String Operations

Difficulty: Beginner

Python strings support several operators that make working with text intuitive. The + operator concatenates (joins) two strings together, creating a new string. The * operator repeats a string a given number of times. These are the two fundamental string operators.

The in and not in operators test whether a substring exists within a string, returning True or False. These membership operators are case-sensitive: "python" in "Python is great" returns False because the cases don't match. For case-insensitive checks, convert both strings to the same case first.

Strings can be compared using comparison operators (==, !=, <, >, <=, >=). String comparison is lexicographic, meaning Python compares characters one by one using their Unicode code point values. Uppercase letters have lower code points than lowercase letters, so "Apple" < "banana" is True (because 'A' has code point 65 while 'b' has code point 98).

A fundamental property of Python strings is immutability. Once a string is created, you cannot change any individual character. Operations like concatenation, replace(), and upper() always produce new string objects. Attempting text[0] = "X" raises a TypeError. If you need to build a string incrementally, use a list of parts and join them at the end for better performance.

Code examples

Concatenation and Repetition

greeting = "Hello" + " " + "World"
line = "-" * 20
border = "*" * 3 + " Title " + "*" * 3

print(greeting)
print(line)
print(border)

The + operator joins strings. The * operator repeats a string. Both create new strings; neither modifies the originals.

Membership Testing with in

text = "Python programming is powerful"

print("Python" in text)
print("python" in text)
print("Java" not in text)
print("python" in text.lower())

The in operator is case-sensitive. 'python' is not found because the original has 'Python'. Converting to lower() makes the check case-insensitive.

String Comparison

print("apple" == "apple")
print("apple" < "banana")
print("Apple" < "apple")
print("abc" < "abd")
print(ord("A"), ord("a"))

Strings are compared character by character using Unicode code points. 'A' (65) < 'a' (97), so uppercase letters sort before lowercase.

String Immutability

text = "Hello"

# Cannot do: text[0] = "J"  -> TypeError
# Instead, create a new string:
new_text = "J" + text[1:]
print(new_text)
print(text)
print(id(text) != id(new_text))

The original string 'Hello' is unchanged. A new string 'Jello' is created by concatenating 'J' with a slice. id() confirms they are different objects.

Key points

String concatenation with + in a loop is inefficient because it creates a new string each iteration. Use ''.join(list) instead for building strings from many parts.
The * operator requires one operand to be a string and the other an integer. Multiplying by 0 or a negative number returns an empty string.
Python caches (interns) small strings, so two variables with the same short string content may share the same object in memory.
Use len() to get the number of characters in a string.

Concepts covered

concatenation, repetition, membership, comparison, immutability