is {name!r}'     # !r conversion, applies repr()
f'His name is {repr(name)}' # equivalent
                            # !s does str(), !a does ascii()
f'length is {len(name)}'    # expression
width=8; prec=3;
f'{3.14159:{width}.{prec}}' # integer formatting
n = 1024
f'{n:x}'                    # '400'
f'{n:4x}'                   # ' 400'
f'{n:04x}'                  # '0400'
f'{n:#x}'                   # '0x400'
f'{n:#6x}'                  # ' 0x400'
f'{n:#06x}'                 # '0x0400'
today = datetime(year=2017, month=1, day=27)
f'{today:%B %d, %Y}'        # date format specifier
f'{n} vs {{n}} vs {{{n}}}'  # '1024 vs {n} vs {1024}'

Raw string literals

r'^foo\.bar$'               # Useful for regex mainly (fix invalid escape sequence)
bar="BAR"
fr'^foo\.{bar}$'            # raw AND formatted string

Operators

+       -       *       **      /       //      %      @
<<      >>      &       |       ^       ~       :=
<       >       <=      >=      ==      !=

Operators and their evaluation order, from highest to lowest:

, [...] {...} `...`                   # Tuple, list & dict. creation; string conv.
s[i] s[i:j] s.attr f(...)             # indexing & slicing; attributes, function calls
+x, -x, ~x                            # Unary operators
x**y                                  # Power
x*y x/y x//y x%y                      # mult, division, floor division (integer division), modulo
x+y x-y                               # addition, substraction
x<<y   x>>y                           # Bit shifting
x&y                                   # Bitwise "and"; also intersection of sets
x^y                                   # Bitwise exclusive or
x|y                                   # Bitwise "or"; also union of sets
x<y  x<=y  x>y  x>=y  x==y x!=y  x<>y # Comparison
x is y   x is not y                   # identity
x in s   x not in s                   # membership
not x                                 # boolean negation
x and y                               # boolean and
x or y                                # boolean or
lambda args: expr                     # anonymous function

Arithmetic operators

1//2                                  # Floor division (PEP-238)

ternary operator

x_sign = 'positive' if (x>=0) else 'negative'

Notes

Use is or notfor testing None

if (p.poll() is None):         # Use 'is' for testing None
    print "None"
if not p.poll():               # ... or 'not'
    print "None"

Delimiters

(       )       [       ]       {       }
,       :       .       ;       @       =       ->
+=      -=      *=      /=      //=     %=      @=
&=      |=      ^=      >>=     <<=     **=

Characers with special meanings as part of other tokens:

' " # \

Data types

Boolean

True            # constant for true
False           # constant for false
bool(x)         # To convert to bool built-in type

Avoid unnecessary call to bool(x).

if x:                     # GOOD
if bool(x):               # BAD
if x is True:             # BAD
if x == True:             # BAD
if bool(x) == True:       # BAD

A valid use:

def count_trues(seq): return sum(bool(x) for x in seq)   # Ensure each item is counted either as 0 or 1

Strings

Strings in Python are immutable objects. There are many differences between Python2 and Python3.

Python 2	Python 3
There are two type of strings: `str` (like `'foo'`) that are bytestring, ie. array of bytes. type('foo') # <type 'str'> `unicode` (like `u'foo'`) that are textual string (Unicode). type(u'foo') # <type 'unicode'>	There are two type of strings: `str` (like `'foo'`) that are textual string (Unicode). type('foo') # <class 'str'> `bytes` (like `b'foo'`) that are bytestring, ie. array of bytes. type(b'foo') # <class 'bytes'> So Python3's `'foo'` is Python2's `u'foo'`, and Python2's `'bar'` is Python3's `b'bar'`.
s.decode() Converts bytes to str (unicode). s.encode() Converts str (unicode) to bytes.	s.decode() `bytes` only. Converts bytes to str (unicode). s.encode() `str` only. Converts str (unicode) to bytes.

b'hello' == 'hello'.encode()          # str to bytes
'hello' == b'hello'.decode()          # bytes to str
"def" in "abcdefgh"                   # substring
s.upper()                             # Change 'uppercase' to 'UPPERCASE'
', '.join(set_3)                      # Join a sequence
map(ord, hex_data)                    # [0xDE, 0xAD, 0xBE, 0xEF]

# Strings function
s="Hello, World"
s.endswith('World')                   # True
s.startswith('Hello')                 # True

Bitstring

See Bitstring module.

List

Nice tutorial: http://effbot.org/zone/python-list.htm

a=[0,3,6]
print a[1]                     # 3

a=[0] * 1000                   # Array with 1000 elements
len(a)                         # Number of elements

b=a                            # This only copy the REFERENCE
b[0]+=1                        # ... this also changes a[0]
b=a[:]                         # This makes a NEW COPY
b=a.copy()                     # PYTHON >3.3

import copy
a=[[1,2],[3,4]]
b=a.deepcopy()                 # Deep copy - MUST for dimension >= 2

a[:]=a[::-1]                   # Reassign element in the list (here in reverse order)
a=a[::-1]                      # Idem, but create a new object

a=[];
a.append(12);                  # Create object before appending
a[len(a):] = [13];             # Same as appending

a=[1,2,3]
a.extend([4,5,6])              # [1,2,3,4,5,6] -- extend with an iterable

l=[1,2,3]
l.pop()                        # 3 - pop last element
l.pop(0)                       # 1 - pop first element - consider deque and popleft() for better perf
del l[0]                       # Delete first element

list("abc")                    # ['a', 'b', 'c']

line = '1234567890'
n = 2
[line[i:i+n] for i in range(0, len(line), n)]   # ['12', '34', '56', '78', '90']

def shiftRow(word, n):
    return word[n:]+word[0:n]
state[i::4] = shiftRow(state[i::4],i)      # Apply shiftRow on 4 bytes distant of 4 each

alist = map(lambda b: sbox[b],alist)

state[:] = [ a ^ b for a,b in zip(state,roundKey) ]    # Ex-oring 2 lists of integers

# Multi-dimensional list
matrix = [[0 for _ in range(5)] for _ in range(5)]     # Initialize bi-dimensional array
matrix = [[0]*5 for _ in range(5)]                     # faster way
# matrix = 5*[5*[0]]                                   # WRONG - 5 times copy of same

