CODEWITHSUNDEEP
python
VPfB
VPfB

Reputation: 17332

How to backport methods like str.removeprefix?

Python 3.9 introduced str.removeprefix(). I'm using it here as an example only.

Let's say, I would like to use this new feature in a library supposed to run on all supported Python versions (also 3.6, 3.7 and 3.8 as of now). I mean a direct usage: mystring.removeprefix('xy_').

I tried this:

if sys.version_info < (3,9):
    def removeprefix_compat(self, prefix):
        # implement removeprefix (or just copy it from PEP-616)
        return 'TODO'
    str.removeprefix = removeprefix_compat

but the result is: TypeError: can't set attributes of built-in/extension type 'str'.

So it seems not possible until winter 2024/2025 (3.8 will be in EOL state). Is it really so?

UPDATE1 (based on links from @buran's comment):

This breaks things:

import json, builtins

class PatchedStr(str):
    pass

builtins.str = PatchedStr

json.loads('"aaa"')
# TypeError: the JSON object must be str, bytes or bytearray, not str

Upvotes: 4

Views: 566

Answers (1)

deceze
deceze

Reputation: 522382

The facts:

  • monkey patching builtins is somewhere between hard and impossible, and that's generally a good thing
  • you need to provide a fallback implementation anyway, so what's the point of conditionally using the builtin method, risking a difference in behaviour between different Python versions

Conclusion:

If you need to support older Python versions, develop as if only the lowest version you want to support is available. I.e. if you set Python 3.6 as your minimum required version, then only use features available in Python 3.6 and treat anything in newer versions as non-existent, because in fact it does not exist in at least one version you want to support.

Upvotes: 3

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