Support Environment Variables in config file

Environment variables can be assigned as values in the configuration
file using the !ENV tag. Resolves #1954.

The behavior is defined in the third-party package pyyaml_env_tag:
https://github.com/waylan/pyyaml-env-tag

docs/about/release-notes.md

@@ -25,6 +25,20 @@ The current and past members of the MkDocs team.
 
 ### Major Additions to Version 1.2
 
+#### Support added for Environment Variables in the configuration file (#1954)
+
+Environments variables may now be specified in the configuration file with the
+`!ENV` tag. The value of the variable will be parsed by the YAML parser and
+converted to the appropriate type.
+
+```yaml
+somekey: !ENV VAR_NAME
+otherkey: !ENV [VAR_NAME, FALLBACK_VAR, 'default value']
+```
+
+See [Environment Variables](../user-guide/configuration.md#environment-variables)
+in the Configuration documentation for details.
+
 #### A `--wait` flag has been added to the `serve` command (#2061)
 
 To delay a rebuild of the site when using the livereload server, use the
@@ -34,7 +48,7 @@ To delay a rebuild of the site when using the livereload server, use the
 mkdocs serve --wait 60
 ```
 
-#### Update `gh-deloy` command (#2170)
+#### Update `gh-deploy` command (#2170)
 
 The vendored (and modified) copy of ghp_import has been replaced with a
 dependency on the upstream library. As of version 1.0.0, [ghp_import] includes a

docs/user-guide/configuration.md

@@ -12,6 +12,43 @@ project directory named `mkdocs.yml`.
 As a minimum, this configuration file must contain the `site_name` setting. All
 other settings are optional.
 
+### Environment Variables
+
+In most cases, the value of a configuration option is set directly in the
+configuration file. However, as an option, the value of a configuration option
+may be set to the value of an environment variable using the `!ENV` tag. For
+example, to set the value of the `site_name` option to the value of the
+variable `SITE_NAME` the YAML file may contain the following:
+
+```yaml
+site_name: !ENV SITE_NAME
+```
+
+If the environment variable is not defined, then the configuration setting
+would be assigned a `null` (or `None` in Python) value. A default value can be
+defined as the last value in a list. Like this:
+
+```yaml
+site_name: !ENV [SITE_NAME, 'My default site name']
+```
+
+Multiple fallback variables can be used as well. Note that the last value is
+not an environment variable, but must be a value to use as a default if none
+of the specified environment variables are defined.
+
+```yaml
+site_name: !ENV [SITE_NAME, OTHER_NAME, 'My default site name']
+```
+
+Simple types defined within an environment variable such as string, bool,
+integer, float, datestamp and null are parsed as if they were defined directly
+in the YAML file, which means that the value will be converted to the
+appropriate type. However, complex types such as lists and key/value pairs
+cannot be defined within a single environment variable.
+
+For more details, see the [pyyaml_env_tag](https://github.com/waylan/pyyaml-env-tag)
+project.
+
 ## Project information
 
 ### site_name

mkdocs/tests/utils/utils_tests.py

@@ -305,6 +305,28 @@ class UtilsTests(unittest.TestCase):
         self.assertTrue(isinstance(config['key'], str))
         self.assertTrue(isinstance(config['key2'][0], str))
 
+    @mock.patch.dict(os.environ, {'VARNAME': 'Hello, World!', 'BOOLVAR': 'false'})
+    def test_env_var_in_yaml(self):
+
+        yaml_src = dedent(
+            '''
+            key1: !ENV VARNAME
+            key2: !ENV UNDEFINED
+            key3: !ENV [UNDEFINED, default]
+            key4: !ENV [UNDEFINED, VARNAME, default]
+            key5: !ENV BOOLVAR
+            '''
+        )
+        config = utils.yaml_load(yaml_src)
+        self.assertIsInstance(config['key1'], str)
+        self.assertEqual(config['key1'], 'Hello, World!')
+        self.assertIsNone(config['key2'])
+        self.assertIsInstance(config['key3'], str)
+        self.assertEqual(config['key3'], 'default')
+        self.assertIsInstance(config['key4'], str)
+        self.assertEqual(config['key4'], 'Hello, World!')
+        self.assertIs(config['key5'], False)
+
     def test_copy_files(self):
         src_paths = [
             'foo.txt',

mkdocs/utils/__init__.py

@@ -17,6 +17,7 @@ import posixpath
 import functools
 from datetime import datetime, timezone
 from urllib.parse import urlparse
+from yaml_env_tag import construct_env_tag
 
 from mkdocs import exceptions
 
@@ -56,6 +57,10 @@ def yaml_load(source, loader=yaml.Loader):
     # will be unicode on translation.
     Loader.add_constructor('tag:yaml.org,2002:str', construct_yaml_str)
 
+    # Attach Environment Variable constructor.
+    # See https://github.com/waylan/pyyaml-env-tag
+    Loader.add_constructor('!ENV', construct_env_tag)
+
     try:
         return yaml.load(source, Loader)
     finally:

requirements/project-min.txt

@@ -5,4 +5,5 @@ Markdown==3.2.1
 PyYAML==5.1
 tornado==4.1
 mdx_gh_links==0.2
-ghp-import==1.0
+ghp-import==1.0
+pyyaml_env_tag==0.1

requirements/project.txt

@@ -5,4 +5,5 @@ Markdown>=3.2.1
 PyYAML>=5.2
 tornado>=5.1.1
 mdx_gh_links>=0.2
-ghp-import>=1.0
+ghp-import>=1.0
+pyyaml_env_tag>=0.1

setup.py

@@ -61,7 +61,8 @@ setup(
         'Markdown>=3.2.1',
         'PyYAML>=3.10',
         'tornado>=5.0',
-        'ghp-import>=1.0'
+        'ghp-import>=1.0',
+        'pyyaml_env_tag>=0.1'
     ],
     python_requires='>=3.6',
     entry_points={