Import behaviour if no Qt bindings are installed

Hi, firstly thanks very much for QtPy - the flexibility it brings has been very useful for a recent Qt gui I’ve been working on. While testing that project, I noticed something which I think is unintended behaviour with qtpy.

This concerns only the case where no Qt bindings are installed. In a clean venv, with only qtpy, pip, wheel and setuptools installed, with the current version of qtpy,

>>> import qtpy
>>> qtpy.API_NAME
'PyQt5'

importing the package succeeds and the API_NAME is set to “PyQt5”, even though there are no Qt bindings installed. Also e.g.:

>>> import qtpy.QtCore
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "E:\test_qtpy\qtpy\qtpy\QtCore.py", line 15, in <module>
    from PyQt5.QtCore import *
ModuleNotFoundError: No module named 'PyQt5'

results in a ModuleNotFoundError because the program is looking for PyQt5, rather than raising a QtBindingsNotFoundError.

• I think that this can be fixed by replacing line 259 in __init__.py with “raise QtBindingsNotFoundError”

• There also appears to be a bug in the definition of QtBindingsNotFoundError: in line 84 in __init__.py, a “self.” needs to be inserted before _msg.

With these two changes:

>>> import qtpy
Traceback (most recent call last):
  File "E:\test_qtpy\lib\site-packages\qtpy\__init__.py", line 252, in <module>
    from PySide6 import __version__ as PYSIDE_VERSION  # analysis:ignore
ModuleNotFoundError: No module named 'PySide6'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "E:\test_qtpy\lib\site-packages\qtpy\__init__.py", line 259, in <module>
    raise QtBindingsNotFoundError
qtpy.QtBindingsNotFoundError: No Qt bindings could be found

• As __init__.py is executed before importing any other modules, an attempted import of any other module will result in the QtBindingsNotFoundError
• I think this means it may be unnecessary for other modules to import the QtBindingsNotFoundError, as this case would be already handled by __init__.py.

Hope this might be useful. Thanks again.

[CAM-Gerlach]

Sounds like a good plan to me 👍

Do you see any advantage on keeping the current behavior?

Nope, not really, other than theoretical backward compatibility with a likely narrow corner case (that is really an unintended bug). I do think at least for 3.0 we should refactor the __init__ along the lines of a simplified #156 to avoid these sorts of issues in the future.

[dalthviz]

So I think there are already a couple of things for v2.2.1, maybe we should fix the missing self of the _msg call on the QtBindingsNotFoundError definition and release v2.2.1 and then restore raising the exception (although now it will be QtBindingsNotFoundError) for v2.3.0?

What do you think @CAM-Gerlach ? Do you see any advantage on keeping the current behavior?