Python:Strings

Strings are immutable, all methods return a new string

Basics

str1 + str2

Return concatenation of str1 and str2

str1 += str2

Append str2 to str1

str1 * 3

Return str1 3 times

Formatting

Basic

str1.replace(old,new[,cnt])

Return str1 with old replaced by new (cnt times).

str1.strip(<chars>)

Return str1 with all trailing and leading <chars> removed. If <chars> is omitted all trailing and leading whitespaces are removed.

str1.rstrip('\r\n')

str1.lstrip('<char>')

Return str1 with all newline characters (windows, mac or unix) stripped from the end of str1, like perl 'chomp' does.

lstrip removes characters from the beginning of str1

Without character specification all whitespaces are removed.

str1.upper() str1.lower() str1.title()

Return str1 in upppercase, lowercase or with only all first characters in uppercase

str1.join(list)

str1.join(map(str,list))

str1.join(str(e) for e in list)

Join list (or set or other sequence) into a string with str1 as separator. The second and third form makes sure all elements are converted to string before they are joined.

str1.split(sep[,max])

Split string into a list on sep into max + 1 elements (remainder is put in last element)

str1.splitlines([keepends])

Split on newline, with 'keepends' the newline is preserved.

str1.center(w)

str1.ljust(w)

str1.rjust(w)

Put spaces around str1 until length 'w' is reached.

str1.expandtabs(size)

Replace tabs by 'size' number of spaces.

Advanced

str1.format(values)

Fill in 'values' in str1-fields ({}). By numbering the fields they can be in a different order than the values.

If values are in a dict, they can be addressed by their key.

Code Example

 
"Value 1: {}, Value2: {}".format(1,2)
"Value 2: {1}, Value1: {0}".format(1,2)

dict1 = {'value1':1, 'value2':2}
"Value 2: {value2}, Value1: {value1}".format(dict1)

{[field]:formatspec}

The format can be specified after the (optional) fieldnumber.

[[fill]align][sign][#][0][width][grouping_option][.precision][type]

Generic format specification. Anything not needed can be left out.

e.g. "{:07d}".format(5) fill out with 0 in front to 7 digits -> '0000005'

"{:010.6f}".format(5.7647) floating point with precision 6 and total width 10 left fill with 0 -> '005.764700'

"{:10,.2f}".format(12345.7647) floating point with precision 2,total width 10 and comma as thousand separator -> ' 12,345.76'

Alignment

Always show sign

<	Left
>	Right
^	Center
=	Padding (after sign)
#	Prepend for x, o and b types

Types

s	String
c	Character
d	decimal
f	Float
%	Percent
o	Octal
x	Hexadecimal
b	Binary
e	Exponent
g	Python chooses between decimal, float or exponent

Datastructures

Below can be used on datatypes like lists and tuples too, not on sets.

json.dumps(dict, indent=4)

Convert a dict into a json string nicely formatted. Indent each level with 4 spaces.

pprint provides similar functionality

Searching

Basic

if <search> in str1

True if <search> is in str1

str1.count(<search>)

Return how many times <search> is in str1

str1.find(<search>)

str1.index(<search>)

Return where <search> is found in str1. If not found -1 with find, throw exception with index.

str1.endswith(<search>)

str.startswith(<search>)

Return True if str1 ends/starts with <search> (else returns False).

Regular Expressions (regexp)

import re

The re modules provides Perl-like Regular Expressions matching for string and byte objects. The re module is standard available (no installation needed).

re1 = re.compile(regexp)

Create regular expression object to use for matching. This is more efficient if the regular expression in used several times in a program.

re.sub(regexp,new,str1)

Return str1 with all parts matching regexp replaced with new.

NOTE: str1 remains unchanged.

NOTE2: re.sub is much more expensive than string.replace

re.split(regexp,str1,max)

Split str1 into a list on regexp, like with split above superfluous elements will be in the max + 1 element.

mo1 = re1.match(str1)

mo1 = re.match(regexp,str1)

Find 'regexp' at the beginning of 'str1'. Return match object if found, else return None-object

mo1 = re.search(regexp,str1)

Find first occurrence of 'regexp' in 'str1'. Return match object if found, else return None-object

lst1 = re.findall(regexp,str1)

Find all occurrences of 'regexp' in 'str1'. Return a list of strings.

mol1 = re.finditer(regexp,str1)

Find all occurrences of 'regexp' in 'str1'. Return a list of match objects.

Match Objects

mo.group()

mo.group(0)

The matched string in match object 'mo'

mo.group(1)

First submatch in the matched string in 'mo'. The first match is the first ( in the expression.

mo.start()

The start position of the matched string in 'mo'

mo.end()

The end position of the matched string in 'mo'

mo.span()

Tuple with start and end position of the matched string in 'mo'

Search Modifieres

re1 = re.search(regexp,str1,modifier)

re1 = re.compile(regexp,modifier)

Modify how matching is done

re.DOTALL

The . matches all characters (default is all characters except newline). Use for searching in web or book pages.

re.I

Ignore case

re.M

Multiline mode, ^ matches all line beginnings and $ all line endings.

Code Example:

import re
str1 = "The thing to cut in pieces"
rel1 = re.compile('h.*n')
print "Matching"
mo1 = rel1.match(str1)

if m:
 print mo1.group()
 print mo1.start()
 print mo1.end()
 print mo1.span()
else:
 print "no match at beginning of string"
print

print "Searching"
mo1 = re.search('t.*n',str1)
if mo1:
 print mo1.group()
 print mo1.start()
 print mo1.end()
 print mo1.span()

print "Searching case insensitive"
mo1 = re.search('h.*n',str1,re.I)
if mo1:
 print mo1.group()
 print mo1.start()
 print mo1.end()
 print mo1.span()


print "findall"
re1 = re.compile('t')
lst1 = re1.findall(str1)
if lst1:
 print lst1
 for str2 in lst1:
  print str2
print


print "finditer"
re1 = re.compile('i.')
mol1 = re1.finditer(str1)
if mol1:
 for mo1 in mol1:
 	print mo1.group()
 	print mo1.start()
 	print mo1.end()
 	print mo1.span()
print