Update option_manager_mixin.py - issue 3839 (#4812)

* Update option_manager_mixin.py

I  added user 'JoshMayberry' 's solution for issue 3839.
#3839

I  added a test to see if pylint handles environment variables in config file locations.
Added it to test_config because it also deals with config files.
To test I added an environment variable containing tmp_path and then unpacked it with os.path.expandvars.

Tmp_path gets converted to an environment variable  which is fed to the read_config_file function to be tested.
Read_config_file succesfully reads the environment variable, if the variable is changed, the function raises an OSError.

CONTRIBUTORS.txt

@@ -531,3 +531,5 @@ contributors:
 * Michal Vasilek: contributor
 
 * Kai Mueller (kasium): contributor
+
+* Sam Vermeiren (PaaEl): contributor

ChangeLog

@@ -101,6 +101,10 @@ Release date: TBA
 
   Closes #4711
 
+* Config files can now contain environment variables
+
+  Closes #3839
+
 
 What's New in Pylint 2.9.6?
 ===========================

pylint/config/option_manager_mixin.py

@@ -258,7 +258,7 @@ def read_config_file(self, config_file=None, verbose=None):
         if config_file is None:
             config_file = self.config_file
         if config_file is not None:
-            config_file = os.path.expanduser(config_file)
+            config_file = os.path.expandvars(os.path.expanduser(config_file))
             if not os.path.exists(config_file):
                 raise OSError(f"The config file {config_file} doesn't exist!")
 

tests/test_config.py

@@ -1,7 +1,11 @@
 # pylint: disable=missing-module-docstring, missing-function-docstring, protected-access
+import os
 import unittest.mock
 
+import pytest
+
 import pylint.lint
+from pylint.config import OptionsManagerMixIn
 
 
 def check_configuration_file_reader(config_file):
@@ -91,3 +95,31 @@ def test_can_read_toml_rich_types(tmp_path):
 """
     )
     check_configuration_file_reader(config_file)
+
+
+def test_can_read_env_variable(tmp_path):
+    # Check that we can read the "regular" INI .pylintrc file
+    # if it has an environment variable.
+    config_file = tmp_path / "pyproject.toml"
+    config_file.write_text(
+        """
+[tool.pylint."messages control"]
+disable = "logging-not-lazy,logging-format-interpolation"
+jobs = "10"
+reports = "yes"
+"""
+    )
+    os.environ["tmp_path_env"] = str(tmp_path / "pyproject.toml")
+    options_manager_mix_in = OptionsManagerMixIn("", "${tmp_path_env}")
+    options_manager_mix_in.read_config_file("${tmp_path_env}")
+
+    def test_read_config_file():
+        with pytest.raises(OSError):
+            options_manager_mix_in.read_config_file("${tmp_path_en}")
+
+    test_read_config_file()
+    options_manager_mix_in.load_config_file()
+    section = options_manager_mix_in.cfgfile_parser.sections()[0]
+    jobs, jobs_nr = options_manager_mix_in.cfgfile_parser.items(section)[1]
+    assert jobs == "jobs"
+    assert jobs_nr == "10"