PEP 616 – String methods to remove prefixes and suffixes
- String methods to remove prefixes and suffixes
- Dennis Sweeney <sweeney.dennis650 at gmail.com>
- Eric V. Smith <eric at trueblade.com>
- Standards Track
- Motivating examples from the Python standard library
- Rejected Ideas
- How to Teach This
- Reference Implementation
- History of Major revisions
This is a proposal to add two new methods,
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
bytearray objects, and
There have been repeated issues on Python-Ideas  ,
Python-Dev    , the Bug Tracker, and
StackOverflow , related to user confusion about the
str.rstrip methods. These users are
typically expecting the behavior of
but they are surprised that the parameter for
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  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.
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
objects, and the return value should always be a base
Methods with the corresponding semantics will be added to the builtin
bytearray objects. If
b is either a
bytearray object, then
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
lenfunction nor to the more expensive
- More descriptive:
The methods give a higher-level API for code readability as opposed to the traditional method of string slicing.
if test_func_name.startswith("test_"): print(test_func_name[5:]) else: print(test_func_name)
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.
if funcname.startswith("context."): self.funcname = funcname.replace("context.", "") self.contextfunc = True else: self.funcname = funcname self.contextfunc = False
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.")
creationDate = header['POT-Creation-Date'] # peel off the escaped newline at the end of string if creationDate.endswith('\\n'): creationDate = creationDate[:-len('\\n')]
creationDate = header['POT-Creation-Date'].removesuffix('\\n')
There were many other such examples in the stdlib.
Expand the lstrip and rstrip APIs
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
Remove multiple copies of a prefix
This is the behavior that would be consistent with the aforementioned
expansion of the
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
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
Accepting a tuple of affixes
It could be convenient to write the
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')) == 'FooBar'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
removeprefix (the same arguments hold for
stripmethods do in Python.
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.
All of the other methods of the string API, e.g.
The explicitness of “prefix” is preferred.
Many of these might have been acceptable, but “remove” is unambiguous and matches how one would describe the “remove the prefix” behavior in English.
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
split() string methods or the
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
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
It may therefore be helpful to emphasize (as the documentation will)
the following differences between the methods:
- The argument is interpreted as a character set.
- The characters are repeatedly removed from the appropriate end of the string.
- The argument is interpreted as an unbroken substring.
- Only at most one copy of the prefix/suffix is removed.
See the pull request on GitHub .
History of Major revisions
- Version 3: Remove tuple behavior.
- Version 2: Changed name to
removesuffix; added support for tuples as arguments
- Version 1: Initial draft with
- GitHub pull request with implementation (https://github.com/python/cpython/pull/18939)
-  (1, 2)
- [Python-Ideas] “New explicit methods to trim strings” (https://email@example.com/thread/RJARZSUKCXRJIP42Z2YBBAEN5XA7KEC3/)
- “Re: [Python-ideas] adding a trim convenience function” (https://firstname.lastname@example.org/thread/SJ7CKPZSKB5RWT7H3YNXOJUQ7QLD2R3X/#C2W5T7RCFSHU5XI72HG53A6R3J3SN4MV)
- “Re: [Python-Dev] strip behavior provides inconsistent results with certain strings” (https://email@example.com/thread/XYFQMFPUV6FR2N5BGYWPBVMZ5BE5PJ6C/#XYFQMFPUV6FR2N5BGYWPBVMZ5BE5PJ6C)
- [Python-Dev] “correction of a bug” (https://firstname.lastname@example.org/thread/AOZ7RFQTQLCZCTVNKESZI67PB3PSS72X/#AOZ7RFQTQLCZCTVNKESZI67PB3PSS72X)
- [Python-Dev] “str.lstrip bug?” (https://email@example.com/thread/OJDKRIESKGTQFNLX6KZSGKU57UXNZYAN/#CYZUFFJ2Q5ZZKMJIQBZVZR4NSLK5ZPIH)
- [Python-Dev] “strip behavior provides inconsistent results with certain strings” (https://firstname.lastname@example.org/thread/ZWRGCGANHGVDPP44VQKRIYOYX7LNVDVG/#ZWRGCGANHGVDPP44VQKRIYOYX7LNVDVG)
- Comment listing Bug Tracker and StackOverflow issues (https://email@example.com/message/GRGAFIII3AX22K3N3KT7RB4DPBY3LPVG/)
This document is placed in the public domain or under the CC0-1.0-Universal license, whichever is more permissive.
Last modified: 2021-12-17 22:49:28 GMT