Python Strings

A string (str) Immutable is a sequence of characters. Strings are how Python represents text, and they support indexing, slicing, looping, and a large set of built-in methods.

At a Glance

Task	Syntax	Example
Create	'text', "text", '''text'''	name = 'Sam'
Length	len(s)	len('hello') gives 5
Index	s[i]	'hello'[1] gives 'e'
Slice	s[start:stop:step]	'hello'[1:4] gives 'ell'
Join	+, join()	'a' + 'b' gives 'ab'
Repeat	*	'ab' * 3 gives 'ababab'
Format	f'{value}'	f'Age: {26}' gives 'Age: 26'
Search	in, find()	'ell' in 'hello' gives True
Transform	.upper(), .strip(), .replace()	' hi '.strip() gives 'hi'

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Creating Strings

Quotes

Single and double quotes behave identically. Choose whichever lets you avoid escaping.

quotes.py

a = 'Hello'
b = "World"
c = "It's a sunny day"              # Single quote inside double quotes
d = 'She said, "Hello World!"'      # Double quotes inside single quotes
e = 'It\'s'                         # Backslash escape
empty = ''                          # The empty string

Escape Sequences

A backslash gives the next character a special meaning.

Sequence	Meaning
\n	New line
\t	Tab
\\	A literal backslash
\'	A literal single quote
\"	A literal double quote

escapes.py

print('Line 1\nLine 2')
# Line 1
# Line 2

print('Name:\tSam')   # Name:    Sam

Raw Strings

Prefix a string with r to turn off escape processing. This is useful for file paths and regular expressions.

raw_strings.py

print('C:\new\table')   # Broken: \n and \t are interpreted as escapes
print(r'C:\new\table')  # C:\new\table

Multi-line Strings

Triple quotes (''' or """) let a string span several lines, preserving the layout exactly as written.

multiline.py

poem = """Roses are red,
Violets are blue."""

print(poem)
# Roses are red,
# Violets are blue.

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Length, Indexing, and Immutability

• len(s) returns the number of characters.
• Zero-based indexing: positions run from 0 to len(s) - 1, so s[0] is the first character.
• Negative indexing counts from the right: s[-1] is the last character and s[-2] is the second to last.

Index positions for 'Hello world'

   H   e   l   l   o       w   o   r   l   d
   0   1   2   3   4   5   6   7   8   9  10
 -11 -10  -9  -8  -7  -6  -5  -4  -3  -2  -1

indexing.py

my_str = 'Hello world'

print(len(my_str))   # 11
print(my_str[0])     # 'H'
print(my_str[-1])    # 'd'
print(my_str[20])    # IndexError: string index out of range

Immutability

Strings cannot be changed in place. You can point a variable at a new string, but you cannot overwrite individual characters.

immutability.py

greeting = 'hello'
greeting = 'Hello'      # Fine: the name now refers to a new string

greeting[0] = 'J'       # TypeError: 'str' object does not support item assignment

To “modify” a string, build a new one:

modify_by_rebuilding.py

greeting = 'hello'

new_greeting = 'J' + greeting[1:]   # 'Jello'
print(new_greeting)

Danger

Every string method returns a new string and leaves the original untouched. Calling text.upper() without assigning the result does nothing useful:

text = 'hello'
text.upper()          # Result is discarded, text is still 'hello'
text = text.upper()   # Correct

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Slicing

Extract part of a string with string[start:stop:step].

• stop is excluded. The slice runs up to, but not including, that index.
• Defaults: omitting start means 0, omitting stop means the end, and omitting step means 1.
• A negative step walks backward, so [::-1] reverses the string.
• Out-of-range slices never raise errors. Python simply stops at the edges.

Expression	Result for 'Hello world'
text[1:4]	'ell'
text[:5]	'Hello'
text[6:]	'world'
text[-5:]	'world'
text[:-6]	'Hello'
text[::2]	'Hlowrd'
text[::-1]	'dlrow olleH'
text[:]	'Hello world' (a full copy)
text[20:]	'' (empty, no error)

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Combining Strings

Operation	Example	Result
Concatenation (+)	'A' + ' ' + 'B'	'A B'
Augmented assignment (+=)	s += '!'	Appends and reassigns
Repetition (*)	'-' * 10	'----------'
Conversion (str())	str(26)	'26'

combining.py

name = 'John Doe'
age = 26

print(name + ' ' + str(age))   # 'John Doe 26'
print(name + ' ' + age)        # TypeError: can only concatenate str (not "int") to str

message = 'Hello'
message += '!'                 # 'Hello!'

print('=' * 20)                # '===================='

Tip

When combining many pieces, prefer ' '.join(parts) or an f-string over repeated +. Both are clearer, and join is the efficient choice when building text in a loop.

