Check for Incompatible os.getenv default value

[renatgalimov]

Steps to reproduce

1. echo import os >> test.py
2. echo 'value = os.getenv("VALUE", [])' >> test.py
3. pylint test.py -rn

Current behavior

$ pylint test.py -rn

-------------------------------------------------------------------
Your code has been rated at 10.00/10 (previous run: 8.00/10, +2.00)

Expected behavior

$ pylint test.py -rn
************* Module testcase
W:  2, 0: Incompatible getenv default value (incompatible-getenv-default)

-------------------------------------------------------------------
Your code has been rated at 7.50/10 (previous run: 10.00/10, -2.50)

pylint --version output

pylint 1.7.2, 
astroid 1.5.3
Python 3.5.3 (default, Sep 11 2017, 12:24:31) 
[GCC 5.4.0]

Just a moment ago I caught a bug caused by the wrong usage of os.getenv default. I would be nice to have a check for this.

[rogalski]

May be generalized to invalid-envvar-value, if implemented, also should be emitted for os.putenv, os.environ.get and os.environ[k] = v assignments.

[PCManticore]

In an ideal world, we would have support for type annotations and type stubs, which would had solved this problem without requiring an extra check, just a new stub for these functions. Until then, we can probably go with this check, as suggested by @rogalski

[renatgalimov]

Thanks. I can do it if nobody already started.

[PCManticore]

Sure thing! Let us know if there is anything we can help with.

[scirelli]

Any idea why I'm now seeing

warning| [invalid-envvar-default] os.getenv default type is builtins.int. Expected str or None.

for

    logger = logging.getLogger('Some Name')
    logger.setLevel(getenv('LOGLEVEL', default=logging.NOTSET))

NOTSET is 0
https://docs.python.org/3/library/logging.html#logging-levels

[scirelli]

Nm. I read the docs. I see what the warning means. getenv will case the default value to a string. The warning is a little confusing.