# Compare - simply use ==
[1,2,3] == [1,2,3]                                     # True
[1,2,3] == [1,2,3,4]                                   # False
[1,2,3] == ['a','b']                                   # False

# ... to remove order and duplicates, use set()
set([1,2,3]) == set([2,1,3,3])                         # True

# Sort
a.sort()

# Sum
a=[8,19,3,17,12,2]
sum(x <= 10 for x in a)
sum(1 for x in a if x <= 10)                          # List comprehension

def count(iterable):
    return sum(1 for _ in iterable)
sub10Count = count(x for x in a if x <= 10)           # Cheap (doesn't create useless list) and readable

# Adding (https://stackoverflow.com/questions/18713321/element-wise-addition-of-2-lists)
                            [sum(x) for x in zip(list1, list2)]     # 177ms
from itertools import izip; [sum(x) for x in izip(list1, list2)]    # 139ms
                            [a + b for a, b in zip(list1, list2)]   # 112ms, most pythonic
from itertools import izip; [a + b for a, b in izip(list1, list2)]  #  71ms, pythonic
from operator import add;   map(add, list1, list2)                  #  44ms

import numpy as np
vector1 = np.array([1, 2, 3])
vector2 = np.array([4, 5, 6])
sum_vector = vector1 + vector2                                      # 25x faster

# Find *first* matching item
["foo", "bar", "baz"].index("bar")                                  # 1  !!! Throws ValueError if item not found
try:
    return L.index(obj)                                             # Fastest method - note: this could return -1
except ValueError:
    # ...
if obj in L:
    return L.index(obj)                                             # Faster if obj not found

# Find all items
[i for i, e in enumerate([1, 2, 1]) if e == 1]                      # [0, 2]
g = (i for i, e in enumerate([1, 2, 1]) if e == 1)
next(g)                                                             # 0
next(g)                                                             # 2

# Sort based on object attribute
ut.sort(key=lambda x: x.count, reverse=True)   # To sort the list in place...
newlist = sorted(ut, key=lambda x: x.count, reverse=True)  # To return a new list, use the sorted() built-in function...

(From stackoverflow [5])

for c in list(sha256.digest()):
    key.append(ord(c))

Dictionary

D = { 'x':42, 'y':3.14, 'z':7 }
D['x']                              # 42
del D[k]                            # Removes from dictionary D the item whose key is k
#Spare matrix
Matrix = {}
Matrix[1,2] = 15                    # This works because 1,2 -- a tuple -- is used as a key

for key in d:                       # Loop over keys in dictionary d
for key, value in d.iteritems():    # Loop over keys and values in dictionary d

Set

S = set()                           # Empty set
S = {1,2,3}                         # Set with some values
S.add(4)                            # Add an element
S.update([2,5])                     # Add a list of element
S.update({2,5})                     # ... any iterable

(elem,) = S                         # get only elem -- fail if S not singleton
elem = next(iter(S))                # get any elem -- work if S singleton or not

Control flow statements

if

if x < 0: print('x is negative')
elif x % 2: print('x is positive and odd')
else: print('x is even and non-negative')

# Better style (PEP 8):
if x < 0:
    print('x is negative')
elif x % 2:
    print('x is positive and odd')
else:
    print('x is even and non-negative')

while

count = 0
while x > 0:
    x //= 2              # truncating division
    count += 1
    print('The approximate log2 is', count)

for

for letter in 'ciao':
    print('give me a', letter, '...')

# target can be a tuple
for key, value in d.items():
    if key and value:        # print only true keys and values
        print(key, value)

# ... or something else (LHS expression)
prototype = [1, 'placemarker', 3]
for prototype[1] in 'xyz': print(prototype)
# prints [1, 'x', 3], then [1, 'y', 3], then [1, 'z', 3]

# Using range():
for i in range(10):
    statement(s)
for i in range(5,10):
    statement(s)

#Using list comprehension:
result1 = [x+1 for x in some_sequence]
#... same as:
result2 = []
for x in some_sequence:
    result2.append(x+1)
# Comprehension list may have 'if', or nested for
result3 = [x+1 for x in some_sequence if x>23]
result5 = [x for sublist in listoflists for x in sublist]

# Dict comprehension
d = {n:n//2 for n in range(5)}
print(d) # prints: {0:0, 1:0, 2:1, 3:1, 4:2] or other order

break

while True:               # this loop can never terminate naturally
    x = get_next()
    y = preprocess(x)
    if not keep_looping(x, y): break
    process(x, y)

continue

for x in some_container:
    if not seems_ok(x): continue

for-else and while-else

for x in some_container:
    if is_ok(x): break # item x is satisfactory, terminate loop
else:
    print('Beware: no satisfactory item was found in container')
    x = None

pass

if condition1(x):
    process1(x)
elif x>23 or condition2(x) and x<5:
    pass                  # nothing to be done in this case
elif condition3(x):
    process3(x)
else:
    process_default(x)

try-except-finally-else / raise

try:
    print(x)
except:
    print("An exception occured")

try:
  print(x)
except NameError:      # Can give many except
  print("Variable x is not defined")
except:
  print("Something else went wrong")

try:
  print(x)
except:
  print("Something  went wrong")
else:   # exec'ed if no error and NO BREAK
  print("try block finished")

try:
  print(x)
except:
  print("Something  went wrong")
finally:   # exec'ed no matter what
  print("the 'try except is finished'")

raise Exception("Sorry, that was wrong")

try:
    i = int(s.strip())
except OSError as err:
    print("OS error: {0}".format(err))
except ValueError:
    print("Could not convert data to an integer.")
except:
    print("Unexpected error:", sys.exc_info()[0])
    raise

with

The with statement is the Python embodiment of the well-known C++ idiom “resource acquisition is initialization" (RAII)

with expression [as varname]:
    statement(s)

Functions

a = 'global'
def afunction():
    global a                         # Use 'global' to change scope of a variable
    a = 'still using global'
    b = 'local'

Docstrings

def toh(cls,s):
    """ Convert a (binary) string into an hexadecimal string.
    >>> mc.toh('ABCD')
    '41424344'
    >>> mc.toh('mycrypto')
    '6d7963727970746f'
    """
    return s.encode('hex')

Docstrings can also be defined at module level. The docstring line must appear before imports:

#! /usr/bin/python3

"""Use int() to convert either binary or hex string to an integer
>>> int('11110000',2)
240
"""

import binascii

Use module doctest to test examples in docstrings:

# Check docstring examples on exec (not on import)
if __name__ == "__main__":
    import doctest
    doctest.testmod()

Classes

Reference: Python docs.

