fix(python): clear error message when trying to serialize function (#824)

A common error seems to be to try to pass a function as a parameter
into a JSII call (have seen 3 occurrences of this in the last week).

Previously, this call would fail when trying to assing the
`__jsii_type__` property to the function object.

Specifically catch this case to give a helpful error message.

Fixes aws/aws-cdk#4064.

Author: rix0rrr

packages/jsii-python-runtime/src/jsii/_kernel/__init__.py

@@ -1,6 +1,7 @@
 import datetime
 import inspect
 import itertools
+from types import FunctionType, MethodType, BuiltinFunctionType, LambdaType
 
 from typing import Any, List, Optional, Type, Union
 
@@ -126,6 +127,11 @@ def _make_reference_for_native(kernel, d):
         return d
     elif isinstance(d, (int, type(None), str, float, bool, datetime.datetime)):
         return d
+    elif isinstance(d, (FunctionType, MethodType, BuiltinFunctionType, LambdaType)):
+        # Whether a given object is a function-like object.
+        # We won't use iscallable() since objects may implement __call__()
+        # but we still want to serialize them as normal.
+        raise JSIIError("Cannot pass function as argument here (did you mean to call this function?): %r" % d)
     else:
         d.__jsii__type__ = "Object"
         kernel.create(Object, d)

packages/jsii-python-runtime/tests/test_python.py

@@ -23,6 +23,11 @@ def test_inheritance_maintained(self):
 
         assert base_names == ['DerivedStruct', 'MyFirstStruct']
 
+    def test_descriptive_error_when_passing_function(self):
+        obj = jsii_calc.Calculator()
+
+        with pytest.raises(JSIIError, match="Cannot pass function as argument here.*"):
+            obj.add(self.test_descriptive_error_when_passing_function)
 
 
 def find_struct_bases(x):