mypy.ini processing should allow variable expansion ("interpolation")

[jtmoon79]

tl;dr I cannot write variable-expanded directory paths within mypy.ini to set the mypy_path. I cannot commit a useful mypy.ini. It would be cool if mypy supported variable expansion ("interpolation") within it's configparser processed file.

Problem example

Given my pipenv-created virtualenv at C:/Users/user1/.virtualenvs/project1-rAPKj25Z, I want to set mypy_path to the virtual environment's Lib/site-packages. I can set

[mypy]
mypy_path: C:/Users/user1/.virtualenvs/project1-rAPKj25Z/Lib/site-packages

but that is not portable (i.e. committable). I would like to write something portable like

[mypy]
mypy_path: %(VIRTUAL_ENV)s/Lib/site-packages

where VIRTUAL_ENV is set by pipenv within the caller's environment (or perhaps a python-dotenv file).

...

This is a feature request. I am not sure if this is outside the expectations for mypy.ini files. However, it seems like a clean way to have more portable mypy.ini files.

Using

• Windows 10 Pro
• Python 3.7.1
• mypy 0.670
• pipenv 2018.11.26

[jtmoon79]

I thought this could easily be solved by

--- a/mypy/main.py                                                                                                                                                                                                                                                                      
+++ b/mypy/main.py
@@ -905,7 +905,7 @@ def parse_config_file(options: Options, filename: Optional[str]) -> None:                                                                                                                                                                                        
    else:
        config_files = tuple(map(os.path.expanduser, defaults.CONFIG_FILES))                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
-    parser = configparser.RawConfigParser()                                                                                                                                                                                                                                        
+    parser = configparser.ConfigParser(os.environ)

given mypy.ini

[mypy]
mypy_path: %(VIRTUAL_ENV)s/Lib/site-packages

that solution worked in toy program

import configparser, os
config = configparser.ConfigParser(os.environ)
config.read("./mypy.ini")
print(config['mypy']['mypy_path'])  # prints C:/Users/user1/.virtualenvs/project1-rAPKj25Z/Lib/site-packages

I tried out that solution within mypy/main.py and it resulted in numerous errors mypy.ini: [mypy]: Unrecognized option: ..., e.g.

mypy.ini: [mypy]: Unrecognized option: aclocal_path = C:\Programs\Git\mingw64\share\aclocal;C:\Programs\Git\usr\share\aclocal
mypy.ini: [mypy]: Unrecognized option: allusersprofile = C:\ProgramData
...

Darn.

Looking closer at the code, the change may not be so trivial.

Relates to commit 2fbb724.

[JelleZijlstra]

For context, do other development tools with config files (e.g., linters) usually support interpolating environment variables? That could help decide whether this is something worth supporting in mypy.

[jtmoon79]

do other development tools with config files (e.g., linters) usually support interpolating environment variables?

@JelleZijlstra this Issue might be broken out into two questions:

• should mypy support configparser interpolating the .ini file?
• how to make .ini file parameter mypy_path more portable (i.e. committable)?

Regarding the mypy_path, it strikes me as necessary to allow some method for inputting "portable paths".

[wakemaster39]

This would be a really helpful feature for situations like pipenv where I need to have a wrapper script to make sure mypy knows where to look for packages to avoid import errors and to ensure that the config is portable for the rest of my team.

[ilevkivskyi]

This is now supported (in most cases), see https://mypy.readthedocs.io/en/latest/config_file.html:

Some flags support user home directory and environment variable expansion. To refer to the user home directory, use ~ at the beginning of the path. To expand environment variables use $VARNAME or ${VARNAME}.

[Mattwmaster58]

@ilevkivskyi

Some flags support user home directory and environment variable expansion

Any hint as to what flags those are?