[New-bugs-announce] [issue36334] pathlib.Path unexpected (wrong) behaviour when combining paths in a for loop

[Mike Davies]

New submission from Mike Davies <michaeljamesdavies at gmail.com>:

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')]

----------
components: Windows
messages: 338156
nosy: Mike Davies, paul.moore, steve.dower, tim.golden, zach.ware
priority: normal
severity: normal
status: open
title: pathlib.Path  unexpected (wrong) behaviour when combining paths in a for loop
type: behavior
versions: Python 3.7

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue36334>
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