String Methods and Length#

Note

Source: Immutability warning, method descriptions, and exercise adapted from the C# edition (basicstringops/stringmethods.rst). Python-specific methods (strip, split, join, startswith, endswith) and the quick-reference table are original additions.

Strings in Python are objects — they bundle data (the characters) with operations (methods). You call a method with dot notation: string_value.method_name(arguments).

All string methods return new strings. Strings are immutable: calling a method never changes the original string.

>>> s = "hello"
>>> s.upper()
'HELLO'
>>> s             # unchanged
'hello'
>>> s = s.upper() # reassign to get the new value
>>> s
'HELLO'

`len()`#

len(s) returns the number of characters in s. It is a built-in function, not a method:

>>> len("coding")
6
>>> len("")
0

Case Conversion#

>>> "Hello World".upper()
'HELLO WORLD'
>>> "Hello World".lower()
'hello world'
>>> "hello world".capitalize()
'Hello world'
>>> "hello world".title()
'Hello World'

Stripping Whitespace#

strip() removes leading and trailing whitespace (spaces, tabs, newlines):

>>> "  hello  ".strip()
'hello'
>>> "  hello  ".lstrip()   # left side only
'hello  '
>>> "  hello  ".rstrip()   # right side only
'  hello'

This is especially useful when reading user input or lines from a file.

Finding Substrings#

find(sub) returns the index of the first occurrence of sub, or -1 if it is not found:

>>> "Bonjour".find("jo")
3
>>> "Bonjour".find("xyz")
-1

index(sub) works the same way but raises a ValueError instead of returning -1 when not found.

Testing Start and End#

>>> "hello.py".endswith(".py")
True
>>> "hello.py".startswith("he")
True
>>> "hello.py".startswith("lo")
False

These return True or False and are often used in if statements.

Replacing Substrings#

replace(old, new) returns a copy of the string with every occurrence of old replaced by new:

>>> "It was the best of times.".replace("best", "worst")
'It was the worst of times.'
>>> "aababc".replace("ab", "X")
'aXXc'

An optional third argument limits how many replacements to make:

>>> "aababc".replace("ab", "X", 1)
'aXabc'

Splitting and Joining#

split() breaks a string into a list of words, splitting on whitespace by default:

>>> "one two three".split()
['one', 'two', 'three']

Pass a delimiter to split on something specific:

>>> "a,b,c".split(",")
['a', 'b', 'c']
>>> "2024-05-01".split("-")
['2024', '05', '01']

join() is the inverse: it assembles a list of strings into one string, inserting a separator between each pair:

>>> ", ".join(["Alice", "Bob", "Carol"])
'Alice, Bob, Carol'
>>> "-".join(["2024", "05", "01"])
'2024-05-01'

Quick Reference#

Expression	Result
`len(s)`	Number of characters
`s.upper()`	Uppercase copy
`s.lower()`	Lowercase copy
`s.capitalize()`	First letter uppercase, rest lower
`s.title()`	Title-case copy
`s.strip()`	Remove leading/trailing whitespace
`s.find(sub)`	Index of first `sub` (-1 if absent)
`s.startswith(pre)`	`True` if `s` begins with `pre`
`s.endswith(suf)`	`True` if `s` ends with `suf`
`s.replace(old, new)`	Copy with `old` → `new`
`s.split(sep)`	List of substrings split by `sep`
`sep.join(lst)`	Join list with `sep` between items

String Methods Exercise#

Predict the output, then check in the REPL:

w = "quickly"
print(len(w))
print(w[len(w) - 2])
print(w[3:5])
print(w[2:])
print(w.find("ti"))
print(w.find("ick"))
k = w.find("c")
print(k, w[k], w[k - 3], w[k:])