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

[poseidon]

On 15/02/17 14:34, Wolfgang Maier wrote:
> 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.

Thank you, Wolfgang.

Did the following: Put a symlink tau4 in /home/poseidon/tau4/swr/py3/src 
pointing to this directory. Removed the symlink in the site-packages. 
Restore the pth file tau4.pth containing the line 
/home/poseidon/tau4/swr/py3/src.

Worx! Thank you for sheding light on this!

Paul


>
> Best,
> Wolfgang
>