An empty class:

class Empty(object):
    pass

e=Empty()

A class with constructor and data members:

class Basic(object):
    __param = None                           # __* denotes a class-private member

    def __init__(self, param):
        self.__param = param
        print "Basic is born with param %s" % param

b1=Basic('foo')
b2=Basic(param='bar')

A class that inherits:

class Child(Parent):
    __param = None

    def __init__(self, param):
        Parent.__init__(self)                # Must call EXPLICITLY parent constructor
        self.__param = param

Class members can be defined as properties:

class Rectangle(object):
    def __init__(self, width, height):
        self.width = width
        self.height = height
    @property
    def area(self):
        '''area of the rectangle'''
        return self.width * self.height
    @area.setter
    def area(self, value):
        scale = math.sqrt(value/self.area)
        self.width *= scale
        self.height *= scale

Classes may have static methods and class methods [6]:

class Rectangle(object):
    max_area = 10       # A class variable shared by all instances

    def __init__(self, width, height):
        self.width = width
        self.height = height

    @staticmethod
    def give_height(area,width):
        return area / width

    @classmethod
    def get_max_height(cls,max_area):
        return cls.max_area

Using modules

References:

importlib, the default implementation for Python import
Relative imports for the billionth time, easy explanation of relative import issues.

Assume we have a module named module.py:

import module;               # Import everything in module.* namespace
from module import *;        # Import everything in current namespace

import sys
sys.path.append('some/custom/path')
import module;               # Import module from a custom path

Use built-in __import__, or better yet importlib.import_module, to import a module whose name is in a string (http://effbot.org/zone/import-string.htm)

mymodule = __import__('mymodule')  # Import module from string - see http://effbot.org/zone/import-string.htm

import importlib
importlib.import_module('mymodule')

Note that modules can be imported anywhere, not just at the start of the file. This allows for loading a module only when necessary.

Scripts vs module

There are two ways to load a python file: as the top-level script, or as a module.
File is loaded as top-level script when it is executed directly (eg. python myfile.py). The __name__ of the top-level script is always __main__.
A module is a file imported with import mymodule. The __name__ is mymodule.
A module can be part of a package (say package.mysubpackage.mymodule). Module in package can do relative import (from .. import blah).

Relative imports

This usually works, at least for a top-level script (see also post above, and section below Get path of current file)

# To import ../../some/package/mymodule.py, relatively to current file
sys.path.append(os.path.dirname(os.path.abspath(__file__)) + '/../../some/package')
import mymodule

Python 2 reference

Print

for i in range(10):
    print i

# Add a comma to remove carriage return
for i in range(10):
    print i,                     # 0 1 2 3 4 5 6 7 8 9

To enable Python 3 print function:

from __future__ import print_function        # Enable v3 print in Python 2.x

Basic I/O in Python

Source: O'Reilly Python in a Nutshell.

String formatting with `format` or formatted-string literals

Source:

https://docs.python.org/3/tutorial/inputoutput.html
See Python in Nutshell, chapter 8 for more information.

Available since Python 3.

# v3 - String formatting
# '{[selector][conversion]:[format_specifier]}'.format(value)
'First: {} second: {}'.format(1, 'two')
'Second: {1} first: {0}'.format(1, 'two')                        # Give positional for all 
'a: {a}, 1st: {}, 2nd: {}, a again: {a}'.format(1, 'two', a=3)   # Give name for some
'a: {a} first:{0} second: {1} first: {0}'.format(1, 'two', a=3)  # Can mix name and positional

# Using sequences and composites:
'p0[1]: {[1]} p1[0]: {[0]}'.format(('zero', 'one'), ('two', 'three'))
'p1[0]: {1[0]} p0[1]: {0[1]}'.format(('zero', 'one'), ('two', 'three'))
'{} {} {a[2]}'.format(1, 2, a=(5, 4, 3))
'First r: {.real} Second i: {a.imag}'.format(1+2j, a=3+4j)

# Field width
'{:^12s}'.format(s)
'{:.>12s}'.format(s)
print('{:,}'.format(12345678))

# Precision specification
'as f: {:.4f}'.format(x)
'as g: {:.4g}'.format(x)
'as s: {:.6s}'.format(s)

# Formatted-string literals
# ... Use {{ or  }} to insert a literal left or right curly brace
table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 7678}
for name, phone in table.items():
    print(f'{name:10} ==> {phone:10d}')

String formatting with `%`

Available in Python 2 and 3.

# format % values
'result = %d' % x               # %d - decimal
'answers: %d %f' % x, y         # %f - float
'%x' % hexval                   # Print hex
'File not found %r' % filename  # !!! USE %r to log possibly erroneous strings !!!

Input parsing

Text output

print(value, ..., sep=' ', end='\n', file=sys.stdout, flush=False)

for i in range(10):
    print(i,"",end="")             # 0 1 2 3 4 5 6 7 8 9 
for i in range(10):
    print(f"{i} ",end="")          # 0 1 2 3 4 5 6 7 8 9 

import sys;
sys.stdout                         # Standard output
sys.stderr                         # Standard error

# Output to a file
print(file=f,'...')
f.write('...')
sys.stdout.write(...)              # Using write with stdout

# Output to stderr
sys.stderr.write(...)              # Using write
print(file=sys.stderr,'...')       # Using print
def eprint(*args, **kwargs):
    print(*args, file=sys.stderr, **kwargs)
eprint('...')                      # Using custom fct

Text input

Standard Library

sys module

Arguments

sys.argv, len(sys.argv)          # Argument list, number of arguments ([0] -> exec name)
if ("-h" in sys.argv) or ("--help" in sys.argv):
    printUsage()
for a in range(len(sys.argv)):
    if sys.argv[a] == "-e":
        # handler

Exit

sys.exit()

io module

To open a file:

# - mode can be 'r', 'w', 'a', 'r+', 'w+', 'a+', ...
#   Default is text 't', add 'b' for binary, 'U' for universal line ending
open(file, mode='r', buffering=-1, encoding=None, errors='strict', newline=None, closefd=True, opener=os.open)

with io.open(...) as f:            # PYTHONIC way, open is a manager
    # ...

for line in io.open(...):          # PYTHONIC way to read line by line, file close automatically
    # ...

