String Methods and Built-in Functions

Python ships with a rich set of built-in functions and methods for working with strings, so most everyday manipulations need no hand-written loop at all. One of them — len() — is a standalone function called as len(s); the rest are methods, called with dot notation on the string itself, e.g. s.upper(). Because strings are immutable, every one of these returns a new value and leaves the original string unchanged.

The commonly used ones follow, each with its behaviour and an example.

len() — length of the string

Returns the number of characters in the given string (spaces and symbols count).

>>> s = 'Hello World!'
>>> len(s)
12

title() — Title Case

Returns the string with the first letter of every word in uppercase and all remaining letters in lowercase.

>>> s = 'india WON the MATCH'
>>> s.title()
'India Won The Match'

lower() — all lowercase

Returns the string with every uppercase letter converted to lowercase.

>>> s = 'india WON the MATCH'
>>> s.lower()
'india won the match'

upper() — all uppercase

Returns the string with every lowercase letter converted to uppercase.

>>> s = 'india WON the MATCH'
>>> s.upper()
'INDIA WON THE MATCH'

count(str, start, end) — how many occurrences

Returns the number of times the substring occurs in the given string. Optional start and end indices restrict the search to that range; if they are omitted, searching runs from index 0 to the length of the string.

>>> s = 'the cat and the dog and the cow'
>>> s.count('the')          # whole string searched
3
>>> s.count('the', 10, 31)  # only within indices 10..30
2

find(str, start, end) — index of first occurrence

Returns the index of the first occurrence of the substring in the given string; the optional start and end limit the search range exactly as in count(). If the substring is not present, it returns -1 (no error).

>>> s = 'the cat and the dog and the cow'
>>> s.find('the')           # first match is at the very start
0
>>> s.find('the', 5, 20)    # first match within indices 5..19
12
>>> s.find('thee')          # absent -> -1
-1

index(str, start, end) — like find(), but raises an error

Same as find(), except that when the substring is not present it raises an exception instead of returning -1.

>>> s = 'the cat and the dog'
>>> s.index('the')
0
>>> s.index('thee')
ValueError: substring not found

endswith() — does it end with...?

Returns True if the string ends with the supplied substring, otherwise False.

>>> s = 'Hello World!'
>>> s.endswith('World!')
True
>>> s.endswith('!')
True
>>> s.endswith('rld')
False

startswith() — does it start with...?

Returns True if the string starts with the supplied substring, otherwise False.

>>> s = 'Hello World!'
>>> s.startswith('He')
True
>>> s.startswith('Hee')
False

The is...() family — testing what a string contains

These methods classify the string's contents and return a Boolean.

isalnum() — True if every character is a letter or a digit (alphanumeric). Whitespace or special symbols anywhere — or an empty string — give False.

>>> 'Result2026'.isalnum()
True
>>> 'Result 2026'.isalnum()    # contains a space
False
>>> 'Result!!'.isalnum()       # contains symbols
False

islower() — True if the string is non-empty and all its alphabets are lowercase, or it has at least one lowercase alphabet and the rest are non-alphabet characters. A string with no alphabet at all gives False.

>>> 'good morning!'.islower()
True
>>> 'good 1234'.islower()
True
>>> 'good ??'.islower()
True
>>> '1234'.islower()           # no alphabet at all
False
>>> 'Good Morning'.islower()   # has uppercase letters
False

isupper() — the mirror image: True if the string is non-empty and all its alphabets are uppercase, or it has at least one uppercase alphabet and the rest are non-alphabet characters.

>>> 'GOOD MORNING!'.isupper()
True
>>> 'GOOD 1234'.isupper()
True
>>> 'GOOD ??'.isupper()
True
>>> '1234'.isupper()
False
>>> 'Good Morning'.isupper()
False

isspace() — True if the string is non-empty and every character is whitespace (blank space, tab, newline or carriage return).

>>> '  \n \t \r'.isspace()
True
>>> 'hi \n'.isspace()          # contains non-space characters
False

istitle() — True if the string is non-empty and in title case, i.e., the first letter of every word is uppercase and the rest lowercase.

>>> 'Hello World!'.istitle()
True
>>> 'hello World!'.istitle()
False

lstrip(), rstrip(), strip() — trimming spaces

• lstrip() returns the string with spaces removed only on the left.
• rstrip() returns the string with spaces removed only on the right.
• strip() returns the string with spaces removed on both the left and the right.

>>> s = '   Hello World!   '
>>> s.lstrip()
'Hello World!   '
>>> s.rstrip()
'   Hello World!'
>>> s.strip()
'Hello World!'

replace(oldstr, newstr) — substitute a substring

Returns a string in which every occurrence of the old substring has been replaced by the new one.

>>> s = 'Hello World!'
>>> s.replace('o', '*')            # every 'o' replaced
'Hell* W*rld!'
>>> s.replace('World', 'Python')   # a whole word replaced
'Hello Python!'
>>> 'Hi World! Hi'.replace('Hi', 'Bye')
'Bye World! Bye'

join() — insert a separator between characters

Called on the separator string, join() returns a string in which the characters of the argument have been joined together with that separator placed between them.

>>> s = 'PYTHON'
>>> sep = '-'          # the separator
>>> sep.join(s)
'P-Y-T-H-O-N'

partition() — split into exactly three parts …