PTH files: Abs paths not working as expected. Symlinks needed?

[Wolfgang Maier]

On 15.02.2017 13:42, poseidon wrote:
> On 15/02/17 12:16, Wolfgang Maier wrote:
>> On 15.02.2017 10:33, poseidon wrote:
>>> In /usr/lib/python3.6/site-packages I wrote a file tau4.pth. It contains
>>> the line
>>>
>>> /home/poseidon/tau4/swr/py3/src
>>>
>>> In /home/poseidon/tau4/swr/py3/src there's an __init__.py file, so it
>>> should be possible to write
>>>
>>> import tau4
>>>
>>> in my programs.
>>
>>
>> No, that's not what you should expect!
>> A path file contains paths to be added at interpreter startup to the
>> package/module search path stored in sys.path. That is, in your example,
>> if you put a file tau4.py or a tau4 directory with the __init__.py file
>> inside into /home/poseidon/tau4/swr/py3/src, *then* you could import
>> tau4.
>>
>>> It works, if I set a symlink to /home/poseidon/tau4/swr/py3/src in the
>>> site-packages dir:
>>>
>>> ln -s /home/poseidon/tau4/swr/py3/src
>>> /usr/lib/python3.6/site-packages/tau4
>>>
>>
>> Well this works because now Python finds (following the symlink) a tau4
>> package (i.e., a directory with that name and an __init__.py file
>> inside) in /usr/lib/python3.6/site-packages. The .pth file is not
>> involved in this at all.
>>
>
> Yes, removed it (symlink still there) and it still works. But then, what
> are pth files for? I'd just place a symlink to the package and am done
> with. The path doesn't seem to be needed in sys.path (where it would go
> if placed in a pth file). If I write
>
> from tau4 import datalogging
>
> that works, too. So no need for the path being in sys.path (i.e. in a
> pth file)?
>

I guess a major point of .pth files is that you only have one or a small 
number of files with a clear purpose polluting the containing directory. 
Of course, you could put symlinks to all your packages and modules into 
site-packages, but what's the point of putting them somewhere else in 
the first place? Also, you cannot create symlinks across devices, but 
.pth files will work.

Best,
Wolfgang