Python quick reference

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Strings

Replace

mystring = mystring.replace('oldtext', 'newtext')

Find

# String found
if mystring.find('text') != -1:
     ...
# String not found
else:

String formatting

(Available starting in 2.6, standard starting in 3.0)

'This is a string: {0}'.format('this too')

Zero-padded numbers

print('{:02d}'.format(2))
02

Lists

Copy a list

copy_of_list1 = list(list1)

Dictionaries

Sorting

for key in sorted(mydict):

Delete a key (and its value)

del mydict[somekey]

Misc

Multi-line output

print 'test this ' \
    'and this'
sys.stderr.write('test this too '
    'and this')

Regular expressions

If you’ll be using the regex more than once in your script:

import re
pattern = re.compile('regex here')
match = pattern.search(string)
if match:
    name = match.group(1)
    address = match.group(2)

To find multiple occurrences of a pattern:

import re
pattern = re.compile('regex here')
matches = pattern.findall(string)
if matches != []:

Backwards-compatible code (determining Python version)

import datetime
import sys
if sys.version_info < (2, 5):
    import time

date = '20100319'
if sys.version_info < (2, 5):
    somedate = datetime.datetime(*(time.strptime(date, '%Y%m%d')[0:6]))
else:
    somedate = datetime.datetime.strptime(date, '%Y%m%d')

Safely open a file

with open('file.txt', 'r') as f:
    output = f.read()

Multiple files:

with open('file1.txt', 'r') as file1, open('file2.txt', 'r') as file2:

Character encoding issues

str.decode(encoding)
- Decodes from the provided encoding to unicode
str.encode(encoding)
- Encodes from unicode (not utf8) to the provided encoding

For example, if you have a utf8 string which has been mistakenly decoded as a cp1252 string to utf8:

'it\xc3\xa2\xe2\x82\xac\xe2\x84\xa2s'

You need to first decode it from utf8 to unicode, then encode it from cp1252 to utf8:

'it\xc3\xa2\xe2\x82\xac\xe2\x84\xa2s'.decode('utf8').encode('cp1252')

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