mypy.ini parsing trailing comma causes duplicate module error

[amin-nejad]

Documentation

I've opted for documentation but this could just as well have been a bug report or a feature request. The issue is with a trailing comma after the last file in a list of files in mypy.ini. e.g.:

[mypy]
files =
    myproject/package1,
    myproject/package2,
    myproject/module1.py,
    myproject/module2.py,

This results in a duplicate module error as follows:

myproject/package1/__init__.py: error: Duplicate module named "myproject.package1" (also at "myproject/package1/__init__.py")
myproject/package1/__init__.py: note: Are you missing an __init__.py? Alternatively, consider using --exclude to avoid checking one of them.
Found 1 error in 1 file (errors prevented further checking)

It appears to somehow read the first element in the list again. Both packages already have __init__.py files. The problem is that this error isn't very informative and it took me a while to realise this was the cause.

Possible solutions:

• Document this here
• Detect trailing comma and flag this to user instead of parsing the config file and returning a cryptic error
• Ignore trailing commas for the final element of a list during parsing of the config file

mypy==0.910

[sumit-158]

can I work on this?

[amin-nejad]

can I work on this?

No objections from me 🙂

[webknjaz]

Just hit this as well. I think the right solution would be to allow trailing commas. It's a bit more obvious with TOML and shouldn't really differ in ini, UX-wise.

I haven't checked, but my hunch is that configparser might be reading this entire value as a giant string, like file1,file2,file3,, splitting it by comma. And then, the last element is an empty string. This translates to just calling MyPy in terminal and passing an empty string on CLI.

I have confirmed that passing that empty string as another file breaks in exactly the same way:

$ MYPY_PATH=src/ mypy --explicit-package-bases myproject/package1 myproject/package2 myproject/module1.py myproject/module2.py ''

Additionally, I've found that you can call it with a double empty string:

$ MYPY_PATH=src/ mypy --explicit-package-bases '' ''

and it would explode the same...

Interestingly, a single empty string argument seems fine:

$ MYPY_PATH=src/ mypy --explicit-package-bases ''

So it seems to me that there are 2 bugs here:

1. MyPy's argument processing breaks when there's more than one file path argument passed and at least one of those paths is an empty string — this deserves a clear error output, regardless of whether it's coming from a config or a CLI arg
2. The configparser-based reader should normalize the INI format to allow for trailing commas and disallow empty strings so that it behaves the same as TOML from the user perspective.

@hauntsaninja could you please change the label from documentation to bug? Also, this is probably a good-first-issue.

[webknjaz]

It just occurred to me that the config parsing bug may affect other settings that are being interpreted as lists. Perhaps, that's worth checking by whoever gets to work on this.

[x612skm]

Hi!, Can I take this up?

[webknjaz]

@hauntsaninja thanks for the labels! Could you assign the issue to me and @x612skm? I'll help him navigate the process.

[brianschubert]

@x612skm @webknjaz Feel free to work on this! (we generally don't assign issues)

You can check out CONTRIBUTING.md and the Developer Guides for contributing guidelines and tips on where to start. Thanks!

[x612skm]

Just for the fyi - I've started brainstorming into this and soon will be hitting a PR after a discussion with @webknjaz . Thanks!