Modify Strings

Python strings are sequences of characters enclosed in quotes. Although strings are immutable — you cannot change them in place — Python gives you a rich set of operators and methods that return new, modified strings. This chapter covers the most important techniques: concatenation, repetition, case conversion, slicing, replacing substrings, stripping whitespace, splitting and joining, and more.

Key Concept: Strings Are Immutable

Before diving in, it is important to understand that every "modification" method returns a new string. The original string is never changed.

greeting = "hello"
upper_greeting = greeting.upper()

print(greeting)        # hello  (unchanged)
print(upper_greeting)  # HELLO  (new string)

This means you must assign the result to a variable (or use it directly) to keep the change.

String Concatenation

Concatenation combines two or more strings into one using the + operator. You can chain as many strings as you need.

Concatenate strings in Python

Concatenating with Variables and Literals

You can mix string variables with string literals:

first_name = "Ada"
last_name  = "Lovelace"
full_name  = first_name + " " + last_name
print(full_name)  # Ada Lovelace

For larger amounts of text, consider format strings or f-strings which are easier to read than long chains of +.

String Repetition

The * operator repeats a string a given number of times. This is useful for generating separators, padding, or simple patterns.

Repeat a string in Python

Changing Case

Python provides several methods to change the capitalisation of a string.

Convert a string to uppercase or lowercase in Python

Case methods are commonly used when comparing user input regardless of how it was typed:

answer = input("Type yes or no: ")
if answer.lower() == "yes":
    print("You said yes!")

Slicing Strings

Slicing extracts part of a string using the [start:stop:step] syntax. The result is a new string containing the characters from index start up to but not including index stop.

 H  e  l  l  o  ,     W  o  r  l  d  !
 0  1  2  3  4  5  6  7  8  9 10 11 12
-13-12-11-10-9 -8 -7 -6 -5 -4 -3 -2 -1

Slice strings in Python

When to Use Negative Indices

Negative indices count from the end of the string. Index -1 is the last character, -2 is the second-to-last, and so on. This is handy when you want the tail of a string without knowing its length:

filename = "report_2024.csv"
extension = filename[-3:]
print(extension)  # csv

For a dedicated deep-dive, see the Slicing Strings chapter.

Replacing Substrings

The replace(old, new) method returns a copy of the string with every occurrence of old replaced by new. Pass a third argument count to limit how many replacements are made.

Replace a part of a string in Python

text = "I like cats. Cats are great. Cats!"
print(text.replace("Cats", "Dogs"))         # replaces all occurrences
print(text.replace("Cats", "Dogs", 1))      # replaces first occurrence only

I like cats. Dogs are great. Dogs!
I like cats. Dogs are great. Cats!

Note that replace() is case-sensitive: "cats" and "Cats" are treated as different substrings.

Stripping Whitespace

Whitespace characters (spaces, tabs, newlines) at the start or end of a string often need to be removed, especially when processing user input or reading files.

raw = "   hello world   "
print(repr(raw.strip()))   # 'hello world'
print(repr(raw.lstrip()))  # 'hello world   '
print(repr(raw.rstrip()))  # '   hello world'

You can also strip specific characters by passing them as an argument:

path = "///usr/local/bin///"
print(path.strip("/"))  # usr/local/bin

Splitting Strings

The split(sep) method splits a string on a separator and returns a list of substrings. With no argument it splits on any whitespace and removes empty strings.

sentence = "Python is easy to learn"
words = sentence.split()
print(words)  # ['Python', 'is', 'easy', 'to', 'learn']

csv_row = "Alice,30,Engineer"
fields = csv_row.split(",")
print(fields)  # ['Alice', '30', 'Engineer']

Pass a second argument maxsplit to limit the number of splits:

data = "one:two:three:four"
print(data.split(":", 2))  # ['one', 'two', 'three:four']

Joining Strings

join() is the reverse of split(). It combines a list of strings into one, placing a separator between each element. The separator is the string on which you call join().

words = ["Python", "is", "fun"]
sentence = " ".join(words)
print(sentence)  # Python is fun

path_parts = ["usr", "local", "bin"]
path = "/".join(path_parts)
print(path)  # usr/local/bin

Using join() is far more efficient than concatenating strings in a loop, because each + creates a new string object. join() allocates the final string only once.

Searching Within Strings

Several methods help you find text inside a string.

find() and index()

find(sub) returns the index of the first occurrence of sub, or -1 if not found. index(sub) does the same but raises ValueError instead of returning -1.

text = "the quick brown fox"
print(text.find("quick"))   # 4
print(text.find("slow"))    # -1

print(text.index("brown"))  # 10

startswith() and endswith()

These return True or False and are a cleaner alternative to slicing when you need to check the beginning or end of a string.

url = "https://www.w3docs.com"
print(url.startswith("https"))   # True
print(url.endswith(".com"))      # True
print(url.startswith("http://")) # False

You can pass a tuple of prefixes/suffixes to check for multiple options at once:

filename = "photo.jpg"
print(filename.endswith((".jpg", ".jpeg", ".png")))  # True

count()

count(sub) returns how many times sub appears in the string (non-overlapping).

text = "banana"
print(text.count("a"))   # 3
print(text.count("an"))  # 2

Checking String Properties

Python has a family of is* methods that return True or False about the content of a string.

print("hello".isalpha())    # True  – all alphabetic
print("hello123".isalnum()) # True  – all alphanumeric
print("12345".isdigit())    # True  – all digits
print("  ".isspace())       # True  – all whitespace
print("Hello World".istitle()) # True – title-case

These are especially useful for input validation.

Practical Example: Cleaning User Input

Real-world string modification often combines several techniques:

raw_input = "  Ada Lovelace  "

# Clean and normalise
name = raw_input.strip()          # remove surrounding spaces
name = name.title()               # ensure proper capitalisation

parts = name.split()              # split into first / last
first, last = parts[0], parts[1]

print(f"First: {first}, Last: {last}")  # First: Ada, Last: Lovelace

Quick Reference

For the complete list of built-in string methods, see String Methods. To learn about escape sequences like \n and \t, see Escape Characters. For powerful text search and manipulation using patterns, see Python Regex.

Practice