[Python-Dev] proposed os.fspath() change

Koos Zevenhoven k7hoven at gmail.com
Wed Jun 15 13:39:03 EDT 2016

My proposal at the point of the first PEP draft solved both of these issues.

That version of the fspath function passed anything right through that
was an instance of the keyword-only `type_constraint`. If not, it
would ask __fspath__, and before returning the result, it would check
that __fspath__ returned an instance of `type_constraint` and
otherwise raise a TypeError. `type_constraint=object` would then have
given the behavior you want. I always wanted fspath to spare the
caller from all the instance checking (most of which it does even

The main problem with setting type_constraint to something broader
than (str, bytes) is that then that parameter would affect the return
type of the function, which would at least complicate the type hinting
issue. Mypy might now support things like

def fspath(path: T, type_constraint: Type[T] = (str, bytes)) -> T: ...

but then again, isinstance and Union are not compatible (for a
reason?), and PEP484 for a reason does not allow tuples like (str,
bytes) in place of Unions.

Anyway, if we were to go back to this behavior, we would need to
decide whether to officially allow a wider type constraint or whether
to leave that to Stack Overflow, so to speak.

-- Koos

On Wed, Jun 15, 2016 at 7:46 PM, Guido van Rossum <guido at python.org> wrote:
> These are really two separate proposals.
> I'm okay with checking the return value of calling obj.__fspath__; that's an
> error in the object anyways, and it doesn't matter much whether we do this
> or not (though when approving the PEP I considered this and decided not to
> insert a check for this). But it doesn't affect your example, does it? I
> guess it's easier to raise now and change the API in the future to avoid
> raising in this case (if we find that raising is undesirable) than the other
> way around, so I'm +0 on this.
> The other proposal (passing anything that's not understood right through) is
> more interesting and your use case is somewhat compelling. Catching the
> exception coming out of os.fspath() would certainly be much messier. The
> question remaining is whether, when this behavior is not desired (e.g. when
> the caller of os.fspath() just wants a string that it can pass to open()),
> the condition of passing that's neither a string not supports __fspath__
> still produces an understandable error. I'm not sure that that's the case.
> E.g. open() accepts file descriptors in addition to paths, but I'm not sure
> that accepting an integer is a good idea in most cases -- it either gives a
> mystery "Bad file descriptor" error or starts reading/writing some random
> system file, which it then closes once the stream is closed.
> On Wed, Jun 15, 2016 at 9:12 AM, Ethan Furman <ethan at stoneleaf.us> wrote:
>> I would like to make a change to os.fspath().
>> Specifically, os.fspath() currently raises an exception if something
>> besides str, bytes, or os.PathLike is passed in, but makes no checks
>> if an os.PathLike object returns something besides a str or bytes.
>> I would like to change that to the opposite: if a non-os.PathLike is
>> passed in, return it unchanged (so no change for str and bytes); if
>> an os.PathLike object returns something that is not a str nor bytes,
>> raise.
>> An example of the difference in the lzma file:
>> Current code (has not been upgraded to use os.fspath() yet)
>> -----------------------------------------------------------
>>     if isinstance(filename, (str, bytes)):
>>         if "b" not in mode:
>>             mode += "b"
>>         self._fp = builtins.open(filename, mode)
>>         self._closefp = True
>>         self._mode = mode_code
>>     elif hasattr(filename, "read") or hasattr(filename, "write"):
>>         self._fp = filename
>>         self._mode = mode_code
>>     else:
>>         raise TypeError(
>>              "filename must be a str or bytes object, or a file"
>>               )
>> Code change if using upgraded os.fspath() (placed before above stanza):
>>     filename = os.fspath(filename)
>> Code change with current os.fspath() (ditto):
>>     if isinstance(filename, os.PathLike):
>>         filename = os.fspath(filename)
>> My intention with the os.fspath() function was to minimize boiler-plate
>> code and make PathLike objects easy and painless to support; having to
>> discover if any given parameter is PathLike before calling os.fspath()
>> on it is, IMHO, just the opposite.
>> There is also precedent for having a __dunder__ check the return type:
>>     --> class Huh:
>>     ...   def __int__(self):
>>     ...     return 'string'
>>     ...   def __index__(self):
>>     ...     return b'bytestring'
>>     ...   def __bool__(self):
>>     ...     return 'true-ish'
>>     ...
>>     --> h = Huh()
>>     --> int(h)
>>     Traceback (most recent call last):
>>       File "<stdin>", line 1, in <module>
>>     TypeError: __int__ returned non-int (type str)
>>     --> ''[h]
>>     Traceback (most recent call last):
>>       File "<stdin>", line 1, in <module>
>>     TypeError: __index__ returned non-int (type bytes)
>>     --> bool(h)
>>     Traceback (most recent call last):
>>       File "<stdin>", line 1, in <module>
>>     TypeError: __bool__ should return bool, returned str
>> Arguments in favor or against?
>> --
>> ~Ethan~
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> --
> --Guido van Rossum (python.org/~guido)
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