Python Strings

Strings are one of the most frequently used data types in Python, representing sequences of text characters. Python strings are immutable, meaning that once created, a string's characters cannot be changed in place — any operation that appears to "modify" a string actually creates and returns a brand-new string.

Python offers rich, readable tools for building and manipulating text, from the modern f-string syntax for embedding expressions directly into text, to a large set of built-in string methods for searching, splitting, joining, and cleaning up text data.

Why it matters

Names, messages, file contents, web pages and API responses are all text, so string handling is in every program. Slicing, splitting and joining are the operations used most, and because strings cannot be changed in place, knowing that each operation returns a new string prevents a very common mistake.

f-strings

Formatted string literals, or f-strings, are written by placing an "f" before the opening quote, and they let you embed any expression directly inside curly braces: f"Hello, {name}! You are {age} years old.". The expression inside the braces is evaluated at runtime and converted to its string form automatically, and f-strings also support format specifiers for things like decimal precision, e.g. f"{price:.2f}".

Indexing and Slicing Strings

Since a string is an ordered sequence of characters, it supports the same indexing and slicing rules as a list: text[0] gets the first character, text[-1] gets the last, and text[2:5] extracts a substring from index 2 up to (but not including) index 5. Because strings are immutable, slicing always returns a new string rather than a view into the original.

Common String Methods

Python strings come with many useful built-in methods, all of which return a new string or value rather than modifying the original.

• split(sep) — breaks a string into a list of substrings using sep as the delimiter (whitespace by default).
• join(iterable) — the reverse of split; joins the elements of an iterable of strings using the original string as the separator, e.g. ", ".join(["a", "b", "c"]).
• strip() — removes leading and trailing whitespace (or specified characters) from a string; lstrip() and rstrip() trim only one side.
• format() — an older but still-used way to embed values into a string, e.g. "{} is {} years old".format(name, age).
• upper(), lower(), replace(old, new), and startswith()/endswith() round out the most commonly used methods.

Examples

name = "Priya"
age = 29
price = 49.5

print(f"Hello, {name}! You are {age} years old.")
print(f"Price: {price:.2f}")

text = "Programming"
print(text[0], text[-1])
print(text[3:7])
print(text[::-1])

Hello, Priya! You are 29 years old.
Price: 49.50
P g
gram
gnimmargorP

sentence = "  the quick brown fox  "
cleaned = sentence.strip()
print(f"'{cleaned}'")

words = cleaned.split()
print(words)

joined = "-".join(words)
print(joined)

print("Python".upper())
print("Python".replace("y", "Y"))

'the quick brown fox'
['the', 'quick', 'brown', 'fox']
the-quick-brown-fox
PYTHON
PYthon

Exercise

Starting from the text below, print the number of words it contains, then the word Python reversed, then the words joined with hyphens.

Expected output: 3, nohtyP, Hello,-Python-World (one per line)

text = "  Hello, Python World  "

# TODO: split the text into words and print how many there are
# TODO: print the second word reversed
# TODO: print the words joined with "-"

text = "  Hello, Python World  "

words = text.split()
print(len(words))
print(words[1][::-1])
print("-".join(words))

Quick quiz

Interview questions