PEP 616: Add str.removeprefix and str.removesuffix methods

[sweeneyde]

BPO	39939
Nosy	@gvanrossum, @doerwalter, @rhettinger, @vstinner, @ericvsmith, @stevendaprano, @elazarg, @miss-islington, @tirkarthi, @sweeneyde
PRs	bpo-39939: Add str.removeprefix and str.removesuffix #18939 WIP: bpo-39939: Use removeprefix() and removesuffix() #19455 bpo-39939: Fix typo in docs: removeprefix is issue 39939 #20473 [3.9] bpo-39939: Fix removeprefix issue number in the What's New in Python 3.9 (GH-20473) #20474
Files	pep-9999.rst: Revised for typos

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2020-04-22.21:16:57.018>
created_at = <Date 2020-03-11.19:11:38.177>
labels = ['interpreter-core', 'type-feature', '3.9']
title = 'PEP 616: Add str.removeprefix and str.removesuffix methods'
updated_at = <Date 2020-05-28.01:24:39.490>
user = 'https://github.com/sweeneyde'

bugs.python.org fields:

activity = <Date 2020-05-28.01:24:39.490>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2020-04-22.21:16:57.018>
closer = 'vstinner'
components = ['Interpreter Core']
creation = <Date 2020-03-11.19:11:38.177>
creator = 'Dennis Sweeney'
dependencies = []
files = ['48989']
hgrepos = []
issue_num = 39939
keywords = ['patch']
message_count = 29.0
messages = ['363958', '364020', '364028', '364277', '364284', '364313', '364581', '364582', '364643', '364657', '364664', '364671', '364681', '364701', '364703', '365036', '366879', '366882', '366886', '366890', '366897', '366960', '366964', '367049', '367052', '367055', '367056', '370158', '370160']
nosy_count = 10.0
nosy_names = ['gvanrossum', 'doerwalter', 'rhettinger', 'vstinner', 'eric.smith', 'steven.daprano', 'elazar', 'miss-islington', 'xtreak', 'Dennis Sweeney']
pr_nums = ['18939', '19455', '20473', '20474']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue39939'
versions = ['Python 3.9']

[sweeneyde]

Following discussion here ( https://mail.python.org/archives/list/python-ideas@python.org/thread/RJARZSUKCXRJIP42Z2YBBAEN5XA7KEC3/ ), there is a proposal to add new methods str.cutprefix and str.cutsuffix to alleviate the common misuse of str.lstrip and str.rstrip.

I think sticking with the most basic possible behavior

    def cutprefix(self: str, prefix: str) -> str:
        if self.startswith(prefix):
            return self[len(prefix):]
        # return a copy to work for bytearrays
        return self[:]

    def cutsuffix(self: str, suffix: str) -> str:
        if self.startswith(suffix):
            # handles the "[:-0]" issue
            return self[:len(self)-len(suffix)]
        return self[:]

would be best (refusing to guess in the face of ambiguous multiple arguments). Someone can do, e.g.

    >>> 'foo.tar.gz'.cutsuffix('.gz').cutsuffix('.tar')
    'foo'

to cut off multiple suffixes. More complicated behavior for multiple arguments could be added later, but it would be easy to make a mistake in prematurely generalizing right now.

In bikeshedding method names, I think that avoiding the word "strip" would be nice so users can have a consistent feeling that "'strip' means character sets; 'cut' means substrings".

[stevendaprano]

To be clear, are you only making a copy of the unchanged object if it is a mutable bytearray, not str or bytes?

[sweeneyde]

Yes:

    >>> x = "A"*10**6
    >>> x.cutprefix("B") is x
    True
    >>> x.cutprefix("") is x
    True

    >>> y = b"A"*10**6
    >>> y.cutprefix(b"B") is y
    True
    >>> y.cutprefix(b"") is y
    True

    >>> z = bytearray(b"A")*10**6
    >>> z.cutprefix(b"B") is z
    False
    >>> z.cutprefix(b"") is z
    False

I'm not sure whether this should be part of the spec or an implementation detail. The (str/bytes).replace method docs don't clarify this, but they have the same behavior:

    >>> x = "A"*10**6
    >>> x.replace("B", "C") is x
    True
    >>> x.replace("", "") is x
    True

    >>> y = b"A"*10**6
    >>> y.replace(b"B", b"C") is y
    True
    >>> y.replace(b"", b"") is y
    True

    >>> z = bytearray(b"A")*10**6
    >>> z.replace(b"B", b"C") is z
    False
    >>> z.replace(b"", b"") is z
    False

[rhettinger]

Guido, do you support this API expansion?

[gvanrossum]

I stopped following the discussion at some point, but I think this is worth adding it -- I have seen this done over and over again, and apparently lots of other people have felt the need too.

I think these names are fine, and about the best we can do (keeping in line with the "feel" of the rest of the string API).

I like the behavior of returning a copy of the string if there's no match (as opposed to failing, which was also brought up). If the original object is immutable this should return the original object, but that should be considered a CPython optimization (IIRC all the string methods are pretty careful about that), but not required by the spec.

FWIW the pseudo code has a copy/paste error: In cutsuffix() it should use endswith() rather than startswith().

[vstinner]

The proposed change will affect many builtin types: bytes, bytearray, str, but also other types like collections.UserString. Would it make sense to summarize what has been said in the python-ideas thread into a PEP? It may good to specify things like:

    >>> x = "A"*10**6
    >>> x.cutprefix("B") is x
    True

The specification can be just "that's an implementation detail" or "CPython implementation specific" :-)

I don't expect such PEP to be long nor controversial, but it may help to write it down.

[sweeneyde]

If no one has started, I can draft such a PEP.

[gvanrossum]

Sounds good.

[sweeneyde]

Here is a draft PEP -- I believe it needs a Core Developer sponsor now?