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Formatting with f-Strings

Prefix a string with f and place expressions inside {}. f-strings convert non-string values automatically and can evaluate any expression.

f_strings.py

name = 'John'
age = 26

print(f'{name} is {age} years old')       # John is 26 years old
print(f'Summing: {5} + {10} = {5 + 10}')  # Summing: 5 + 10 = 15
print(f'Shout: {name.upper()}')           # Shout: JOHN
print(f'{age = }')                        # age = 26 (debugging aid, Python 3.8+)

Format Specifiers

Add : after the expression to control how the value is displayed.

Specifier	Example	Result
.2f	f'{3.14159:.2f}'	'3.14'
,	f'{1234567:,}'	'1,234,567'
.1%	f'{0.256:.1%}'	'25.6%'
05d	f'{42:05d}'	'00042'
>8	f"{'hi':>8}"	' hi' (right-aligned)
<8	f"{'hi':<8}"	'hi ' (left-aligned)
^8	f"{'hi':^8}"	' hi ' (centered)
!r	f'{name!r}'	"'John'" (shows the repr)

Other Formatting Styles

f-string (preferred)

print(f'{name} is {age}')

The clearest and fastest option. Use this by default.

.format()

print('{} is {}'.format(name, age))
print('{n} is {a}'.format(n=name, a=age))

Useful when the template string is stored separately from the values.

% (legacy)

print('%s is %d' % (name, age))

Found in older code. Avoid it in new code.

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Membership and Comparison

in and not in test whether one string appears inside another and return a Boolean.

membership_and_comparison.py

sentence = 'The quick brown fox'

print('quick' in sentence)       # True
print('slow' not in sentence)    # True
print('Quick' in sentence)       # False, matching is case-sensitive

print('apple' == 'Apple')        # False
print('apple' < 'banana')        # True, compared alphabetically by character code
print('Zebra' < 'apple')         # True, all uppercase letters sort before lowercase

Note

For case-insensitive comparison, normalize both sides first: a.lower() == b.lower(). For non-English text, casefold() is more thorough than lower().

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String Methods

Call a method with dot notation: variable.method(). Remember that every method returns a new value.

Changing Case

Method	Result	Example
upper()	All uppercase	'hi'.upper() gives 'HI'
lower()	All lowercase	'HI'.lower() gives 'hi'
capitalize()	First character upper, the rest lower	'hELLO'.capitalize() gives 'Hello'
title()	First letter of every word upper	'hello world'.title() gives 'Hello World'
swapcase()	Inverts the case of every letter	'Hi'.swapcase() gives 'hI'

Cleaning and Replacing

Method	Result	Example
strip()	Removes leading and trailing whitespace	' hi '.strip() gives 'hi'
lstrip() / rstrip()	Strips only the left or right side	' hi '.rstrip() gives ' hi'
strip(chars)	Removes any of the listed characters from both ends	'!!hi!!'.strip('!') gives 'hi'
replace(old, new)	Replaces every occurrence	'a-b-c'.replace('-', '/') gives 'a/b/c'
replace(old, new, n)	Replaces only the first n occurrences	'a-b-c'.replace('-', '/', 1) gives 'a/b-c'
removeprefix(p) 3.9+	Removes a prefix if present	'unhappy'.removeprefix('un') gives 'happy'
removesuffix(s) 3.9+	Removes a suffix if present	'report.txt'.removesuffix('.txt') gives 'report'

Searching and Counting

Method	Result	Example
find(sub)	Index of the first match, or -1 if absent	'hello'.find('l') gives 2
index(sub)	Like find(), but raises ValueError if absent	'hello'.index('z') raises an error
rfind(sub)	Index of the last match, or -1	'hello'.rfind('l') gives 3
count(sub)	Non-overlapping occurrences	'banana'.count('an') gives 2
startswith(p)	Whether the string begins with p	'hello'.startswith('he') gives True
endswith(s)	Whether the string ends with s	'hello'.endswith('lo') gives True

Tip

startswith() and endswith() accept a tuple of options: filename.endswith(('.png', '.jpg')).

