Description

Builder106

opened on Nov 10, 2026

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shutil.which() accepts a bytes command and returns a bytes path. It also accepts path-like command objects, but a path-like object whose __fspath__() method returns bytes raises TypeError instead of behaving like the equivalent direct bytes value.

import shutil


class BytesPath:
    def __fspath__(self):
        return b"python3"


print(shutil.which(b"python3"))
print(shutil.which(BytesPath()))

On CPython main, the first call returns a bytes path and the second fails:

b'/usr/bin/python3'
TypeError: Can't mix strings and bytes in path components

which() checks whether cmd is bytes before resolving the path-like object. It then decodes PATH to str, while os.path.split() has resolved the command to bytes. Calling os.fspath(cmd) before choosing the string or bytes path keeps the command and search path on the same type.

CPython versions tested on

• CPython main branch (3.16.0a2+)

Operating systems tested on

• Linux ARM64

Linked PRs

• gh-161788: Handle path-like commands in shutil.which() #161789

Activity

1. aisk

   added

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   on Nov 11, 2026
2. picnixz commented on Nov 11, 2026

   

   picnixz

   on Nov 11, 2026

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   I don't think it's a bug. It accepts path-like objects by change I think, not by contract. Supporting bytes was a new feature because it also returned bytes. But looking at older issues, it's not clear whether shutil.which() should support path-like objects (see #90883, #67430 as well)
3. picnixz

   added

   type-featureA feature request or enhancementA feature request or enhancement

   pendingThe issue will be closed if no feedback is providedThe issue will be closed if no feedback is provided

   and removed

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Nov 11, 2026
4. picnixz

   changed the title [-]shutil.which() fails for bytes-backed path-like commands[/-] [+]Support path-like objects in shutil.which() [/+] on Nov 11, 2026
5. Builder106 commented on Nov 11, 2026

   

   Builder106

   on Nov 11, 2026

   ContributorAuthor

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   @picnixz

   Oh right, hadn't considered that path-like support might be accidental. I was thinking the return type could follow whatever os.fspath() returns, but I do see your point.

   Do you think it's worth supporting that, or should we leave path-like commands unsupported?
6. picnixz commented on Nov 11, 2026

   

   picnixz

   on Nov 11, 2026

   Member

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   Considering how hard it was for adding the bytes support, I'd rather not add pathlike support. I'd prefer that every functions that seems to work with pathlike in shutil to be considered accidental and decide later whether to add pathlike support entirely or not. Semantically, it's a bit hard to determine whether cmd should be considered a path or not in the first place (e.g., shutil.which('ls') returns a path but this doesn't mean that the command itself should be a path).

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