Document get_bindings

Might want to look into making pytest handle Sphinx-formatted doctests
to reduce the code repetition here.

injector/__init__.py

@@ -863,14 +863,43 @@ class Injector:
         return dependencies
 
 
-def get_bindings(callable):
+def get_bindings(callable: Callable) -> Dict[str, type]:
+    """Get bindings of injectable parameters from a callable.
+
+    If the callable is not decorated with :func:`inject` an empty dictionary will
+    be returned.  Otherwise the returned dictionary will contain a mapping
+    between parameter names and their types with the exception of parameters
+    excluded from dependency injection with :func:`noninjectable`. For example::
+
+        >>> def function1(a: int) -> None:
+        ...     pass
+        ...
+        >>> get_bindings(function1)
+        {}
+        >>> @inject
+        ... def function2(a: int) -> None:
+        ...     pass
+        ...
+        >>> get_bindings(function2)
+        {'a': int}
+        >>> @inject
+        ... @noninjectable('b')
+        ... def function3(a: int, b: str) -> None:
+        ...     pass
+        ...
+        >>> get_bindings(function3)
+        {'a': int}
+
+    This function is used internally so by calling it you can learn what exactly
+    Injector is going to try to provide to a callable.
+    """
     if not hasattr(callable, '__bindings__'):
         return {}
 
-    if callable.__bindings__ == 'deferred':
+    if cast(Any, callable).__bindings__ == 'deferred':
         read_and_store_bindings(callable, _infer_injected_bindings(callable))
     noninjectables = getattr(callable, '__noninjectables__', set())
-    return {k: v for k, v in callable.__bindings__.items() if k not in noninjectables}
+    return {k: v for k, v in cast(Any, callable).__bindings__.items() if k not in noninjectables}
 
 
 class _BindingNotYetAvailable(Exception):

injector_test.py

@@ -29,6 +29,7 @@ from injector import (
     Scope,
     InstanceProvider,
     ClassProvider,
+    get_bindings,
     inject,
     multiprovider,
     noninjectable,
@@ -1417,3 +1418,23 @@ class Data:
 
     injector = Injector([configure])
     assert injector.get(Data).name == 'data'
+
+
+def test_get_bindings():
+    def function1(a: int) -> None:
+        pass
+
+    assert get_bindings(function1) == {}
+
+    @inject
+    def function2(a: int) -> None:
+        pass
+
+    assert get_bindings(function2) == {'a': int}
+
+    @inject
+    @noninjectable('b')
+    def function3(a: int, b: str) -> None:
+        pass
+
+    assert get_bindings(function3) == {'a': int}