Allow specifying method as a string

flax/linen/module.py

@@ -1363,7 +1363,7 @@ def apply(self,
             variables: VariableDict,
             *args,
             rngs: Optional[RNGSequences] = None,
-            method: Optional[Callable[..., Any]] = None,
+            method: Union[Callable[..., Any], str, None] = None,
             mutable: CollectionFilter = False,
             capture_intermediates: Union[bool, Callable[['Module', str], bool]] = False,
             **kwargs) -> Union[Any, Tuple[Any, FrozenVariableDict]]:
@@ -1382,6 +1382,11 @@ def apply(self,
 
       encoded = model.apply({'params': params}, x, method=model.encode)
 
+    You can also pass a string to a callable attribute of the module. For
+    example, the previous can be written as::
+
+      encoded = model.apply({'params': params}, x, method='encode')
+
     Note ``method`` can also be a function that is not defined in
     ``Transformer``. In that case, the function should have at least one
     argument representing an instance of the Module class::
@@ -1401,7 +1406,8 @@ def other_fn(instance, ...):
         The "params" PRNG sequence is used to initialize parameters.
       method: A function to call apply on. This is generally a function in the
         module. If provided, applies this method. If not provided, applies the
-        ``__call__`` method of the module.
+        ``__call__`` method of the module. A string can also be provided to
+        specify a method by name.
       mutable: Can be bool, str, or list. Specifies which collections should be
                treated as mutable: ``bool``: all/no collections are mutable.
                ``str``: The name of a single mutable collection. ``list``: A
@@ -1420,7 +1426,13 @@ def other_fn(instance, ...):
     """
     Module._module_checks(self)
 
-    if method is None:
+    if isinstance(method, str):
+      attribute_name = method
+      method = getattr(self, attribute_name)
+      if not callable(method):
+        class_name = type(self).__name__
+        raise TypeError(f'\'{class_name}.{attribute_name}\' must be a callable, got {type(method)}.')
+    elif method is None:
       method = self.__call__
     method = _get_unbound_fn(method)
     return apply(
@@ -1433,7 +1445,7 @@ def other_fn(instance, ...):
   def init_with_output(self,
                        rngs: Union[PRNGKey, RNGSequences],
                        *args,
-                       method: Optional[Callable[..., Any]] = None,
+                       method: Union[Callable[..., Any], str, None] = None,
                        mutable: CollectionFilter = DenyList('intermediates'),
                        capture_intermediates: Union[bool, Callable[['Module', str], bool]] = False,
                        **kwargs) -> Tuple[Any, FrozenVariableDict]:
@@ -1443,7 +1455,8 @@ def init_with_output(self,
       rngs: The rngs for the variable collections.
       *args: Named arguments passed to the init function.
       method: An optional method. If provided, applies this method. If not
-        provided, applies the ``__call__`` method.
+        provided, applies the ``__call__`` method. A string can also be'
+        provided to specify a method by name.
       mutable: Can be bool, str, or list. Specifies which collections should be
         treated as mutable: ``bool``: all/no collections are mutable.
         ``str``: The name of a single mutable collection. ``list``: A
@@ -1468,7 +1481,14 @@ def init_with_output(self,
             'RNGs should be of shape (2,) or KeyArray in Module '
             f'{self.__class__.__name__}, but rngs are: {rngs}')
       rngs = {'params': rngs}
-    if method is None:
+
+    if isinstance(method, str):
+      attribute_name = method
+      method = getattr(self, attribute_name)
+      if not callable(method):
+        class_name = type(self).__name__
+        raise TypeError(f'\'{class_name}.{attribute_name}\' must be a callable, got {type(method)}.')
+    elif method is None:
       method = self.__call__
     method = _get_unbound_fn(method)
     return init_with_output(
@@ -1482,7 +1502,7 @@ def init_with_output(self,
   def init(self,
            rngs: Union[PRNGKey, RNGSequences],
            *args,
-           method: Optional[Callable[..., Any]] = None,
+           method: Union[Callable[..., Any], str, None] = None,
            mutable: CollectionFilter = DenyList('intermediates'),
            capture_intermediates: Union[bool, Callable[['Module', str], bool]] = False,
            **kwargs) -> FrozenVariableDict:
@@ -1499,7 +1519,8 @@ def init(self,
       rngs: The rngs for the variable collections.
       *args: Named arguments passed to the init function.
       method: An optional method. If provided, applies this method. If not
-        provided, applies the ``__call__`` method.
+        provided, applies the ``__call__`` method. A string can also be
+        provided to specify a method by name.
       mutable: Can be bool, str, or list. Specifies which collections should be
         treated as mutable: ``bool``: all/no collections are mutable.
         ``str``: The name of a single mutable collection. ``list``: A

tests/linen/linen_module_test.py

@@ -826,6 +826,7 @@ def __call__(self, x):
   def test_module_apply_method(self):
 
     class Foo(nn.Module):
+      not_callable: int = 1
 
       @nn.compact
       def __call__(self):
@@ -849,8 +850,23 @@ def test(self):
     with self.assertRaisesRegex(errors.ApplyModuleInvalidMethodError, msg):
       Foo().apply({}, method=lambda: True)
 
-    with self.assertRaisesRegex(errors.ApplyModuleInvalidMethodError, msg):
+    # string method names are also allowed.
+    Foo().apply({}, method='test')
+    # test same for init.
+    Foo().init({}, method='test')
+
+    # non-existent attribute names will yield AttributeError.
+    with self.assertRaisesRegex(AttributeError, "allowed_apply_fn"):
       Foo().apply({}, method='allowed_apply_fn')
+      # test same for init.
+      Foo().init({}, method='allowed_apply_fn')
+
+    # attributes which are not callables yield TypeError.
+    with self.assertRaisesRegex(TypeError, "'Foo.not_callable' must be a callable"):
+      Foo().apply({}, method='not_callable')
+      # test same for init.
+      Foo().init({}, method='not_callable')
+
 
   def test_call_unbound_compact_module_methods(self):
     dense = Dense(3)