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strip(), lstrip(), and rstrip()

Introduction

In coding interviews involving parsing, input strings often come with unwanted leading or trailing whitespace. Python provides three built-in methods to clean the edges of a string: strip(), lstrip(), and rstrip().

How They Work

By default, these methods remove all whitespace (spaces, tabs, newlines) from the edges of a string.

  • strip(): Removes from both ends.
  • lstrip(): Removes from the left end only.
  • rstrip(): Removes from the right end only.
text = "   hello \n"

print(repr(text.strip()))  # 'hello'
print(repr(text.lstrip())) # 'hello \n'
print(repr(text.rstrip())) # '   hello'

Stripping Specific Characters

You can pass a string argument to these methods. Python will remove any characters that appear in the argument string from the edges.

text = "---hello---"
print(text.strip("-")) # "hello"

Common Interview Mistake: Stripping is a Set of Characters

A very common misunderstanding is thinking that strip("abc") removes the exact substring "abc". It does not. It removes any character 'a', 'b', or 'c' from the edges until it hits a character not in the set.

text = "abccbaHELLOabccba"

# This removes all a's, b's, and c's from the ends.
print(text.strip("abc")) # "HELLO"
If you want to remove an exact prefix or suffix (e.g., Python 3.9+), use removeprefix() or removesuffix().

When to Use Them

Use strip() heavily when: - Reading lines from a file or standard input. - Parsing CSVs or unformatted user input. - Ensuring a string is perfectly clean before checking for a palindrome or passing it to an algorithm.

Time and Space Complexity

  • Time Complexity: \(O(N)\) where \(N\) is the length of the string. It iterates from the edges until it finds a non-matching character.
  • Space Complexity: \(O(N)\) to create the newly stripped string copy.

Summary

  • Use strip() to clean whitespace from both sides of a string.
  • Passing arguments to strip() removes a set of characters, not an exact substring.
  • Remember that strip() returns a new string and does not modify the original.