f = io.open(...)                   # BAD. No guarantee that f gets closed

File operations:

f.close()
f.flush()
str = f.read(size=-1)              # bytestring in bynary mode, text string otherwise.
str = f.readline(size=-1)
lst = f.readlines(size=-1)
lst = [l.strip() for l in open(...)] # To get rid of '\n', trailing spaces...
with open(filename) as f:
    mylist = f.read().splitlines() # To get rid of '\n' only
f.write(s)
f.writelines(lst)                  # Same as: for line in lst: f.write(line)

Iterations:

for line in f:
    # ...                          # !!! 'break' and 'next(t)' interferes with file's position
                                   # f.readline() is ok.

os and filesystem operations

import os
os.remove(path)                 # Remove a file
os.unlink(path)                 # ... idem
os.rmdir(path)                  # Remove an (empty) directory
os.path.dirname(path)
os.path.basename(path)

import shutil
shutil.rmtree(path, ignore_errors=False, onerror=None)   # Remove a directory and all its content

import os.path
os.path.isfile(fname)            # True if fname exists and is a file

if not os.path.exists(directory):
    os.makedirs(directory)       # Create directory if does not exists

try:                             # Avoid race condition if directory created by another process
    os.makedirs(path)            # But we could fix solution above as well
except OSError:                  # This one always trigger an exception in nominal case
    if not os.path.isdir(path):  
        raise

Scanning a directory

import glob
tests = glob.glob('tests/tests_*.py')
for t in tests:
    print("tests %s" % t)

# https://stackoverflow.com/questions/6773584/how-is-pythons-glob-glob-ordered
import os
sorted(glob.glob('*.png'))                         # Sort by name
sorted(glob.glob('*.png'), key=os.path.getmtime)   # Sort by modification time
sorted(glob.glob('*.png'), key=os.path.getsize)    # Sort by size

Executing a command in subshell:

os.system(f"diff -rq {dir1} {dir2} >/dev/null 2>/dev/null")   # Return code are multiplied by 256

It is however recommended to use subsystem.call rather than os.system.

argparse module

See excellent argparse tutorial.

# Parse command line
parser = argparse.ArgumentParser()
parser.add_argument("-p", "--port", type=int, default=PORT, help="server port number")
group = parser.add_mutually_exclusive_group()
group.add_argument("-a", "--attach", action="store_true", help="don't start a new server but attach to a running one")
group.add_argument("-t", "--target", default=TARGET, help="path to server executable")
parser.add_argument("test", nargs='+', help="path to python module containing tests to run")
args = parser.parse_args()

bench = TestBench(target=args.target, port=args.port, attach=args.attach)

for s in args.test:
    print("test %s" % s)

random module

import random
IV = []
for i in range(16):
    IV.append(random.randint(0, 255))

datetime module

import datetime
print datetime.datetime.today()  
print datetime.datetime.now()    # similar, but possibly more accurate
print datetime.date.now()        # date only

bitstring module

from bitstring import *
s  = Bits('0x8081828384858687')
s  = Bits(hex='8081828384858687')
s  = Bits(bytes=b'\x80\x81\x82\x83\x84\x85\x86\x87')
sa = BitArray('0x8081828384858687')    # same as Bits, but mutable

s << 8                           # Logical shift
s[8:] + '0x00'                   # ... same as above
s <<= 8                          # ... (with mutation)
sa.rol(8)                        # Cyclic shift (with mutation)
s[8:] + s[:7]                    # ... same as above

See bitstring [7] (manual)

Named Tuple

Source: SO

Using named tuples from collections:

# Python 2 and 3
from collections import namedtuple
MyStruct = namedtuple("MyStruct", "field1 field2 field3")

m = MyStruct("foo", "bar", "baz")
m = MyStruct(field1="foo", field2="bar", field3="baz")

Since Python 3.6, improved NamedTuple:

# Python 3.6
from typing import NamedTuple

class User(NamedTuple):
    name: str

class MyStruct(NamedTuple):
    foo: str
    bar: int
    baz: list
    qux: User

my_item = MyStruct('foo', 0, ['baz'], User('peter'))

Since Python 3.7, Data Classes:

# Python 3.7
from dataclasses import dataclass

@dataclass
class Point:
    x: float
    y: float
    z: float = 0.0

p = Point(1.5, 2.5)

print(p)  # Point(x=1.5, y=2.5, z=0.0)

parsing module

Source: https://pypi.org/project/parse/, https://stackoverflow.com/questions/2175080/sscanf-in-python

Parse is the opposite of format.

from parse import parse

parsed=parse('{} fish',"blue fish")
print(parsed[0])                       # 'blue'

min,max,letter,pwd=parse('{}-{} {}: {}',"4-8 n: noon")
print(f"{min}, {max}, {letter}, {pwd}")

regex module

Source: w3schools

See also parse module for more advanced parsing

# It's a good habit to put re string in raw r'...' strings!

import re
re.search(r'^The.*Spain$', "The rain in Spain")    # re.Match object
re.match(r'The.*Spain$', "The rain in Spain")      # match always start from beg.

re.findall(r'.ai', "The rain in Spain")            # ['rai', 'pai']
re.split(r'\s', "The rain in Spain")               # ['The', 'rain', 'in', 'Spain']
re.sub(r'\s', "_", "The rain in Spain")            # The_rain_in_Spain
re.sub(r'\s', "_", "The rain in Spain", 2)         # The_rain_in Spain

# re.Match object
re.search(r'.ai', "The rain in Spain").group()     # rai
re.search(r'.ai', "The rain in Spain").span()      # (4, 7)
re.search(r'.ai', "The rain in Spain").string      # The rain in Spain

# re.sub
x = re.sub(r'\s', "_", "The rain in Spain")        # The_rain_in_Spain

# re.compile
r = re.compile(r'.ai')
r.search("The rain in Spain").group()              # rai

# flags
a="foo\nbar"
re.match(r'foo\nbar',a) is not None           # True
re.match(r'foo\nbar$',a) is not None          # True
re.match(r'foo$\nbar',a) is not None          # False - ^ and $ only match first/last
re.match(r'fo.*ar$',a) is not None            # False - . doesn't match \n
re.match(r'fo.*ar$',a,re.S) is not None       # True - re.S: . match also \n
re.match(r'foo$\nbar$',a,re.M) is not None    # True - re.M: ^ and $ match any \n
re.match(r'f.*^bar$',a,re.S|re.M) is not None # True

Subsystem

Execute a command in a subshell (better than os.system):

import subsystem
code = subsystem.call("diff", "-rq", dir1, dir2, stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)

Here, no specific handling is necessary if dir1 or dir2 contains special characters.

