python-peps/pep-0616.rst

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


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 [#pyid]_ [3]_,
Python-Dev [4]_ [5]_ [6]_ [7]_, the Bug Tracker, and
StackOverflow [#confusion]_, 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 [#pyid]_ 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 with roughly the
following behavior::

    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:
        if suffix and self.endswith(suffix):
            return self[:-len(suffix)]
        else:
            return self[:]

Note that ``self[:]`` might not actually make a copy -- if the affix
is empty or not found, and if ``type(self) is str``, then these methods
may, but are not required to, make the optimization of returning ``self``.
However, when called on instances of subclasses of ``str``, these
methods should return base ``str`` objects, not ``self``.

Without the check for the truthiness of ``suffix``,
``s.removesuffix('')`` would be mishandled and always return the empty
string due to the unintended evaluation of ``self[:-0]``.

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.  Although the methods
on the immutable ``str`` and ``bytes`` types may make the aforementioned
optimization of returning the original object, ``bytearray.removeprefix()``
and ``bytearray.removesuffix()`` should *always* return a copy, never the
original object.

To test whether any affixes were removed during the call, users
may use the constant-time behavior of comparing the lengths of
the original and new strings::

    >>> string = 'Python String Input'
    >>> new_string = string.removeprefix('Py')
    >>> modified = (len(string) != len(new_string))
    >>> modified
    True

Users may also continue using ``startswith()`` and ``endswith()``
methods for control flow instead of testing the lengths as above.

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:

1. Less fragile:

   The code will not depend on the user to count the length of a literal.

2. More performant:

   The code does not require a call to the Python built-in ``len``
   function nor to the more expensive ``str.replace()`` method.

3. 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_concurrent_futures.py
--------------------------

In the following example, the meaning of the code changes slightly,
but in context, it behaves the same.

- Current::

    if name.endswith(('Mixin', 'Tests')):
        return name[:-5]
    elif name.endswith('Test'):
        return name[:-4]
    else:
        return name

- Improved::

    return (name.removesuffix('Mixin')
                .removesuffix('Tests')
                .removesuffix('Test'))


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 len(s) != len(s := s.removeprefix(prefix)): pass
    >>> s
    'Bar'

or the more obvious and readable alternative::

    >>> 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.


Reference Implementation
========================

See the pull request on GitHub [#pr]_.


References
==========

.. [#pr] GitHub pull request with implementation
   (https://github.com/python/cpython/pull/18939)
.. [#pyid] [Python-Ideas] "New explicit methods to trim strings"
   (https://mail.python.org/archives/list/python-ideas@python.org/thread/RJARZSUKCXRJIP42Z2YBBAEN5XA7KEC3/)
.. [3] "Re: [Python-ideas] adding a trim convenience function"
   (https://mail.python.org/archives/list/python-ideas@python.org/thread/SJ7CKPZSKB5RWT7H3YNXOJUQ7QLD2R3X/#C2W5T7RCFSHU5XI72HG53A6R3J3SN4MV)
.. [4] "Re: [Python-Dev] strip behavior provides inconsistent results with certain strings"
   (https://mail.python.org/archives/list/python-ideas@python.org/thread/XYFQMFPUV6FR2N5BGYWPBVMZ5BE5PJ6C/#XYFQMFPUV6FR2N5BGYWPBVMZ5BE5PJ6C)
.. [5] [Python-Dev] "correction of a bug"
   (https://mail.python.org/archives/list/python-dev@python.org/thread/AOZ7RFQTQLCZCTVNKESZI67PB3PSS72X/#AOZ7RFQTQLCZCTVNKESZI67PB3PSS72X)
.. [6] [Python-Dev] "str.lstrip bug?"
   (https://mail.python.org/archives/list/python-dev@python.org/thread/OJDKRIESKGTQFNLX6KZSGKU57UXNZYAN/#CYZUFFJ2Q5ZZKMJIQBZVZR4NSLK5ZPIH)
.. [7] [Python-Dev] "strip behavior provides inconsistent results with certain strings"
   (https://mail.python.org/archives/list/python-dev@python.org/thread/ZWRGCGANHGVDPP44VQKRIYOYX7LNVDVG/#ZWRGCGANHGVDPP44VQKRIYOYX7LNVDVG)
.. [#confusion] Comment listing Bug Tracker and StackOverflow issues
   (https://mail.python.org/archives/list/python-ideas@python.org/message/GRGAFIII3AX22K3N3KT7RB4DPBY3LPVG/)


Copyright
=========

This document is placed in the public domain or under the
CC0-1.0-Universal license, whichever is more permissive.



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