-2 and --python-version 2.7 arguments differ in behaviour

[sqwishy]

This is on mypy 0.620 on Python 3.6.

My understanding is that the command arguments -2 and --python-version 2.7 are
supposed to do the same thing. But in infer_python_version_and_executable()
python_version is different on the given options and I think this affects how it finds a
value for python_executable.

If I'm understanding this correctly, I think it might be an issue with the arguments
having different dest values? -2 sets a constant on the wrong namespace?

> python3 -m pdb (which mypy) -2 .
> /home/sqwishy/.local/bin/mypy(4)<module>()
-> import re
(Pdb) b mypy/main.py:278
Breakpoint 1 at /home/sqwishy/.local/lib/python3.6/site-packages/mypy/main.py:278
(Pdb) c
> /home/sqwishy/.local/lib/python3.6/site-packages/mypy/main.py(278)infer_python_version_and_executable()
-> if special_opts.python_executable is not None and special_opts.python_version is not None:
(Pdb) pp vars(special_opts)
...
 'python_executable': None,
 'python_version': None,
 'silent_imports': False,
...

> python3 -m pdb (which mypy) --python-version 2.7 .
> /home/sqwishy/.local/bin/mypy(4)<module>()
-> import re
(Pdb) b mypy/main.py:278
Breakpoint 1 at /home/sqwishy/.local/lib/python3.6/site-packages/mypy/main.py:278
(Pdb) c
> /home/sqwishy/.local/lib/python3.6/site-packages/mypy/main.py(278)infer_python_version_and_executable()
-> if special_opts.python_executable is not None and special_opts.python_version is not None:
(Pdb) pp vars(special_opts)
...
 'python_executable': None,
 'python_version': (2, 7),
 'silent_imports': False,
...

I would expect special_opts.python_version to be the same in both cases.

[gvanrossum]

Why are you digging in mypy's internals? Is there any externally observable behavior that differs?

[sqwishy]

I think the site-packages ends up being different. I could be wrong. I'm just not entirely sure how to show this. Sorry for being unclear.

[gvanrossum]

OK, I'll wait until you have something more concrete.

[sqwishy]

My problem was that I was getting Cannot find module named ... and it turned out I forgot a py.typed file because I can't read good. I was looking through mypy because I thought I was doing something wrong but wasn't sure what. Now that I know that...

I have two 2.7 packages, A and B. A just has one function with comment annotations and is installed into a 2.7 virtualenv with pip -e so there is an egg link. B imports A and calls that function with the wrong signature. When running mypy -2 (with Python 3.6) with my virtualenv sourced, I get Cannot find module named 'A'. but with mypy --python-version 2.7 I get what I would expect, a warning about using the wrong signature.

I hope that example makes more sense than what I wrote previously. I've also attached a gzipped
tar (mypy5576.tar.gz) with those testing packages (it's super small) in case that helps.

[gvanrossum]

@ethanhs Can you repro this?

[sqwishy]

It also seems if I write a mypy.ini that sets python_version: 2.7 it behaves like mypy -2.

I think because infer_python_version_and_executable() is reading python_version from special_opts but parse_config_file() set it on options? The result is that options.python_executable is set to a path to python 3 interpreter, not a python2.7 interpreter.

[gvanrossum]

OK, I can definitely repro this. I essentially did this:

• untar the tarball
• create a Python 2 virtalenv and activate it
• pip install -e a
• cd b

Then I ran mypy two ways:

$ my --python-version 2.7 pkgb
pkgb/__main__.py:6: error: Argument 1 to "this_is_a_typed_function" has incompatible type "int"; expected "unicode"

That's expected. But with -2:

$ my -2 pkgb
pkgb/__main__.py:1: error: Cannot find module named 'pkga'
pkgb/__main__.py:1: note: (Perhaps setting MYPYPATH or using the "--ignore-missing-imports" flag would help)

@sqwishy If you have an idea for a fix it may go quicker.

[gvanrossum]

(In any case we decided it's not a blocker for the 0.630 release.)

[sqwishy]

Since the original pull request had some negative outcomes, I've reopened a new one to handle just the behavioural inconsistency between how mypy runs given the -2 and --python-version 2.7 command line arguments -- which is what this issue title speaks to.

I have opened a issue #5620 concerning how this problem relates to the configuration file as well, which I mentioned earlier here.

Hopefully this reduces and clarifies the scope of this stuff and makes things a little easier for you guys. Thanks for your patience.