Cryptography

Package pycrypto

from Crypto.Cipher import AES
def toh(s):
    return s.encode('hex')
def tos(h):
    return h.replace(' ','').decode('hex')
def aes(k,p):
    a=AES.new(tos(k))
    return toh(a.encrypt(tos(p)))
def aesinv(k,c):
    a=AES.new(tos(k))
    return toh(a.decrypt(tos(c)))
def sxor(h1,h2):
    return toh(''.join(chr(ord(a) ^ ord(b)) for a,b in zip(tos(h1),tos(h2))))

Example of use:

ipython

run mycrypto                    # Assuming script in current dir and named 'mycrypto.py'
key='00112233 44556677 8899aabb ccddeeff'
p0='00000100 80000000 00000000 00000000'
c0=aes(key,p0)
p1='aaaaaaaa bbbbbbbb cccccccc dddddddd'
c1=aes(key,sxor(c0,p1))

Modular inverse [8]

# Using gmpy - FASTEST
import gmpy
gmpy.invert(1234567, p)                      # 1000000 loops, best of 3: 737 ns per loop (p 1024-bit)
gmpy.divm(1, 1234567, p)                     # 1000000 loops, best of 3: 933 ns per loop (p 1024-bit)

# Using egcd function - NO DEPS, BUT SLOWER
def egcd(a, b):
    if a == 0:
        return (b, 0, 1)
    else:
        g, y, x = egcd(b % a, a)
        return (g, x - (b // a) * y, y)

def modinv(a, m):
    g, x, y = egcd(a, m)
    if g != 1:
        raise Exception('modular inverse does not exist')
    else:
        return x % m
timeit modinv(1234567,p)                     # 100000 loops, best of 3: 13.6 us per loop (p 1024-bit)

# Using pow() - SIMPLEST BUT SLOWEST
timeit pow(1234567,p-2,p)                    # 100 loops, best of 3: 4.22 ms per loop

modular exponentiation

from gmpy import mpz
def power_mod(a, b, n):
    return long(pow(mpz(a),b,n))

Package hashlib

# Example from https://www.quickprogrammingtips.com/python/how-to-calculate-sha256-hash-of-a-file-in-python.html
import hashlib

filename = input("Enter the input file name: ")
sha256_hash = hashlib.sha256()
with open(filename,"rb") as f:
    # Read and update hash string value in blocks of 4K
    for byte_block in iter(lambda: f.read(4096),b""):
        sha256_hash.update(byte_block)
    print(sha256_hash.hexdigest())

Doctest

The doctest module searches for pieces of text that look like interactive Python sessions, and then executes those sessions to verify that they work exactly as shown.

See example below.

# file dc.py

def toh(s):
    """ Convert a (binary) string into an hexadecimal string.
    >>> toh('DOH!')
    '444f4821'
    """
    return s.encode('hex')

if __name__ == "__main__":
    import doctest
    doctest.testmod()

Run the tests with:

python dc.py

Testing

pytest

See pytest.

Tips

Simple HTTP Server

It's very easy to setup an ad-hoc HTTP server with Python. Just open a shell in a folder with some contents to share, and type:

python -m SimpleHTTPServer

More available at http://docs.python.org/2/library/internet.html (see BaseHTTPServer and CGIHTTPServer).

Detect interactive mode

References: [9], [10]

Started with	First method	Second method	Third method	Fourth method
	`import __main__ as main print hasattr(main, '__file__')`	`def in_ipython(): try: __IPYTHON__ except NameError: return False return True`	`import sys print hasattr(sys, 'ps1'):`	`import sys print bool(sys.flags.interactive)`
`python mymod.py`	True	-	-	-
`python -i mymod.py`	True	-	-	True
`python` then `import mymod`	-	-	True	-
`ipython mymod.py`	True	True	-	-
`ipython -i mymod.py`	True	True	-	-
`ipython` then `run mymod.py`	True	True	-	-
`ipython` then `run -i mymod.py`	True	True	-	-
`ipython` then `import mymod`	-	True	-	-
`ipython -i` then `import mymod`	-	True	-	-

Find duplicates in list

From stackoverflow [11]

import collections

def fastest():                         # 134 us - Fastest
    seen = set()
    seen_add = seen.add                                            # To avoid lookup 'add' ever time an item is inserted
    seen_twice = set( x for x in l if x in seen or seen_add(x) )   # adds all elements it doesn't know yet to seen and all other to seen_twice
    return list( seen_twice )                                      # turn the set into a list (as requested)

def compact():                         # 415 us
    return [x for x, y in collections.Counter(l).items() if y > 1]

def slowest():                         # 19.2 ms
    return list(set([x for x in l if l.count(x) > 1]))

Start post-mortem debugger on exception

From stackoverflow [12]

>>> import pdb
>>> pdb.pm()

Miscellaneous

Detect whether a variable is defined

Note it is bad practice to define a variable conditionally [13]. An interesting use case is to run code and define variable conditionally based on interactive status.

# Using try ... except
try: myvar
except NameError: print "variable 'myvar' IS defined"

# Using vars() / globals()
'myvar' in vars() or 'myvar' in globals()
# ...pedantic...
'myvar' in vars(__builtins__)

Analyse memory usage

Dowser

See [14] — seems better suited to find memory leaks, not to analyse usage for memory hungry applications

memory_profiler

See [15]
Install

sudo pip install -U memory_profiler
sudo pip install psutil

Add @profile decorator

@profile
def primes(n): 
    ...

Run the profiler

 python -m memory_profiler primes.py

The Pythonic way

Type import this in a Python interpreter, you get this:

The Zen of Python, by Tim Peters

Beautiful is better than ugly.
Explicit is better than implicit.
Simple is better than complex.
Complex is better than complicated.
Flat is better than nested.
Sparse is better than dense.
Readability counts.
Special cases aren't special enough to break the rules.
Although practicality beats purity.
Errors should never pass silently.
Unless explicitly silenced.
In the face of ambiguity, refuse the temptation to guess.
There should be one-- and preferably only one --obvious way to do it.
Although that way may not be obvious at first unless you're Dutch.
Now is better than never.
Although never is often better than *right* now.
If the implementation is hard to explain, it's a bad idea.
If the implementation is easy to explain, it may be a good idea.
Namespaces are one honking great idea -- let's do more of those!

