Added IncorrectPostInitOverrideError to capture incorrect post init overrides.

flax/errors.py

@@ -571,7 +571,7 @@ def __init__(self):
 
 class InvalidInstanceModuleError(FlaxError):
   """
-  This error occurs when you are trying to call `.init()`, `.init_with_output()` or `.apply()
+  This error occurs when you are trying to call `.init()`, `.init_with_output()`, `.apply() or `.bind()`
   on the Module class itself, instead of an instance of the Module class.
   For example, the error will be raised when trying to run this code::
 
@@ -588,6 +588,29 @@ def __call__(self, x):
   def __init__(self):
     super().__init__('Can only call init, init_with_output or apply methods on an instance of the Module class, not the Module class itself')
 
+class IncorrectPostInitOverrideError(FlaxError):
+  """
+  This error occurs when you overrode `.__post_init__()` without calling `super().__post_init__()`.
+  For example, the error will be raised when trying to run this code::
+
+    from flax import linen as nn
+    import jax.numpy as jnp
+    import jax
+    class A(nn.Module):
+      x: float
+      def __post_init__(self):
+        self.x_square = self.x ** 2
+        # super().__post_init__() <-- forgot to add this line
+      @nn.compact
+      def __call__(self, input):
+        return input + 3
+
+    r = A(x=3)
+    r.init(jax.random.PRNGKey(2), jnp.ones(3))
+  """
+  def __init__(self):
+    super().__init__('Overrode `.__post_init__()` without calling `super().__post_init__()`')
+
 
 class InvalidCheckpointError(FlaxError):
   """

flax/linen/module.py

@@ -1136,6 +1136,16 @@ def is_initializing(self) -> bool:
       raise ValueError("Can't check if running under init() on unbound modules")
     return self.scope.get_flag('initializing', False)
 
+  def _module_checks(self):
+    """Run standard runtime checks."""
+
+    if not isinstance(self, Module):
+      raise errors.InvalidInstanceModuleError()
+
+    overridden_post_init = self.__post_init__ != Module.__post_init__
+    if overridden_post_init and not hasattr(self, "_id"):
+      raise errors.IncorrectPostInitOverrideError()
+
   @traceback_util.api_boundary
   def bind(self,
            variables: VariableDict,
@@ -1191,6 +1201,8 @@ def __call__(self, x):
     Returns:
       A copy of this instance with bound variables and RNGs.
     """
+    Module._module_checks(self)
+
     del args
     scope = core.bind(variables, rngs=rngs, mutable=mutable)
     return self.clone(parent=scope)
@@ -1255,8 +1267,7 @@ def other_fn(instance, ...):
       mutable, returns ``(output, vars)``, where ``vars`` are is a dict
       of the modified collections.
     """
-    if not isinstance(self, Module):
-      raise errors.InvalidInstanceModuleError()
+    Module._module_checks(self)
 
     if method is None:
       method = self.__call__
@@ -1298,8 +1309,7 @@ def init_with_output(self,
       `(output, vars)``, where ``vars`` are is a dict of the modified
       collections.
     """
-    if not isinstance(self, Module):
-      raise errors.InvalidInstanceModuleError()
+    Module._module_checks(self)
 
     if not isinstance(rngs, dict):
       if not core.scope._is_valid_rng(rngs):
@@ -1354,8 +1364,7 @@ def init(self,
     Returns:
       The initialized variable dict.
     """
-    if not isinstance(self, Module):
-      raise errors.InvalidInstanceModuleError()
+    Module._module_checks(self)
 
     _, v_out = self.init_with_output(
         rngs,

tests/linen/linen_module_test.py

@@ -1772,6 +1772,23 @@ def __call__(self, x):
       B.init_with_output(k, x)
     with self.assertRaises(errors.InvalidInstanceModuleError):
       B.apply({}, x)   # similar issue w. apply called on class instead of instance.
+    with self.assertRaises(errors.InvalidInstanceModuleError):
+      B.bind({}, x)   # similar issue w. apply called on class instead of instance.
+
+  def test_throws_incorrect_post_init_override_error(self):
+
+    class A(nn.Module):
+      x: float
+      def __post_init__(self):
+        self.x_square = self.x ** 2
+      @nn.compact
+      def __call__(self, input):
+        return input + 3
+
+    r = A(x=3)
+
+    with self.assertRaises(errors.IncorrectPostInitOverrideError):
+      r.init(jax.random.PRNGKey(2), jnp.ones(3))
 
 class LeakTests(absltest.TestCase):