PEP 616 – String methods to remove prefixes and suffixes

PEP: 616
Title: String methods to remove prefixes and suffixes
Author: Dennis Sweeney <sweeney.dennis650 at gmail.com>
Sponsor: Eric V. Smith <eric at trueblade.com>
Status: Accepted
Type: Standards Track
Created: 19-Mar-2020
Python-Version: 3.9
Post-History: 20-Mar-2020

Abstract
Rationale
Specification
Motivating examples from the Python standard library
Rejected Ideas
How to Teach This
Reference Implementation
History of Major revisions
References
Copyright

Abstract

This is a proposal to add two new methods, removeprefix() and removesuffix(), to the APIs of Python’s various string objects. These methods would remove a prefix or suffix (respectively) from a string, if present, and would be added to Unicode str objects, binary bytes and bytearray objects, and collections.UserString.

Rationale

There have been repeated issues on Python-Ideas 2 3, Python-Dev 4 5 6 7, the Bug Tracker, and StackOverflow 8, related to user confusion about the existing str.lstrip and str.rstrip methods. These users are typically expecting the behavior of removeprefix and removesuffix, but they are surprised that the parameter for lstrip is interpreted as a set of characters, not a substring. This repeated issue is evidence that these methods are useful. The new methods allow a cleaner redirection of users to the desired behavior.

As another testimonial for the usefulness of these methods, several users on Python-Ideas 2 reported frequently including similar functions in their code for productivity. The implementation often contained subtle mistakes regarding the handling of the empty string, so a well-tested built-in method would be useful.

The existing solutions for creating the desired behavior are to either implement the methods as in the Specification below, or to use regular expressions as in the expression re.sub('^' + re.escape(prefix), '', s), which is less discoverable, requires a module import, and results in less readable code.

Specification

The builtin str class will gain two new methods which will behave as follows when type(self) is type(prefix) is type(suffix) is str:

def removeprefix(self: str, prefix: str, /) -> str:
    if self.startswith(prefix):
        return self[len(prefix):]
    else:
        return self[:]

def removesuffix(self: str, suffix: str, /) -> str:
    # suffix='' should not call self[:-0].
    if suffix and self.endswith(suffix):
        return self[:-len(suffix)]
    else:
        return self[:]

When the arguments are instances of str subclasses, the methods should behave as though those arguments were first coerced to base str objects, and the return value should always be a base str.

Methods with the corresponding semantics will be added to the builtin bytes and bytearray objects. If b is either a bytes or bytearray object, then b.removeprefix() and b.removesuffix() will accept any bytes-like object as an argument. The two methods will also be added to collections.UserString, with similar behavior.

Motivating examples from the Python standard library

The examples below demonstrate how the proposed methods can make code one or more of the following:

Less fragile:
The code will not depend on the user to count the length of a literal.
More performant:
The code does not require a call to the Python built-in len function nor to the more expensive str.replace() method.
More descriptive:
The methods give a higher-level API for code readability as opposed to the traditional method of string slicing.

find_recursionlimit.py

Current:

if test_func_name.startswith("test_"):
    print(test_func_name[5:])
else:
    print(test_func_name)

Improved:

print(test_func_name.removeprefix("test_"))

deccheck.py

This is an interesting case because the author chose to use the str.replace method in a situation where only a prefix was intended to be removed.

Current:

if funcname.startswith("context."):
    self.funcname = funcname.replace("context.", "")
    self.contextfunc = True
else:
    self.funcname = funcname
    self.contextfunc = False

Improved:

if funcname.startswith("context."):
    self.funcname = funcname.removeprefix("context.")
    self.contextfunc = True
else:
    self.funcname = funcname
    self.contextfunc = False

Arguably further improved:

self.contextfunc = funcname.startswith("context.")
self.funcname = funcname.removeprefix("context.")

cookiejar.py

Current:

def strip_quotes(text):
    if text.startswith('"'):
        text = text[1:]
    if text.endswith('"'):
        text = text[:-1]
    return text

Improved:

def strip_quotes(text):
    return text.removeprefix('"').removesuffix('"')

test_i18n.py

Current:

creationDate = header['POT-Creation-Date']

# peel off the escaped newline at the end of string
if creationDate.endswith('\\n'):
    creationDate = creationDate[:-len('\\n')]

Improved:

creationDate = header['POT-Creation-Date'].removesuffix('\\n')

There were many other such examples in the stdlib.