Detect Python 2 or Python 3 dependency

For instance, does gdb uses python 2 or 3?

ldd $(which gdb)|grep python
# libpython3.5m.so.1.0 => /usr/lib/x86_64-linux-gnu/libpython3.5m.so.1.0 (0x00007f442a960000)

Find character in a string

The fastest and simplest is to use in operator, like

if '.' in name:
    # ...

To detect more characters, we must use a regex [16]:

>>> import re
>>> def special_match(strg, search=re.compile(r'[^a-z0-9.]').search):
...     return not bool(search(strg))
>>> special_match("az09.")
True
>>> special_match("az09.\n")
False

Note:

search is faster than using match.
If using match, there is no need to use ^...$ to force a full match.
Regex should use raw string r'...'.
If using the regex multiple times, compile it once and reuse later!

Detect Python version, location...

From pwndbg [17]:

# Find the Python version
PYVER=$(python -c 'import platform; print(".".join(platform.python_version_tuple()[:2]))')
PYTHON=$(python -c 'import sys; print(sys.executable)')
PYTHON="${PYTHON}${PYVER}"

# Find the Python site-packages that we need to use
SITE_PACKAGES=$(python -c 'import site; print(site.getsitepackages()[0])')
# or to get user site
SITE_PACKAGES=$(python -c 'import site; print(site.getusersitepackages())')

Using script above, one can install a module using pip for the given python/site installation.

# Install Python dependencies using pip
sudo ${PYTHON} -m pip install --target ${SITE_PACKAGES} -Ur requirements.txt

Display random distribution with seaborn

seaborn is a powerful python toolkit to visualize statistical data.

Assume a data file like

head -n 5 samples
# 19.2
# 6.6
# 7.9
# 5.5
# 3.6
# ...

To visualize into seaborn:

# First setup seaborn - https://seaborn.pydata.org/tutorial/distributions.html
%matplotlib gtk

import numpy as np
import pandas as pd
from scipy import stats, integrate
import matplotlib.pyplot as plt
import seaborn as sns
sns.set(color_codes=True)
np.random.seed(sum(map(ord, "distributions")))

# Then load our file - https://stackoverflow.com/questions/36343646/reading-a-text-file-and-converting-string-to-float
file_in = open('../samples','r')
for z in file_in.read().split('\n'):
    if z: y.append(float(z))
file_in.close()

# Then tell seaborn to show the distribution. If 
sns.distplot(y)

# Normally the graph should pop up automatically. If not:
# plt.show()
# sns.plt.show();

Convert bytes to str and vice-versa

Python v2 and v3 have different types of strings.

In v2, the type str is a sequence of bytes, while unicode are for Unicode text strings.
In v3, the type str are for Unicode text strings, and bytes is a sequence of bytes, also known as bytestring or byte string.

# Python v3
isinstance(s,str)          # True if s is a unicode text string
isinstance('abc',str)      # True
isinstance(b,bytes)        # True if b is a bytestring
isinstance(b'abc',bytes)   # True
s.encode()                 # Convert a text string (str) to bytes
b.decode()                 # Convert a bytestring (bytes) to str

XOR strings together

In Python 2.x [18]:

def sxor(s1,s2):
    return ''.join(chr(ord(a) ^ ord(b)) for a,b in zip(s1,s2))

In Python 3.x:

def bytes_xor(a, b) :
    return bytes(x ^ y for x, y in zip(a, b))

Various conversion

Binary 00110101

# Or use bin to convert an integer into binary literal string ('0b' prefix)
>>> bin(173)
'0b10101101'
# Binary literals are regular integers
>>> 0b101111
47
# Use int(..., 2) to convert a binary string into integer
>>> print int('01010101111',2)
687
>>> print int('11111111',2)
255

Reverse a string

>>> 'hello world'[::-1]
'dlrow olleh'

Reload a module in interactive python

There is reload command:

Python3 >= 3.4: importlib.reload(some_module)
Python3 < 3.4: imp.reload(some_module)
Python2: reload(some_module) built-in

For instance

import importlib
import some_module

# hack hack...

importlib.reload(some_module)           # Reload module

However

reload does not reload dependencies.
It does not work when module is loaded like from some_module import *.

Usually it's simpler to do:

python3 -c 'from some_module import *'
# >>> hack hack...
# >>> <CTRL-D>
python3 -c 'from some_module import *'
# >>> ....

Benchmark an algorithm

From the shell, using the timeit module:

python -mtimeit -s'l=[[1,2,3],[4,5,6], [7], [8,9]]*99' '[item for sublist in l for item in sublist]'
# 10000 loops, best of 3: 143 usec per loop
python -mtimeit -s'l=[[1,2,3],[4,5,6], [7], [8,9]]*99' 'sum(l, [])'
# 1000 loops, best of 3: 969 usec per loop
python -mtimeit -s'l=[[1,2,3],[4,5,6], [7], [8,9]]*99' 'reduce(lambda x,y: x+y,l)'
# 1000 loops, best of 3: 1.1 msec per loop

Or directly in Python, using timeit.Timer:

>>> timeit.Timer(
        '[item for sublist in l for item in sublist]',
        'l=[[1, 2, 3], [4, 5, 6, 7, 8], [1, 2, 3, 4, 5, 6, 7]] * 10000'
    ).timeit(100)
2.0440959930419922

Flatten a list of lists (of lists...)

from SO:

l = [[1, 2, 3], [4, 5, 6], [7], [8, 9]]

# Fastest - using iconcat
functools.reduce(operator.iconcat, a, [])

# Fastest - using itertools
list(itertools.chain(*list2d))
list(itertools.chain.from_iterable(list2d))  # Since Python 2.6, no unpacking needed

# Using list comprehension - very fast
flat_list = [item for sublist in l for item in sublist]

# Using sum and monoid - fastest for small list, very compact
sum(l, [])

# Using lambda, slowest
reduce(lambda x,y: x+y,l)

See also this blogspot, for a non-recursive solution that can process even deeply nested lists.

