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NOTE this kinda overlaps with the module design doc, should be unified in the future.

Consider a toy package/module structure with minimal code, wihout any actual data parsing, just for demonstration purposes.

  • main package structure

    • my/twitter/gdpr.py Extracts Twitter data from GDPR archive.
    • my/twitter/all.py Merges twitter data from multiple sources (only gdpr in this case), so data consumers are agnostic of specific data sources used. This will be overriden by overlay.
    • my/twitter/common.py Contains helper function to merge data, so they can be reused by overlay's all.py.
    • my/reddit.py Extracts Reddit data – this won't be overridden by the overlay, we just keep it for demonstration purposes.

  • overlay package structure

    • my/twitter/talon.py Extracts Twitter data from Talon android app.
    • my/twitter/all.py Override for all.py from main package – it merges together data from gpdr and talon modules.

Installing (editable install)

NOTE: this was tested with python 3.10 and pip 23.3.2.

To install, we run:

pip3 install --user -e overlay/
pip3 install --user -e main/

As a result, we get:

pip3 list | grep hpi
hpi-main           0.0.0       /project/main/src
hpi-overlay        0.0.0       /project/overlay/src

cat ~/.local/lib/python3.10/site-packages/easy-install.pth
/project/overlay/src
/project/main/src

(the order above is important, so overlay takes precedence over main TODO link)

Verify the setup:

$ python3 -c 'import my; print(my.__path__)'
_NamespacePath(['/project/overlay/src/my', '/project/main/src/my'])

This basically means that modules will be searched in both paths, with overlay taking precedence.

Testing (editable install) 

$ python3 -c 'import my.reddit as R; print(R.upvotes())'
[main] my.reddit hello
['reddit upvote1', 'reddit upvote2']

Just as expected here, my.reddit is imported from the main package, since it doesn't exist in overlay.

Let's theck twitter now:

$ python3 -c 'import my.twitter.all as T; print(T.tweets())'
[overlay] my.twitter.all hello
[main] my.twitter.common hello
[main] my.twitter.gdpr hello
[overlay] my.twitter.talon hello
['gdpr tweet 1', 'gdpr tweet 2', 'talon tweet 1', 'talon tweet 2']

As expected, my.twitter.all was imported from the overlay. As you can see it's merged data from gdpr (from main package) and talon (from overlay package).

So far so good, let's see how it works with mypy.

Mypy support (editable install)

To check that mypy works as expected I injected some statements in modules that have no impact on runtime, but should trigger mypy, like this trigger_mypy_error: str = 123:

Let's run it:

$ mypy --namespace-packages --strict -p my
overlay/src/my/twitter/talon.py:9: error: Incompatible types in assignment (expression has type "int", variable has type "str")
[assignment]
    trigger_mypy_error: str = 123
                              ^
Found 1 error in 1 file (checked 4 source files)

Hmm, this did find the statement in the overlay, but missed everything from main (e.g. reddit.py and gdpr.py should have also triggered the check).

First, let's check which sources mypy is processing:

$ mypy --namespace-packages --strict -p my -v 2>&1 | grep BuildSource
LOG:  Found source:           BuildSource(path='/project/overlay/src/my', module='my', has_text=False, base_dir=None)
LOG:  Found source:           BuildSource(path='/project/overlay/src/my/twitter', module='my.twitter', has_text=False, base_dir=None)
LOG:  Found source:           BuildSource(path='/project/overlay/src/my/twitter/all.py', module='my.twitter.all', has_text=False, base_dir=None)
LOG:  Found source:           BuildSource(path='/project/overlay/src/my/twitter/talon.py', module='my.twitter.talon', has_text=False, base_dir=None)

So seems like mypy is not processing anything from main package at all?

At this point I cloned mypy, put a breakpoint, and found out this is the culprit: https://github.com/python/mypy/blob/1dd8e7fe654991b01bd80ef7f1f675d9e3910c3a/mypy/modulefinder.py#L288

This basically returns the first path where it finds my package, which happens to be the overlay in this case. So everything else is ignored?

It even seems to have a test for a similar usecase, which is quite sad. https://github.com/python/mypy/blob/1dd8e7fe654991b01bd80ef7f1f675d9e3910c3a/mypy/test/testmodulefinder.py#L64-L71

For now, I opened an issue in mypy repository https://github.com/python/mypy/issues/16683

But ok, maybe mypy treats main as an external package somhow but still type checks it properly? Let's see what's going on with imports:

$ mypy --namespace-packages --strict -p my --follow-imports=error
overlay/src/my/twitter/talon.py:9: error: Incompatible types in assignment (expression has type "int", variable has type "str")
[assignment]
    trigger_mypy_error: str = 123
                              ^
overlay/src/my/twitter/all.py:3: error: Import of "my.twitter.common" ignored  [misc]
    from .common import merge
    ^
overlay/src/my/twitter/all.py:6: error: Import of "my.twitter.gdpr" ignored  [misc]
        from . import gdpr
    ^
overlay/src/my/twitter/all.py:6: note: (Using --follow-imports=error, module not passed on command line)
overlay/src/my/twitter/all.py: note: In function "tweets":
overlay/src/my/twitter/all.py:8: error: Returning Any from function declared to return "List[str]"  [no-any-return]
        return merge(gdpr, talon)
        ^
Found 4 errors in 2 files (checked 4 source files)

Nope – looks like it's completely unawareof main, and what's worst, by default (without tweaking --follow-imports), these errors would be suppressed.

What if we don't install at all?

Instead of editable install let's try running mypy directly over source files

First let's only check main package:

$ MYPYPATH=main/src mypy --namespace-packages --strict -p my
main/src/my/twitter/gdpr.py:9: error: Incompatible types in assignment (expression has type "int", variable has type "str")  [assignment]
    trigger_mypy_error: str = 123
                              ^~~
main/src/my/reddit.py:11: error: Incompatible types in assignment (expression has type "int", variable has type "str")  [assignment]
    trigger_mypy_error: str = 123
                              ^~~
Found 2 errors in 2 files (checked 6 source files)

As expected, it found both errors.

Now with overlay as well:

$ MYPYPATH=overlay/src:main/src mypy --namespace-packages --strict -p my
overlay/src/my/twitter/all.py:6: note: In module imported here:
main/src/my/twitter/gdpr.py:9: error: Incompatible types in assignment (expression has type "int", variable has type "str")  [assignment]
    trigger_mypy_error: str = 123
                              ^~~
overlay/src/my/twitter/talon.py:9: error: Incompatible types in assignment (expression has type "int", variable has type "str")
[assignment]
    trigger_mypy_error: str = 123
                              ^~~
Found 2 errors in 2 files (checked 4 source files)

Interesting enough, this is slightly better than the editable install (it detected error in gdpr.py as well). But still no reddit.py error.

TODO possibly worth submitting to mypy issue tracker as well…

Overall it seems that properly type checking modules in overlays (especially the ones actually overriding/extending base modules) is kinda problematic.