Python

Links

• The Python Tutorial
• Python 2.7.6 docs

• Nice example of generating / testing regex in Python (with nice / small test framework) [1]

Shell

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

Reference

• range

Basic

for i in range(10):
    print i                      # carriage return

for i in range(10):
    print i,                     # no carriage return

a = 'global'
def afunction():
    global a
    a = 'still using global'
    b = 'local'

s.upper()                        # string s to uppercase

# 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...

[2],

Modules

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

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: [3], [4]

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 else: 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	-	-

Miscellaneous

Detect whether a variable is defined

Note it is bad practice to define a variable conditionally [5]. 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__)

Traps

Frequent mistakes. Beware the snake can bite you!

Confuse a method and a property in a test

SOLUTION: 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.

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

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] == None         # --> True      This works, but is unusual and likely bad practice

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

Unicode

Set source file encoding

Add any of these lines [6]:

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

Write the BOM

See [7]

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 [8]:

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"