pathlib.Path unexpected (wrong) behaviour when combining paths in a for loop

[MikeDavies]

BPO	36334
Nosy	@pfmoore, @tjguk, @zware, @eryksun, @zooba

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2019-03-18.04:08:02.973>
created_at = <Date 2019-03-18.02:12:20.452>
labels = ['invalid', 'type-bug', '3.7', 'OS-windows']
title = 'pathlib.Path  unexpected (wrong) behaviour when combining paths in a for loop'
updated_at = <Date 2019-03-18.04:08:02.971>
user = 'https://bugs.python.org/MikeDavies'

bugs.python.org fields:

activity = <Date 2019-03-18.04:08:02.971>
actor = 'eryksun'
assignee = 'none'
closed = True
closed_date = <Date 2019-03-18.04:08:02.973>
closer = 'eryksun'
components = ['Windows']
creation = <Date 2019-03-18.02:12:20.452>
creator = 'Mike Davies'
dependencies = []
files = []
hgrepos = []
issue_num = 36334
keywords = []
message_count = 2.0
messages = ['338156', '338160']
nosy_count = 6.0
nosy_names = ['paul.moore', 'tim.golden', 'zach.ware', 'eryksun', 'steve.dower', 'Mike Davies']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue36334'
versions = ['Python 3.7']

[MikeDavies]

I wish to create a list of pathlib.Paths by combining two lists of pathlib.Paths. I use two for loops to make every combination.

However the output is not what one would expect (see no 'Program Files' is visible in the output).

########## INPUT:
pa = [   
    Path('C:/Program Files'),
    Path('C:/')
]
pb = [
    Path('/one/two/three.exe'),
    Path('/four.exe') 
    ]
print( [a/b for a in pa for b in pb] )

########### OUTPUT:
[WindowsPath('C:/one/two/three.exe'), WindowsPath('C:/four.exe'), WindowsPath('C:/one/two/three.exe'), WindowsPath('C:/four.exe')]

This is true whether I use for loops or list comprehensions.

To get the expected output I need to change the Paths to strings, combine them, then convert them back to paths like this:

########### INPUT:
print( [Path(str(a)+str(b)) for a in pa for b in pb] )
########### OUTPUT:
[WindowsPath('C:/Program Files/one/two/three.exe'), WindowsPath('C:/Program Files/four.exe'), WindowsPath('C:/one/two/three.exe'), WindowsPath('C:/four.exe')]

Interestingly if I print only 'a' I get the expected answer:
########### INPUT:
print( [a for a in pa for b in pb] )
########### OUTPUT:
[WindowsPath('C:/Program Files'), WindowsPath('C:/Program Files'), WindowsPath('C:/'), WindowsPath('C:/')]

And the same is true if I print only 'b':
########### INPUT:
print( [b for a in pa for b in pb] )
########### OUTPUT:
[WindowsPath('/one/two/three.exe'), WindowsPath('/four.exe'), WindowsPath('/one/two/three.exe'), WindowsPath('/four.exe')]

Additionally in some cases it does give the correct answer. Here is a similar example where the answer is correct:
########### INPUT:
pa = [Path('C:/'), Path('D:/')]
pb = [Path('a.exe'), Path('b.exe')]
print( [a/b for a in pa for b in pb] )
########### OUTPUT:
[WindowsPath('C:/a.exe'), WindowsPath('C:/b.exe'), WindowsPath('D:/a.exe'), WindowsPath('D:/b.exe')]

[eryksun]

The drive is retained when a rooted path is joined to a drive-absolute or UNC-absolute path. This is documented behavior 1:

When several absolute paths are given, the last is taken as an
anchor (mimicking os.path.join()’s behaviour):

        >>> PurePath('/etc', '/usr', 'lib64')
        PurePosixPath('/usr/lib64')
        >>> PureWindowsPath('c:/Windows', 'd:bar')
        PureWindowsPath('d:bar')

    However, in a Windows path, changing the local root doesn’t
    discard the previous drive setting:
    
        >>> PureWindowsPath('c:/Windows', '/Program Files')
        PureWindowsPath('c:/Program Files')

If you want to simply append directories to a path, use a relative path. For example:

    >>> Path('C:/Program Files') / Path('one/two/three.exe')
    WindowsPath('C:/Program Files/one/two/three.exe')