Splitting and Joining

Method	Result	Example
split()	Splits on any whitespace, ignoring extra spaces	'a b c'.split() gives ['a', 'b', 'c']
split(sep)	Splits at each separator	'a,b,,c'.split(',') gives ['a', 'b', '', 'c']
split(sep, n)	Splits at most n times	'a,b,c'.split(',', 1) gives ['a', 'b,c']
splitlines()	Splits at line breaks	'a\nb'.splitlines() gives ['a', 'b']
partition(sep)	Splits into (before, sep, after) at the first match	'key=value'.partition('=') gives ('key', '=', 'value')
join(items)	Glues strings together using the host string as separator	'-'.join(['a', 'b']) gives 'a-b'

split_and_join.py

words = 'one two three'.split()   # ['one', 'two', 'three']
print('-'.join(words))            # 'one-two-three'

print('-'.join([1, 2, 3]))        # TypeError: items must be strings
print('-'.join(str(n) for n in [1, 2, 3]))  # '1-2-3'

Testing Content

Each of these returns True or False. All return False for an empty string.

Method	True when the string…	Example
isdigit()	contains only digits	'42'.isdigit() is True, '-5'.isdigit() is False
isalpha()	contains only letters	'abc'.isalpha() is True
isalnum()	contains only letters and digits	'abc123'.isalnum() is True
isspace()	contains only whitespace	' '.isspace() is True
isupper()	has at least one letter and all letters are uppercase	'HELLO'.isupper() is True
islower()	has at least one letter and all letters are lowercase	'hello'.islower() is True

Padding and Alignment

Method	Example	Result
zfill(width)	'7'.zfill(3)	'007'
center(width, fill)	'hi'.center(8, '*')	'***hi***'
ljust(width, fill)	'hi'.ljust(5, '.')	'hi...'
rjust(width, fill)	'hi'.rjust(5, '.')	'...hi'

Chaining Methods

Because each method returns a string, you can chain them left to right.

chaining.py

raw_text = '  Hello World  '

print(raw_text.strip().lower().replace(' ', '-'))  # 'hello-world'

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Looping Over Strings

A string is a sequence, so you can iterate over its characters directly.

iteration.py

for char in 'abc':
    print(char)           # a, b, c

for index, char in enumerate('abc'):
    print(index, char)    # 0 a, 1 b, 2 c

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Putting It Together

1. Normalize the input. Use strip(), split(), and lower() or title() to remove noise and unify the case.

2. Validate it. Use in, startswith(), isdigit(), or isalpha() to confirm the content is what you expect.

3. Transform it. Use replace(), slicing, and join() to reshape the text.

4. Present it. Use an f-string to format the final output.

clean_name.py

raw_name = '  jOHN   dOE  '

clean_name = ' '.join(raw_name.split()).title()
print(f'Hello, {clean_name}!')   # Hello, John Doe!

palindrome.py

def is_palindrome(text):
    cleaned = ''.join(ch.lower() for ch in text if ch.isalnum())
    return cleaned == cleaned[::-1]

print(is_palindrome('A man, a plan, a canal: Panama'))  # True
print(is_palindrome('Python'))                           # False

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Common Mistakes

Mistake	Problem	Fix
s[0] = 'H'	TypeError, strings are immutable	s = 'H' + s[1:]
s.upper() with no assignment	The new string is discarded	s = s.upper()
'Age: ' + 26	TypeError, cannot add str and int	f'Age: {26}' or 'Age: ' + str(26)
s[len(s)]	IndexError, the last index is len(s) - 1	s[-1]
'-'.join([1, 2])	TypeError, join needs strings	'-'.join(map(str, [1, 2]))
s.strip('abc') expecting a substring	strip removes any of those characters, not the exact text	Use removeprefix() or removesuffix()
'hello' in 'Hello World'	Matching is case-sensitive	Compare .lower() versions
"they're".title()	Gives "They'Re"	Use string.capwords() for simple cases
'42'.isdigit() to validate numbers	Rejects negatives and decimals	Try int() or float() in a try / except

Best Practices Checklist

1. Pick the quote style that avoids escaping, and stay consistent within a project.
2. Use f-strings for formatting instead of + or %.
3. Build text from many pieces with join(), not repeated concatenation.
4. Remember that strings are immutable, and always capture the result of a method.
5. Prefer slicing over manual index arithmetic, and use negative indices to count from the end.
6. Normalize with strip() and lower() before comparing user input.
7. Use find() when absence is acceptable and index() when it should be an error.
8. Use raw strings (r'...') for Windows paths and regular expressions.