Browser Link: https://www.python.org/dev/peps/pep-0616/ 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: 30-Aug-2002 Abstract ======== This is a proposal to add two new methods, ``cutprefix`` and ``cutsuffix``, to the APIs of Python's various string objects. In particular, the methods would be added to Unicode ``str`` objects, binary ``bytes`` and ``bytearray`` objects, and ``collections.UserString``. If ``s`` is one these objects, and ``s`` has ``pre`` as a prefix, then ``s.cutprefix(pre)`` returns a copy of ``s`` in which that prefix has been removed. If ``s`` does not have ``pre`` as a prefix, an unchanged copy of ``s`` is returned. In summary, ``s.cutprefix(pre)`` is roughly equivalent to ``s[len(pre):] if s.startswith(pre) else s``. The behavior of ``cutsuffix`` is analogous: ``s.cutsuffix(suf)`` is roughly equivalent to ``s[:-len(suf)] if suf and s.endswith(suf) else s``. Rationale ========= There have been repeated issues [#confusion]_ on the Bug Tracker and StackOverflow related to user confusion about the existing ``str.lstrip`` and ``str.rstrip`` methods. These users are typically expecting the behavior of ``cutprefix`` and ``cutsuffix``, 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, and 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 own code for productivity. The implementation often contained subtle mistakes regarding the handling of the empty string (see `Specification`_). Specification ============= The builtin ``str`` class will gain two new methods with roughly the following behavior:: def cutprefix(self: str, pre: str, /) -> str: if self.startswith(pre): return self[len(pre):] return self[:] def cutsuffix(self: str, suf: str, /) -> str: if suf and self.endswith(suf): return self[:-len(suf)] return self[:] The only difference between the real implementation and the above is that, as with other string methods like ``replace``, the methods will raise a ``TypeError`` if any of ``self``, ``pre`` or ``suf`` is not an instace of ``str``, and will cast subclasses of ``str`` to builtin ``str`` objects. Note that without the check for the truthyness of ``suf``, ``s.cutsuffix('')`` 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.cutsuffix()`` and ``b.cutprefix()`` will accept any bytes-like object as an argument. Note that the ``bytearray`` methods return a copy of ``self``; they do not operate in place. The following behavior is considered a CPython implementation detail, but is not guaranteed by this specification:: >>> x = 'foobar' * 10**6 >>> x.cutprefix('baz') is x is x.cutsuffix('baz') True >>> x.cutprefix('') is x is x.cutsuffix('') True That is, for CPython's immutable ``str`` and ``bytes`` objects, the methods return the original object when the affix is not found or if the affix is empty. Because these types test for equality using shortcuts for identity and length, the following equivalent expressions are evaluated at approximately the same speed, for any ``str`` objects (or ``bytes`` objects) ``x`` and ``y``:: >>> (True, x[len(y):]) if x.startswith(y) else (False, x) >>> (True, z) if x != (z := x.cutprefix(y)) else (False, x) The two methods will also be added to ``collections.UserString``, where they rely on the implementation of the new ``str`` methods. 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. More descriptive: The methods give a higher-level API for code readability, as opposed to the traditional method of string slicing. refactor.py ----------- - Current:: if fix_name.startswith(self.FILE_PREFIX): fix_name = fix_name[len(self.FILE_PREFIX):] - Improved:: fix_name = fix_name.cutprefix(self.FILE_PREFIX) c_annotations.py: ----------------- - Current:: if name.startswith("c."): name = name[2:] - Improved:: name = name.cutprefix("c.") find_recursionlimit.py ---------------------- - Current:: if test_func_name.startswith("test_"): print(test_func_name[5:]) else: print(test_func_name) - Improved:: print(test_finc_name.cutprefix("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.cutprefix("context.") self.contextfunc = True else: self.funcname = funcname self.contextfunc = False - Arguably further improved:: self.contextfunc = funcname.startswith("context.") self.funcname = funcname.cutprefix("context.") test_i18n.py ------------ - Current:: if test_func_name.startswith("test_"): print(test_func_name[5:]) else: print(test_func_name) - Improved:: print(test_finc_name.cutprefix("test_")) - Current:: if creationDate.endswith('\\n'): creationDate = creationDate[:-len('\\n')] - Improved:: creationDate = creationDate.cutsuffix('\\n') shared_memory.py ---------------- - Current:: reported_name = self._name if _USE_POSIX and self._prepend_leading_slash: if self._name.startswith("/"): reported_name = self._name[1:] return reported_name - Improved:: if _USE_POSIX and self._prepend_leading_slash: return self._name.cutprefix("/") return self._name build-installer.py ------------------ - Current:: if archiveName.endswith('.tar.gz'): retval = os.path.basename(archiveName[:-7]) if ((retval.startswith('tcl') or retval.startswith('tk')) and retval.endswith('-src')): retval = retval[:-4] - Improved:: if archiveName.endswith('.tar.gz'): retval = os.path.basename(archiveName[:-7]) if retval.startswith(('tcl', 'tk')): retval = retval.cutsuffix('-src') Depending on personal style, ``archiveName[:-7]`` could also be changed to ``archiveName.cutsuffix('.tar.gz')``. test_core.py ------------ - Current:: if output.endswith("\n"): output = output[:-1] - Improved:: output = output.cutsuffix("\n") 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.cutprefix('"').cutsuffix('"') - Current:: if line.endswith("\n"): line = line[:-1] - Improved:: line = line.cutsuffix("\n") fixdiv.py --------- - Current:: def chop(line): if line.endswith("\n"): return line[:-1] else: return line - Improved:: def chop(line): return line.cutsuffix("\n") 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.cutsuffix('Mixin').cutsuffix('Tests').cutsuffix('Test') msvc9compiler.py ---------------- - Current:: if value.endswith(os.pathsep): value = value[:-1] - Improved:: value = value.cutsuffix(os.pathsep) test_pathlib.py --------------- - Current:: self.assertTrue(r.startswith(clsname + '('), r) self.assertTrue(r.endswith(')'), r) inner = r[len(clsname) + 1 : -1] - Improved:: self.assertTrue(r.startswith(clsname + '('), r) self.assertTrue(r.endswith(')'), r) inner = r.cutprefix(clsname + '(').cutsuffix(')') 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'.cutprefix(('foo,))`` for the common use case of a single prefix. Allow multiple prefixes ----------------------- Some users discussed the desire to be able to remove multiple prefixes, calling, for example, ``s.cutprefix('From: ', 'CC: ')``. However, this adds ambiguity about the order in which the prefixes are removed, especially in cases like ``s.cutprefix('Foo', 'FooBar')``. After this proposal, this can be spelled explicitly as ``s.cutprefix('Foo').cutprefix('FooBar')``. 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 the following:: >>> s = 'foo' * 100 + 'bar' >>> while s != (s := s.cutprefix("foo")): pass >>> s 'bar' The above can be modififed by chaining multiple ``cutprefix`` calls together to achieve the full behavior of the ``lstrip``/``rstrip`` generalization, while being explicit in the order of removal. While the proposed API could later be extended to include some of these use cases, to do so before any observation of how these methods are used in practice would be premature and may lead to choosing the wrong behavior. Raising an exception when not found ----------------------------------- There was a suggestion that ``s.cutprefix(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. 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 ``cutprefix`` (the same arguments hold for ``cutsuffix``) ``ltrim`` "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 select the behavior. ``cut_prefix`` All of the other methods of the string API, e.g. ``str.startswith()``, use ``lowercase`` rather than ``lower_case_with_underscores``. ``cutleft``, ``leftcut``, or ``lcut`` The explicitness of "prefix" is preferred. ``removeprefix``, ``deleteprefix``, ``withoutprefix``, etc. All of these might have been acceptable, but they have more characters than ``cut``. Some suggested that the verb "cut" implies mutability, but the string API already contains verbs like "replace", "strip", "split", and "swapcase". ``stripprefix`` Users may benefit from the mnemonic 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] Discussion on Python-Ideas (https://mail.python.org/archives/list/python-ideas@python.org/thread/RJARZSU...) .. [#confusion] Comment listing Bug Tracker and StackOverflow issues (https://mail.python.org/archives/list/python-ideas@python.org/message/GRGAFI...) Copyright ========= This document is placed in the public domain or under the CC0-1.0-Universal license, whichever is more permissive. .. Local Variables: mode: indented-text indent-tabs-mode: nil sentence-end-double-space: t fill-column: 70 coding: utf-8 End: