Behaviour of os.path.join

[BlindAnagram]

On 26/05/2020 18:51, Mats Wichmann wrote:
> On 5/26/20 10:57 AM, BlindAnagram wrote:
>> On 26/05/2020 16:59, Mats Wichmann wrote:
>>> On 5/26/20 8:56 AM, BlindAnagram wrote:
>>>> I came across an issue that I am wondering whether I should report as an
>>>> issue.  If I have a directory, say:
>>>>
>>>>   base='C:\\Documents'
>>>>
>>>> and I use os.path.join() as follows:
>>>>
>>>>   join(base, '..\\..\\', 'build', '')
>>>>
>>>> I obtain as expected from the documentation:
>>>>
>>>> 'C:\\Documents\\..\\..\\build\\'
>>>>
>>>> But if I try to make the directory myself (as I tried first):
>>>>
>>>>   join(base, '..\\..\\', 'build', '\\')
>>>>
>>>> I obtain:
>>>>
>>>> 'C:\\'
>>>>
>>>> The documentation says that an absolute path in the parameter list for
>>>> join will discard all previous parameters but '\\' is not an absoute path!
>>>
>>> But it is - an absolute path is one that starts with the pathname separator.
>>
>> On Windows, when part of a string representing a file or directory path,
>> '\\' acts as a directory separator.  It may imply, as you say, that it
>> is a path on the current drive but it also behaves as a move to a
>> sub-directory of what has appeared to its left.  And this is how it is
>> treated in other os.path  functions. For example:
>>
>> base = "C:\\Documents\\Finance\\"
>> abspath(base)
>> 'C:\\Documents\\Finance'
>>
>> where it can be seen that '\\' is being treated as a directory
>> separator.  In my view it is not sensible to have some functions
>> treating '\\' in one way and others differently.  I would hence expect
>> join() to accept '\\'  as an addition to a file path on Windows and not
>> a signal of an absolute path.
> 
> Unlike the string join method, which would behave as you seem to want,
> the os.path.join method "knows" it is working on paths, so it's going to
> assign some meaning to the content of the pieces.   If it was up to me I
> might not have chosen the approach Python did of "if we see a piece that
> looks like an absolute path, discard what came before", but that's the
> one that was chosen, presumably for good reasons, and it is so
> documented. And the pathlib library module chose to retain that
> behavior: "When several absolute paths are given, the last is taken as
> an anchor (mimicking os.path.join()’s behaviour)". So it's just
> something to get used to?

I agree with much of what you say.

But I believe that it is more natural and less 'dangerous' when a
functions behaviour matches that which its name implies, that is, to
join things together, not to throw one of them away!

And, I don't presume that there were good reasons since it might simply
have been an oversight, which is why I raised it (even Python developers
are not perfect :-)