Rejected Ideas

Expand the lstrip and rstrip APIs

Because lstrip takes a string as its argument, it could be viewed as taking an iterable of length-1 strings. The API could, therefore, be generalized to accept any iterable of strings, which would be successively removed as prefixes. While this behavior would be consistent, it would not be obvious for users to have to call 'foobar'.lstrip(('foo',)) for the common use case of a single prefix.

Remove multiple copies of a prefix

This is the behavior that would be consistent with the aforementioned expansion of the lstrip/rstrip API – repeatedly applying the function until the argument is unchanged. This behavior is attainable from the proposed behavior via by the following:

>>> s = 'Foo' * 100 + 'Bar'
>>> prefix = 'Foo'
>>> while s.startswith(prefix): s = s.removeprefix(prefix)
>>> s
'Bar'

Raising an exception when not found

There was a suggestion that s.removeprefix(pre) should raise an exception if not s.startswith(pre). However, this does not match with the behavior and feel of other string methods. There could be required=False keyword added, but this violates the KISS principle.

Accepting a tuple of affixes

It could be convenient to write the test_concurrent_futures.py example above as name.removesuffix(('Mixin', 'Tests', 'Test')), so there was a suggestion that the new methods be able to take a tuple of strings as an argument, similar to the startswith() API. Within the tuple, only the first matching affix would be removed. This was rejected on the following grounds:

This behavior can be surprising or visually confusing, especially when one prefix is empty or is a substring of another prefix, as in 'FooBar'.removeprefix(('', 'Foo')) == 'Foo' or 'FooBar text'.removeprefix(('Foo', 'FooBar ')) == 'Bar text'.
The API for str.replace() only accepts a single pair of replacement strings, but has stood the test of time by refusing the temptation to guess in the face of ambiguous multiple replacements.
There may be a compelling use case for such a feature in the future, but generalization before the basic feature sees real-world use would be easy to get permanently wrong.

Alternative Method Names

Several alternatives method names have been proposed. Some are listed below, along with commentary for why they should be rejected in favor of removeprefix (the same arguments hold for removesuffix).

ltrim, trimprefix, etc.:
“Trim” does in other languages (e.g. JavaScript, Java, Go, PHP) what strip methods do in Python.
lstrip(string=...)
This would avoid adding a new method, but for different behavior, it’s better to have two different methods than one method with a keyword argument that selects the behavior.
remove_prefix:
All of the other methods of the string API, e.g. str.startswith(), use lowercase rather than lower_case_with_underscores.
removeleft, leftremove, or lremove:
The explicitness of “prefix” is preferred.
cutprefix, deleteprefix, withoutprefix, dropprefix, etc.:
Many of these might have been acceptable, but “remove” is unambiguous and matches how one would describe the “remove the prefix” behavior in English.
stripprefix:
Users may benefit from remembering that “strip” means working with sets of characters, while other methods work with substrings, so re-using “strip” here should be avoided.

How to Teach This

Among the uses for the partition(), startswith(), and split() string methods or the enumerate() or zip() built-in functions, a common theme is that if a beginner finds themselves manually indexing or slicing a string, then they should consider whether there is a higher-level method that better communicates what the code should do rather than merely how the code should do it. The proposed removeprefix() and removesuffix() methods expand the high-level string “toolbox” and further allow for this sort of skepticism toward manual slicing.

The main opportunity for user confusion will be the conflation of lstrip/rstrip with removeprefix/removesuffix. It may therefore be helpful to emphasize (as the documentation will) the following differences between the methods:

(l/r)strip:
- The argument is interpreted as a character set.
- The characters are repeatedly removed from the appropriate end of the string.
remove(prefix/suffix):
- The argument is interpreted as an unbroken substring.
- Only at most one copy of the prefix/suffix is removed.

Reference Implementation

See the pull request on GitHub 1.

History of Major revisions

Version 3: Remove tuple behavior.
Version 2: Changed name to removeprefix/removesuffix; added support for tuples as arguments
Version 1: Initial draft with cutprefix/cutsuffix