feat: dynamic configuration (#30)

We allow for placeholders in prefixes and variable name to allow
for the definition of dynamic configuration.

envier/env.py

@@ -69,11 +69,11 @@ def _retrieve(self, env: "Env", prefix: str) -> T:
         source = env.source
 
         full_name = prefix + _normalized(self.name)
-        raw = source.get(full_name)
+        raw = source.get(full_name.format(**env.dynamic))
         if raw is None and self.deprecations:
             for name, deprecated_when, removed_when in self.deprecations:
                 full_deprecated_name = prefix + _normalized(name)
-                raw = source.get(full_deprecated_name)
+                raw = source.get(full_deprecated_name.format(**env.dynamic))
                 if raw is not None:
                     deprecated_when_message = (
                         " in version %s" % deprecated_when
@@ -217,9 +217,15 @@ def __init__(
         self,
         source: t.Optional[t.Dict[str, str]] = None,
         parent: t.Optional["Env"] = None,
+        dynamic: t.Optional[t.Dict[str, str]] = None,
     ) -> None:
         self.source = source or os.environ
         self.parent = parent
+        self.dynamic = (
+            {k.upper(): v.upper() for k, v in dynamic.items()}
+            if dynamic is not None
+            else {}
+        )
 
         self._full_prefix: str = (
             parent._full_prefix if parent is not None else ""

tests/test_env.py

@@ -344,3 +344,16 @@ def validate(value):
             Config()
     else:
         assert Config().foo == value
+
+
+def test_env_dynamic(monkeypatch):
+    monkeypatch.setenv("FOO_PFX_BAR_VAR", "24")
+
+    class Config(Env):
+        __prefix__ = "foo.{prefix}"
+
+        foobar = Env.var(int, "{dyn}.var", default=42)
+
+    config = Config(dynamic={"prefix": "pfx", "dyn": "bar"})
+
+    assert config.foobar == 24