isso: config: Add support for environment variables in config

- Updated `IssoParser` to support environment variable substitution.
- Enhanced documentation to include details on using environment variables in the configuration file.
- Added unit tests to verify the functionality of environment variable substitution in `IssoParser`.

Closes #397

docs/docs/reference/server-config.rst

@@ -1,6 +1,7 @@
 Server Configuration
 ====================
 
+
 The Isso configuration file is an `INI-style`__ textfile. It reads integers,
 booleans, strings and lists. Here's the default isso configuration:
 `isso.cfg <https://github.com/isso-comments/isso/blob/master/isso/isso.cfg>`_. A
@@ -572,3 +573,24 @@ Booleans
    ``on``/``off`` etc.
 
 .. _getboolean: https://docs.python.org/3/library/configparser.html#configparser.ConfigParser.getboolean
+
+Environment Variables
+^^^^^^^^^^^^^^^^^^^^
+.. versionadded:: 0.13.1
+
+You can use environment variables in the configuration file. This is useful for keeping sensitive information, such as passwords, out of the configuration file itself. Environment variables are referenced using the ``$VAR_NAME`` or ``${VAR_NAME}`` syntax.
+
+Example:
+
+.. code-block:: ini
+   :caption: ``isso.cfg``
+
+    [general]
+    dbpath = /var/lib/isso/comments.db
+    host = https://example.tld/
+
+    [smtp]
+    username = $SMTP_USERNAME
+    password = ${SMTP_PASSWORD}
+
+In this example, the values for ``username`` and ``password`` will be taken from the environment variables ``SMTP_USERNAME`` and ``SMTP_PASSWORD``, respectively.

isso/config.py

@@ -2,6 +2,7 @@
 
 import datetime
 import logging
+import os
 import pkg_resources
 import re
 
@@ -82,6 +83,7 @@ class IssoParser(ConfigParser):
         * Parse human-readable timedelta such as "15m" as "15 minutes"
         * Handle indented lines as "lists"
         * Disable string interpolation ('%s') for values
+        * Support environment variable substitution
     """
 
     def __init__(self, *args, **kwargs):
@@ -95,6 +97,10 @@ def __init__(self, *args, **kwargs):
         return super(IssoParser, self).__init__(
             allow_no_value=True, interpolation=None, *args, **kwargs)
 
+    def get(self, section, key, **kwargs):
+        value = super(IssoParser, self).get(section, key, **kwargs)
+        return os.path.expandvars(value)
+
     def getint(self, section, key):
         try:
             delta = timedelta(self.get(section, key))

isso/tests/test_config.py

@@ -2,6 +2,7 @@
 
 import unittest
 import io
+import os
 
 from isso import config
 
@@ -34,3 +35,51 @@ def test_parser(self):
         # Section.get() should function the same way as plain IssoParser
         foosection = parser.section("foo")
         self.assertEqual(foosection.get("password"), '%s%%foo')
+
+    def test_parser_with_environment_variables(self):
+
+        parser = config.IssoParser()
+        parser.read_file(io.StringIO("""
+            [foo]
+            bar = $TEST_ENV_VAR
+            baz = ${TEST_ENV_VAR2}
+        """))
+
+        # Set environment variables
+        os.environ['TEST_ENV_VAR'] = 'test_value'
+        os.environ['TEST_ENV_VAR2'] = 'another_test_value'
+
+        # Test environment variable expansion
+        self.assertEqual(parser.get("foo", "bar"), 'test_value')
+        self.assertEqual(parser.get("foo", "baz"), 'another_test_value')
+
+        # Test Section.get() with environment variables
+        foosection = parser.section("foo")
+        self.assertEqual(foosection.get("bar"), 'test_value')
+        self.assertEqual(foosection.get("baz"), 'another_test_value')
+
+        # Clean up environment variables
+        del os.environ['TEST_ENV_VAR']
+        del os.environ['TEST_ENV_VAR2']
+
+    def test_parser_with_missing_environment_variables(self):
+
+        parser = config.IssoParser()
+        parser.read_file(io.StringIO("""
+            [foo]
+            bar = $MISSING_ENV_VAR
+        """))
+
+        self.assertEqual(parser.get("foo", "bar"), '$MISSING_ENV_VAR')
+
+    def test_parser_with_special_characters_in_environment_variables(self):
+
+        os.environ['SPECIAL_ENV_VAR'] = 'value_with_$pecial_characters'
+        parser = config.IssoParser()
+        parser.read_file(io.StringIO("""
+            [foo]
+            bar = $SPECIAL_ENV_VAR
+        """))
+
+        self.assertEqual(parser.get("foo", "bar"), 'value_with_$pecial_characters')
+        del os.environ['SPECIAL_ENV_VAR']