Detect last element in a for loop

From SO:

def lookahead(iterable):
    """Pass through all values from the given iterable, augmented by the
    information if there are more values to come after the current one
    (False), or if it is the last value (True).
    """
    # Get an iterator and pull the first value.
    it = iter(iterable)
    last = next(it)
    # Run the iterator to exhaustion (starting from the second value).
    for val in it:
        # Report the *previous* value (more to come).
        yield last, False
        last = val
    # Report the last value.
    yield last, True

for i, has_more in lookahead(range(3)):
    print(i, has_more)

Swap two variables

The pythonic way [19]:

a,b = b,a

Print to stderr

# For Python 2:
# from __future__ import print_function
# import sys

def eprint(*args, **kwargs):
    print(*args, file=sys.stderr, **kwargs)

Note that stderr is not buffered, so no need to flush [20].

Get product of all elements in a list

import numpy
L={10,20,30}
int(numpy.prod(L))      # 6000

Check that a variable is an integer

isinstance(1, int)      # True
isinstance(1.1, int)    # False

Get path to current script

From StackOverflow:

Python 3

# Directory of the script being run
import pathlib
pathlib.Path(__file__).parent.absolute()

# Current working directory
import pathlib
pathlib.Path().absolute()

Python 2 and 3

# Directory of the script being run
import os
os.path.dirname(os.path.abspath(__file__))
os.path.dirname(__file__)                      # BAD!!! is empty if __file__ has no dir component

# Current working directory
import os
os.path.abspath(os.getcwd())

Otherwise, a convoluted solution using inspect, when we cannot use __file__:

import os
import inspect

def dummy_func():
    pass

# We can not use __file__ to get the local file path, use another method that uses 'inspect' module
filepath = os.path.dirname(os.path.abspath(inspect.getsourcefile(dummy_func)))
filepath = filepath.replace( '\\', '/')

View methods / attributes of any object

# https://stackoverflow.com/questions/34439/finding-what-methods-a-python-object-has

dir(object)
help(object)

Do's and don't's

foo = 'abcdef'
l = list(foo)                     # DO

foo = 'abcdef'
l = [c for c in foo]              # don't

foo = list(...)
g = map(blah,foo]                 # DO

foo = list(...)
g = [blah(i) for i in foo]        # don't

A = [[0]*5 for _ in range(5)]     # DO

A = [[0]*5]*5                     # don't

Traps

Frequent mistakes. Beware the snake can bite you!

Confuse a method and a property in a test

if A.isdummy():            # This will fail isdummy is a property
if A.isdummy:              # Always True if isdummy is a method

Note that property should only be used to extend the behaviour of a class variable. Properties are designed to make it safe to publish variables in class interface, and get rid of useless mutator/accessor (see Python in a Nutshell, Why properties are important). Don't use property as replacement of a method when designing a new class.

Stick to a convention. Like always define methods like isxyyz() or hasabc() as methods. Note that defining them as property would raise an exception if used as a function, and hence might be safer.

Mix `0` with `None` in a sequence

Testing whether an element is defined is more difficult.

a = [0,None,None,None]
bool(a[0])           # --> False
bool(a[1])           # --> False !!! How can we tell them apart?
a[1] is None         # --> True      This works

Mixing property and normal getter

SOLUTION: prefix all getter method with get, like getvalue()

b = a.prop           # Using a property, OR
b = a.getprop()      # Using a getter

Forget that, in a python function, arguments are always passed by value

def f(x, y):
    x = 23
    y.append(42)
a = 77
b = [99]
f(a, b)
print a, b                 # prints: 77 [99, 42]

To reassing a list in a function, use a[:] construct, like:

def f(a):
   a[:]=a[::-1]             # This will NOT create a new list, but reassign elements in the original list

Use bytes, not string of characters

Characters can be unicode and take more than one byte.

b'abc'
bytes('abc')

Mixing string and bytestring (v3)

buf = b'abc\n'
if buf.find(b'\n'):        # MUST use BYTESTRING here
    # ....
str = 'abc\n'
if str.find('\n'):         #  MUST use STRING here
    # ....

Forget `self.` when using class members

class MyClass(object):
    buf = b''

    def UpdateBuf(self,new_buf):
        buf = new_buf                 # WRONG!
        self.buf = new_buf            # CORRECT!

Examples

Read a file line by line

Sources: [21]

Shortest version with autoclose and universal line ending (mode "u"):

for line in open("path/to/file.txt","U"): # U: universal line ending
    print(line.strip())                   # or strip('\r')

Slightly longer version with with:

with open("path/to/file.txt") as f:       # assume read-text mode "rt"
    for line in f:
        print(line.strip())               # or strip('\r')

Counting line number:

with open('path/to/file.txt') as f:
    for cnt, line in enumerate(f):
        print(f"Line {cnt}: {line.strip()}")

The long old way:

try:
    f = open("path/to/file.txt")
    line = f.readline()
    cnt = 1
    while line:
        print(f"Line {cnt}: {line.strip()}")
        line = f.readline()
        cnt += 1
finally:
    f.close()

Read a list of integers from a file

# https://stackoverflow.com/questions/6583573/how-to-read-numbers-from-file-in-python

# Using for loop
a=[]
with open('input.txt') as f:
    for line in f:
        a.append(int(line))

# Using list comprehension
a=[int(line) for line in open('input.txt')]

Simple TCP server

import socket, socketserver
import sys 
import itertools

SERV_ADDR="0.0.0.0"
SERV_ADDR=2222

class Handler(socketserver.BaseRequestHandler):
    messages = b""
    def handle(self):
        token = uint8(0)
        client = self.request
        client.setblocking(True)

        try:
            while True:
                buf = client.recv(1)
                # buf = client.recv(len)
                # client.send(buf)
                pass
        except socket.error as msg:
            pass

        client.close()
        return

port = SERV_PORT
if len(sys.argv) > 1:
    port = int(sys.argv[1])
server = socketserver.TCPServer((SERV_ADDR, port), Handler)
server.serve_forever()

Docstrings and Doctest

Specifications: pep-0257

To write good module docstrings, "think about somebody doing help(yourmodule) at the interactive interpreter's prompt — what do they want to know?" [22].
See pep-0257 for more recommendations

Using doctest

You can include tests, in the form of examples, in your Python modules' docstrings [23].

