Remove a prefix from a string

python

functions

best-practices

regular-expressions

byAlex Kataev·Jan 27, 2025

Efficiently remove a string's prefix with str.removeprefix() in Python 3.9+. When dealing with older versions, it's as simple as slicing and conducting a prefix check:

# Python 3.9+: No need for surgery, just use removeprefix
s = "prefix_text"
print(s.removeprefix("prefix_"))  # 'text'

# Python <3.9: A slice of bread, sorry, I meant code
s = "prefix_text"
prefix = "prefix_"
print(s[len(prefix):] if s.startswith(prefix) else s)  # 'text'

Dive into prefix removal

Stripping a prefix off a string might appear like a piece of cake, but just like baking, it requires careful follow-through of best practices, and handling of potential edge cases.

Custom functions for portability

In versions preceding Python 3.9, reusing prefix removal logic calls for creating a custom function like this:

def remove_prefix(text, prefix):
    # startswith: The unsung hero in the land of strings
    if text.startswith(prefix):
        return text[len(prefix):]
    # The foresight to return original text if it's not prefixed: Priceless!
    return text

# Usage:
s = "prefix_text"
print(remove_prefix(s, "prefix_"))  # 'text'

<center>🟨 Please board only if your text is prefixed 🟨</center>

Regular expressions for power

Facing complex string patterns or need more flexibility? Welcome, regular expressions:

import re

def remove_prefix_regex(text, prefix):
    escaped_prefix = re.escape(prefix)  # What's a rabbit's favorite music? Hip Hop (re-escape)
    return re.sub(f"^{escaped_prefix}", '', text)

# Usage:
s = "prefix_text"
print(remove_prefix_regex(s, "prefix_"))  # 'text'

Regex gives you additional powers with an added complexity cape.

Multiple conviction: `lstrip()`

Lured towards lstrip()? Don't give in, it's a trap!

# Misleading use of lstrip
s = "prefix_text"
print(s.lstrip("prefix_"))  # Unsuspected result!

Beware, lstrip() deals with individual characters in the prefix, not the collective substring.

Now we're talking Regex

Time to delve into complex scenarios where your prefix might include regex-special characters or user inputs!

import re

def robust_remove_prefix(text, prefix):
    # regex - when strings refuse to act normal
    pattern = "^" + re.escape(prefix)
    regex = re.compile(pattern)  # compile it for performance, like your workout
    return regex.sub('', text)

# Usage:
s = "^some+prefix_text"
print(robust_remove_prefix(s, "^some+prefix_"))  # 'text'

Regular expressions are tough, but can be very powerful with prefix removal!

Finer details of prefix removal

Let's analyze the slicing method a bit more:

Investigate if the string starts with the prefix using text.startswith(prefix). It's like the entry check at a club.
If the prefix is present (bouncer's nod), we slice it off: text[len(prefix):].
Access denied? No problem, return the string as is.

Adopting this approach ensures your code is both efficient and easy-to-read.

Bulletproofing your prefix removal

When it comes to prefix removal, be a pro and follow these best practices:

Use str.removeprefix(). It's Python's gift to 3.9+ users.
Working with older Python versions? Wrap the s.startswith(prefix) slicing logic in a custom function.
Trust in Regular expressions for complex patterns.
Both lstrip() and replace() can lead to errors. Better to avoid them.
Document your decisions. You, or someone else, will be thankful in future.