Choosable dependency

[Thomas Jollans]

On 06/03/2021 12:00, Manfred Lotz wrote:
> Let us say I have a package which reads a TOML file.
>
> I want to give the user of my package the choice to decide if he wants
> to use the toml, tomlkit or rtoml package.
>
> So, in case the user chose to use rtoml then there should be an import
> only for rtoml, aso.
>
> How could I achieve this?
>
I expect in this case the best thing to do is to simply try to import 
your favourite TOML module (rtoml or what have you), and if that doesn't 
work import a fallback – accepting that there might be two TOML parser 
modules loaded in the same process in some cases.

But this dilemma is fairly common in the Qt GUI world, as there are two 
competing Python bindings for the Qt framework (and multiple 
nearly-compatible versions of both) and in that case all packages need 
to use the same one. So you could have a look at what modules depending 
on Qt do.

For example:

https://pyqtgraph.readthedocs.io/en/latest/how_to_use.html#pyqt-and-pyside

PyQtGraph checks if any supported Qt bindings are already imported and 
uses those. If not, it imports its own favourite.

The rules are fairly complex in their case (they're supporting 6 
almost-but-not-quite-compatible backends), so they have an internal 
wrapper module

https://github.com/pyqtgraph/pyqtgraph/blob/master/pyqtgraph/Qt.py

and do something like

from . import Qt

in every file.

Matplotlib does it the other way around:

https://matplotlib.org/stable/api/matplotlib_configuration_api.html#matplotlib.use

there the user imports matplotlib and then optionally calls a function 
to select a backend. Then every function within matplotlib that uses the 
backend has to defer to some wrapper (not sure where or how)


Hope that helps

Thomas


-- 
Dr. Thomas Jollans

e ✉ tjol at tjol.eu