python os.path.join on windows ignores first path element?

Question

Consider the following:

>>> from django.conf import settings
>>> import os
>>> settings.VIRTUAL_ENV
'C:/Users/Marcin/Documents/oneclickcos'
>>> settings.EXTRA_BASE
'/oneclickcos/'
>>> os.path.join(settings.VIRTUAL_ENV,settings.EXTRA_BASE)
'/oneclickcos/'

As you can imagine, I neither expect nor want the concatenation of 'C:/Users/Marcin/Documents/oneclickcos' and '/oneclickcos/' to be '/oneclickcos/'.

Oddly enough, reversing the path components once again shows python ignoring the first path component:

>>> os.path.join(settings.EXTRA_BASE,settings.VIRTUAL_ENV)
'C:/Users/Marcin/Documents/oneclickcos'

While this works something like expected:

>>> os.path.join('/foobar',settings.VIRTUAL_ENV,'barfoo')
'C:/Users/Marcin/Documents/oneclickcos\\barfoo'

I am of course, running on Windows (Windows 7), with the native python.

Why is this happening, and what can I do about it?

Answer 1

That's pretty much how os.path.join is defined (quoting the docs):

If any component is an absolute path, all previous components (on Windows, including the previous drive letter, if there was one) are thrown away

And I'd say it's usually a good thing, as it avoids creating invalid paths. If you want to avoid this behavior, don't feed it absolute paths. Yes, starting with a slash qualifies as absolute path. A quick and dirty solution is just removing the leading slash (settings.EXTRA_BASE.lstrip('/') if you want to do it programmatically).

Answer 2

Remove the leading / from the second string:

>>> os.path.join('C:/Users/Marcin/Documents/oneclickcos', 'oneclickos/')
'C:/Users/Marcin/Documents/oneclickcos\\oneclickos/'

This is because os.path.join discards all previous components once it meets an absolute path, and /oneclickos/ is an absolute path.

Here's an excerpt from the doc of os.path.join:

Join one or more path components intelligently. If any component is an absolute path, all previous components (on Windows, including the previous drive letter, if there was one) are thrown away, and joining continues. [...]