Python String Methods

3String Methods: explained step by step

A string method is a function that belongs to a piece of text and is called with a dot: name.upper(). Python has dozens of them, and together they cover most text work: cleaning user input, splitting lines of data, searching, padding and formatting. This page goes beyond the basics in Python strings and groups the methods by the job they do.

Methods return new strings

Strings are immutable, so no method can change the text it is called on. Each one returns a new string (or a number, boolean or list) and leaves the original alone. If you do not store the result, it is lost.

title = "  python basics  "
title.upper()
print(repr(title))

title = title.strip().upper()
print(repr(title))

The first call computed " PYTHON BASICS " and threw it away. Assigning the result back, as in the second call, is how you "change" a string.

Changing letter case

Five methods change case. lower() and upper() are the workhorses for comparing text without caring about capitals, title() capitalises every word, capitalize() only the first letter of the whole string, and swapcase() flips each letter.

text = "hELLO wORLD"
print(text.lower())
print(text.upper())
print(text.title())
print(text.capitalize())
print(text.swapcase())
print("Straße".casefold() == "STRASSE".casefold())

casefold() is a stronger lower() meant for comparisons, and it handles letters such as the German ß. Use it when users may type text in any language.

Removing whitespace and unwanted characters

strip() removes whitespace (spaces, tabs, newlines) from both ends; lstrip() and rstrip() handle one side. Give them a string and they remove any of those characters instead. Nothing in the middle is touched.

line = "  total: 42\n"
print(repr(line.strip()))
print(repr(line.rstrip()))
print("***sale***".strip("*"))
print("2024-report.txt".removesuffix(".txt"))

Careful: strip("abc") removes any mix of a, b and c, not the word "abc". For a fixed prefix or suffix use removeprefix() and removesuffix().

Splitting text into pieces and joining it back

split() breaks a string into a list. With no argument it splits on any run of whitespace and ignores extra spaces. With an argument it splits exactly on that separator, so doubled separators produce empty strings. rsplit() splits from the right, splitlines() splits on line breaks, and join() is the opposite of split().

sentence = "  red   green blue "
print(sentence.split())
print(sentence.split(" "))
print("2024-05-17".split("-"))
print("a=b=c".rsplit("=", 1))
print("line one\nline two".splitlines())
print(" / ".join(["Mon", "Tue", "Wed"]))

join() is called on the separator, and every item must already be a string. This is the reason "-".join(numbers) fails until you convert with str(); see Python casting.

Replacing text

replace(old, new) swaps every occurrence. An optional third argument limits how many replacements happen.

phone = "555-123-4567"
print(phone.replace("-", ""))
print(phone.replace("-", " ", 1))
print("a,b;c".replace(",", ";").split(";"))

Searching: find, index, count and startswith

find() returns the position of the first match or -1. index() does the same but raises a ValueError when nothing matches, which is the right choice when a missing piece means the data is broken. count() counts non-overlapping matches, rfind() searches from the right, and startswith() / endswith() accept a tuple of options.

email = "maya@example.com"
print(email.find("@"))
print(email.find("#"))
print(email.count("a"))
print(email.endswith((".com", ".org")))
print("report.final.csv".rfind("."))
try:
    email.index("#")
except ValueError as error:
    print("ValueError:", error)

You can combine find() with slicing to cut text around a position. For anything more pattern-like, such as "digits followed by a dash", reach for regular expressions.

Checking what a string contains

The is... methods return True or False and are handy for validating input. They require at least one character, so an empty string is never isdigit().

print("2024".isdigit(), "20.5".isdigit())
print("Asha".isalpha(), "Asha7".isalpha(), "Asha7".isalnum())
print("   ".isspace(), "".isspace())
print("hello".islower(), "Hello".istitle())

Note that "20.5".isdigit() is False because of the dot. To validate decimals, try float() inside try/except instead.

Padding, aligning and splitting in three

zfill(width) pads numbers with zeros, ljust, rjust and center pad with spaces (or any one character you pass), and partition(sep) splits at the first separator into exactly three parts: before, separator, after. That fixed shape makes it perfect for key=value lines.

print("42".zfill(5))
print("Item".ljust(8, ".") + "$5".rjust(5))
print("menu".center(12, "-"))
print("theme=dark=blue".partition("="))
key, _, value = "volume=7".partition("=")
print(key, value)

For richer layouts, f-strings can align text too: f"{name:<8}{price:>6}".

Chaining methods and a real example

Because each method returns a string, you can chain calls left to right. A classic use is building a URL slug from a title.

title = "  10 Tips: Clean Python Code!  "
slug = title.strip().lower().replace(":", "").replace("!", "").replace(" ", "-")
print(slug)

table = str.maketrans("", "", ":!?,.")
print(title.strip().lower().translate(table).replace(" ", "-"))

str.maketrans() and translate() replace or delete many single characters in one pass, which is neater than a long chain of replace() calls.