Python Strings In A Nutshell

Strings are enclosed within single (' '), double (" "), or triple (''' ''' or """ """) quotes.

single_quoted = 'This is a single-quoted string.'
double_quoted = "This is a double-quoted string."
triple_quoted = '''This is a triple-quoted string.'''

Immutability

They cannot be changed in-place after they are created. For example, you can’t change a string by assigning to one of its
positions, but you can always build a new one and assign it to the same name.

Concatenation and Slicing

Python allows you to concatenate strings using the + operator, making it easy to combine text elements.

first_name = "John"
last_name = "Doe"
full_name = first_name + " " + last_name  # full_name is "John Doe"

text = "Hello, World!"
first_char = text[0]  # first_char is 'H'
substring = text[7:12]  # substring is 'World'

Every object in Python is classified as either immutable (unchangeable) or not. In terms of the core types, numbers, strings, and tuples are immutable; lists and dictionaries are
not.

String Operations

>>> match = re.match('/(.*)/(.*)/(.*)', '/usr/home/lumberjack')
>>> match.groups()
('usr', 'home', 'lumberjack')

>>> S[-1] # The last item from the end in S
'm'
>>> S[-2] # The second to last item from the end
'a'

A negative index is simply added to the string’s size,

>>> S[-1] # The last item in S
'm'
>>> S[len(S)-1] # Negative indexing, the hard way
'm'

Sequences also support a more general form of indexing,slicing, which is a way to extract an entire section.

>>> S # A 4-character string
'Spam'
>>> S[1:3] # Slice of S from offsets 1 through 2 (not 3)
'pa'

The general form, X[I:J], means “give me everything in X from offset I up to but not including offset J.” The result is returned in a new object. The second of the preceding operations, for instance, gives us all the characters in string S from offsets 1 through 2 (that is, 3 – 1) as a new string. The effect is
to slice or “parse out” the two characters in the middle.
In a slice, the left bound defaults to zero, and the right bound defaults to the length of
the sequence being sliced. This leads to some common usage variations:

Addition for(+)numbers, and concatenation for strings. This is a general property of Python that is called polymorphism.

Methods In String

>>> S.find('pa') # Find the offset of a substring
1
>>> S
'Spam'
>>> S.replace('pa', 'XYZ') # Replace occurrences of a substring with another
'SXYZm'
>>> S
'Spam'

>>> S[1:] # Everything past the first (1:len(S))
'pam'
>>> S # S itself hasn't changed
'Spam'
>>> S[0:3] # Everything but the last
'Spa'
>>> S[:3] # Same as S[0:3]
'Spa'
>>> S[:-1] # Everything but the last again, but simpler (0:-1)
'Spa'
>>> S[:] # All of S as a top-level copy (0:len(S))
'Spam'

Strings also support concatenation with the plus sign and repetition :

>>> S
Spam'
>>> S + 'xyz' # Concatenation
Strings | 81
'Spamxyz'
>>> S # S is unchanged
'Spam'
>>> S * 8 # Repetition
'SpamSpamSpamSpamSpamSpamSpamSpam'

>>> line = 'aaa,bbb,ccccc,dd'
>>> line.split(',') # Split on a delimiter into a list of substrings
['aaa', 'bbb', 'ccccc', 'dd']
>>> S = 'spam'
>>> S.upper() # Upper- and lowercase conversions
'SPAM'
>>> S.isalpha() # Content tests: isalpha, isdigit, etc.
True
>>> line = 'aaa,bbb,ccccc,dd\n'
>>> line = line.rstrip() # Remove whitespace characters on the right side
>>> line
'aaa,bbb,ccccc,dd'

Pattern Matching

This module has analogous calls for searching, splitting, and replacement, but because we can use patterns to specify substrings, we can be much more general:

>>> import re
>>> match = re.match('Hello[ \t]*(.*)world', 'Hello Python world')
>>> match.group(1)
'Python '

This example searches for a substring that begins with the word “Hello,” followed by zero or more tabs or spaces, followed by arbitrary characters to be saved as a matched group, terminated by the word “world.”The following pattern, picks out three groups separated by slashes:

>>> match = re.match('/(.*)/(.*)/(.*)', '/usr/home/lumberjack')
>>> match.groups()
('usr', 'home', 'lumberjack')