Support default version for env version source

backend/src/hatchling/version/source/env.py

@@ -19,8 +19,14 @@ def get_version_data(self) -> dict:
             raise TypeError(message)
 
         if variable not in os.environ:
-            message = f'environment variable `{variable}` is not set'
-            raise RuntimeError(message)
+            default = self.config.get('default')
+            if default is None:
+                message = f'environment variable `{variable}` is not set and the `default` option was not set'
+                raise RuntimeError(message)
+            if not isinstance(default, str):
+                message = 'option `default` must be a string'
+                raise TypeError(message)
+            return {'version': default}
 
         return {'version': os.environ[variable]}
 

docs/plugins/version-source/env.md

@@ -22,3 +22,4 @@ source = "env"
 | Option | Description |
 | --- | --- |
 | `variable` (required) | The name of the environment variable |
+| `default` | The value to use for the version if the specified environment variable is not set. |

tests/backend/version/source/test_env.py

@@ -27,8 +27,22 @@ def test_variable_not_available(isolation):
         source.get_version_data()
 
 
+def test_variable_not_available_with_default(isolation):
+    source = EnvSource(str(isolation), {'variable': 'ENV_VERSION', 'default': '0.0.0'})
+
+    with EnvVars(exclude=['ENV_VERSION']):
+        assert source.get_version_data()['version'] == '0.0.0'
+
+
 def test_variable_contains_version(isolation):
     source = EnvSource(str(isolation), {'variable': 'ENV_VERSION'})
 
     with EnvVars({'ENV_VERSION': '0.0.1'}):
         assert source.get_version_data()['version'] == '0.0.1'
+
+
+def test_variable_contains_version_with_default(isolation):
+    source = EnvSource(str(isolation), {'variable': 'ENV_VERSION', 'default': '0.0.0'})
+
+    with EnvVars({'ENV_VERSION': '0.0.1'}):
+        assert source.get_version_data()['version'] == '0.0.1'