Apply variable expansion to default values

If a default includes a reference to a variable, expand that reference.
See test_environs.test_default_expands.
Note: type for subs_default is required because of a bug in mypy:
  python/mypy#5423

Author: Richard Cohn

environs/__init__.py

@@ -72,19 +72,15 @@ def method(
             field = field_or_factory(missing=missing, **kwargs)
         else:
             field = field_or_factory(subcast=subcast, missing=missing, **kwargs)
-        parsed_key, raw_value, proxied_key = self._get_from_environ(name, ma.missing)
+        parsed_key, value, proxied_key = self._get_from_environ(name, field.missing)
         self._fields[parsed_key] = field
         source_key = proxied_key or parsed_key
-        if raw_value is ma.missing and field.missing is ma.missing:
+        if value is ma.missing:
             if self.eager:
                 raise EnvError('Environment variable "{}" not set'.format(proxied_key or parsed_key))
             else:
                 self._errors[parsed_key].append("Environment variable not set.")
                 return None
-        if raw_value or raw_value == "":
-            value = raw_value
-        else:
-            value = field.missing if field.missing is not ma.missing else None
         if preprocess:
             value = preprocess(value, subcast=subcast, **kwargs)
         try:
@@ -438,10 +434,12 @@ def _get_from_environ(
         if hasattr(value, "strip"):
             expand_match = self.expand_vars and _EXPANDED_VAR_PATTERN.match(value)
             if expand_match:  # Full match expand_vars - special case keep default
-                proxied_key = expand_match.groups()[0]
-                subs_default = expand_match.groups()[1]
+                proxied_key: _StrType = expand_match.groups()[0]
+                subs_default: typing.Optional[_StrType] = expand_match.groups()[1]
                 if subs_default is not None:
                     default = subs_default[2:]
+                elif value == default:  # if we have used default, don't use it recursively
+                    default = ma.missing
                 return (key, self._get_from_environ(proxied_key, default, proxied=True)[1], proxied_key)
             expand_search = self.expand_vars and _EXPANDED_VAR_PATTERN.search(value)
             if expand_search:  # Multiple or in text match expand_vars - General case - default lost

tests/test_environs.py

@@ -47,6 +47,8 @@ def test_call(self, set_env, env):
         set_env({"STR": "foo", "INT": "42"})
         assert env("STR") == "foo"
         assert env("NOT_SET", "mydefault") == "mydefault"
+        with pytest.raises(environs.EnvError, match='Environment variable "NOT_SET" not set'):
+            assert env("NOT_SET")
 
     def test_call_with_default(self, env):
         assert env("NOT_SET", default="mydefault") == "mydefault"
@@ -761,6 +763,19 @@ def test_recursive_expands(self, env, set_env):
         )
         assert env.str("PGURL") == "postgres://gnarvaja:secret@localhost"
 
+    def test_default_expands(self, env, set_env):
+        set_env(
+            {
+                "MAIN": "${SUBSTI}",
+                "SUBSTI": "substivalue",
+            }
+        )
+        assert env.str("NOT_SET", "${SUBSTI}") == "substivalue"
+        assert env.str("NOT_SET", "${MAIN}") == "substivalue"
+        assert env.str("NOT_SET", "${NOT_SET2:-set2}") == "set2"
+        with pytest.raises(environs.EnvError, match='Environment variable "NOT_SET2" not set'):
+            assert env.str("NOT_SET", "${NOT_SET2}")
+
     def test_escaped_expand(self, env, set_env):
         set_env({"ESCAPED_EXPAND": r"\${ESCAPED}", "ESCAPED": "fail"})
         assert env.str("ESCAPED_EXPAND") == r"${ESCAPED}"