Play around with function annotations to implement a crude type checker.

func_annotations.py

@@ -0,0 +1,58 @@
+#!/usr/bin/env python3
+# Play around with function annotations to implement a crude type checker.
+
+import inspect
+
+
+def typechecked(func):
+    wrapped_func = func
+
+    def typecheck(*args, **kvargs):
+        parameters = inspect.signature(wrapped_func).parameters
+        wants = [(par, parameters[par].annotation) for par in parameters]
+        for want, arg in zip(wants, args):
+            name, class_ = want
+            if not isinstance(arg, class_):
+                raise TypeError("arg {} is not "
+                                "an instance of {}".format(name, class_))
+
+        return_ = wrapped_func(*args, **kvargs)
+        class_ = wrapped_func.__annotations__["return"]
+        if not isinstance(return_, class_):
+                raise TypeError("return value is not "
+                                "an instance of {}".format(class_))
+        return return_
+
+    typecheck.__doc__ = wrapped_func.__doc__
+    return typecheck
+
+
+@typechecked
+def greet(name: str, age: int, fnord: str=None) -> str:
+    """Greet someone.
+
+    >>> greet("Mike", 12)
+    'Hello Mike, you are 12 years old'
+    >>> greet("Florian", "eins")
+    Traceback (most recent call last):
+      File "func_annotations.py", line 14, in typecheck
+        "is not an instance of {}".format(name, class_))
+    TypeError: arg age is not an instance of <class 'int'>
+    """
+    return 'Hello {0}, you are {1} years old'.format(name, age)
+
+
+@typechecked
+def test(foo: int) -> str:
+    """ Just a test.
+
+    >>> test(1)
+    Traceback (most recent call last):
+      File "func_annotations.py", line 21, in typecheck
+        "is not an instance of {}".format(class_))
+    TypeError: return value is not an instance of <class 'str'>
+    """
+    return 1
+
+import doctest
+doctest.testmod()