For instance, here file sxor.py. It contains:

A function with a docstring, and example of use with some test values.
A footer code that will call doctest.testmod() function if the module is loaded as main file.

import binascii

def sxor(s1,s2):
    """Xor two strings together.
    >>> sxor('abcd','1234')
    'b9f9'
    """
    s1=binascii.unhexlify(s1)
    s2=binascii.unhexlify(s2)
    return binascii.hexlify(bytes(a ^ b for a,b in zip(s1,s2))).decode()

# Footer to trigger doctest automatically.
# Alternatively, trigger it with:
# 
#     python -m doctest sxor.py
#
if __name__ == "__main__":
    import doctest
    doctest.testmod()

Now, we can run the tests with:

python3 sxor.py

No output means there was no errors. Use -v to get more output:

python3 sxor.py -v
# Trying:
#     sxor('abcd','1234')
# Expecting:
#     'b9f9'
# ok
# 1 items had no tests:
#     __main__
# 1 items passed all tests:
#    1 tests in __main__.sxor
# 1 tests in 2 items.
# 1 passed and 0 failed.
# Test passed.

Instead of using the footer code, one may call doctest from the command line (since Python 2.6):

python3 -m doctest sxor.py

Create new packages / modules

Links

How to make an awesome Python package in 2021

Recommend using flit for packaging, tox for linters and tests, etc. Very nice writeups. See more on HN.

Poetry

Recommended on HN.

pip can install packages from GitHub:

pip install git+https://myg.it/repo.git

Standard method

Reference: https://packaging.python.org/tutorials/packaging-projects/
setup.cfg: https://setuptools.readthedocs.io/en/latest/userguide/declarative_config.html

Create project template as in reference above
Add the files in src/.
Install build

python3 -m pip install --upgrade build

Build the package

python3 -m build

Libraries

Big numbers

gmpy based on GMP
libnum a lighter bignum library, but compatible with pypy.

Unicode

Set source file encoding

Add any of these lines [24]:

# -*- coding: utf-8 -*-
# vim: set fileencoding=utf-8 :

Write the BOM

See [25]

import codecs

file = codecs.open("lol", "w", "utf-8")
file.write(u'\ufeff')                          # or use unicode name: u'\N{ZERO WIDTH NO-BREAK SPACE}'
file.close()

# Using https://docs.python.org/2/library/codecs.html#module-encodings.utf_8_sig
with codecs.open("test_output", "w", "utf-8-sig") as temp:
    temp.write("hi mom\n")

Handling unicode

Some recommends to always process unicode internally, and decode on input and encode on output [26]:

line = line.decode('utf-8')
# ...treat line as unicode...
print line.encode('utf-8')

But this is error prone. So another solution proposed is to redefine sys.stdout:

import sys
import codecs
sys.stdout = codecs.getwriter('utf8')(sys.stdout)

An hackish way (not recommended):

# -*- coding: utf-8 -*-
import sys
reload(sys)
sys.setdefaultencoding('utf-8')
print u"åäö"

Python 2 to Python 3

Use python v3 print in v2

from __future__ import print_function

This way print() will not print () in v2.

Coding style

From PEP 8, Coding Style.

Use pycodestyle to check code conformance:

pip install pycodestyle
pycodestyle optparse.py

Use autopep8 to format existing code:

pip install autopep8
autopep8 --in-place optparse.py

Naming conventions

lower_case_variable = None

def lower_case_func():
    # ...

class ClassNameAreCapsWord:
    # ...

Some good/bad practices

# BAD - superfluous 'pass'
class InvalidAttribute(AttributeError):
    """Used to indicate attributes that could never be valid"""
    pass
# GOOD
class InvalidAttribute(AttributeError):
    """Used to indicate attributes that could never be valid"""


# BAD
f = open('file.txt')
a = f.read()
print a
f.close()
# GOOD
with open('file.txt') as f:
    for line in f:
        print line

# BAD
my_very_big_string = """For a long time I used to go to bed early. Sometimes, \
    when I had put out my candle, my eyes would close so quickly that I had not even \
    time to say “I’m going to sleep.”"""

from some.deep.module.inside.a.module import a_nice_function, another_nice_function, \
    yet_another_nice_function
# GOOD
my_very_big_string = (
    "For a long time I used to go to bed early. Sometimes, "
    "when I had put out my candle, my eyes would close so quickly "
    "that I had not even time to say “I’m going to sleep.”"
)

from some.deep.module.inside.a.module import (
    a_nice_function, another_nice_function, yet_another_nice_function)

Troubleshooting

Troubleshooting a missing library

Use python -v -c "import mylibrary" to troubleshoot a module.
Look at the log for the loaded libraries.
Some libraries are statically linked in python and might be missing. Use ldd to see the linked libraries, and report missing ones.

ldd /path/to/your/_hashlib.so
# linux-gate.so.1 =>  (0xf77c3000)
# libssl.so.6 => not found
# libcrypto.so.6 => not found
# libpython2.7.so.1.0 => not found
# libpthread.so.0 => /lib/i386-linux-gnu/libpthread.so.0 (0xf776a000)
# libc.so.6 => /lib/i386-linux-gnu/libc.so.6 (0xf75b3000)
# /lib/ld-linux.so.2 (0x5659b000)

Python

References

Books

Links

Tutorials

Shell

Testing

Examples

Install

Virtual Environments

Update Python

Install pip and setuptools

Install module online

Install modules offline

Install python2 pip on Debian Bullseye

Interactive mode

Python

iPython / Jupyter

Reloading modules automatically

Python variants

iPy

Pypy

Python 3 Reference

Keywords

Literals

Operators

Delimiters

Data types

Boolean

Strings

Bitstring

List

Dictionary

Set

Control flow statements

if

while

for

break

continue

for-else and while-else

pass

try-except-finally-else / raise

with

Functions

Docstrings

Classes

Using modules

Python 2 reference

Print

Basic I/O in Python

String formatting with format or formatted-string literals

String formatting with %

Input parsing

Text output

Text input

Standard Library

sys module

Arguments

Exit

io module

os and filesystem operations

argparse module

random module

datetime module

bitstring module

Named Tuple

parsing module

regex module

Subsystem

Cryptography

Doctest

Testing

pytest

Tips

Simple HTTP Server

Detect interactive mode

Find duplicates in list

Start post-mortem debugger on exception

Miscellaneous

String formatting with `format` or formatted-string literals

String formatting with `%`

Mix `0` with `None` in a sequence

Forget `self.` when using class members