Experiment with PEP 593-based way to declare what's (non)injectable

There's an implementation of PEP 593 draft in typing_extensions and mypy
supports it already.

See also:

* typing module discussion (pre-PEP): python/typing#600
* python-ideas discussion (pre-PEP): https://mail.python.org/pipermail/python-ideas/2019-January/054908.html
* The actual PEP: https://www.python.org/dev/peps/pep-0593/
* typing-sig PEP discussion (ongoing): https://mail.python.org/archives/list/[email protected]/thread/CZ7N3M3PGKHUY63RWWSPTICVOAVYI73D/

Author: jstasiak

injector/__init__.py

@@ -23,20 +23,10 @@
 import types
 from abc import ABCMeta, abstractmethod
 from collections import namedtuple
-from typing import (
-    Any,
-    Callable,
-    cast,
-    Dict,
-    Generic,
-    get_type_hints,
-    List,
-    overload,
-    Tuple,
-    Type,
-    TypeVar,
-    Union,
-)
+from typing import Any, Callable, cast, Dict, Generic, List, overload, Tuple, Type, TypeVar, Union
+
+# Ignoring errors here as typing_extensions stub doesn't know about those things yet
+from typing_extensions import _AnnotatedAlias, Annotated, get_type_hints  # type: ignore
 
 TYPING353 = hasattr(Union[str, int], '__origin__')
 
@@ -79,6 +69,80 @@ def wrapper(*args: Any, **kwargs: Any) -> Any:
 lock = threading.RLock()
 
 
+_inject_marker = object()
+_noinject_marker = object()
+
+InjectT = TypeVar('InjectT')
+Inject = Annotated[InjectT, _inject_marker]
+"""An experimental way to declare injectable dependencies utilizing a `PEP 593`_ implementation
+in `typing_extensions`. This API is likely to change when `PEP 593`_ stabilizes.
+
+Those two declarations are equivalent::
+
+    @inject
+    def fun(t: SomeType) -> None:
+        pass
+
+    def fun(t: Inject[SomeType]) -> None:
+        pass
+
+The advantage over using :func:`inject` is that if you have some noninjectable parameters
+it may be easier to spot what are they. Those two are equivalent::
+
+    @inject
+    @noninjectable('s')
+    def fun(t: SomeType, s: SomeOtherType) -> None:
+        pass
+
+    def fun(t: Inject[SomeType], s: SomeOtherType) -> None:
+        pass
+
+.. seealso::
+
+    Function :func:`get_bindings`
+        A way to inspect how various injection declarations interact with each other.
+
+.. versionadded:: 0.18.0
+
+.. _PEP 593: https://www.python.org/dev/peps/pep-0593/
+.. _typing_extensions: https://pypi.org/project/typing-extensions/
+"""
+
+NoInject = Annotated[InjectT, _noinject_marker]
+"""An experimental way to declare noninjectable dependencies utilizing a `PEP 593`_ implementation
+in `typing_extensions`. This API is likely to change when `PEP 593`_ stabilizes.
+
+Since :func:`inject` declares all function's parameters to be injectable there needs to be a way
+to opt out of it. This has been provided by :func:`noninjectable` but `noninjectable` suffers from
+two issues:
+
+* You need to repeat the parameter name
+* The declaration may be relatively distance in space from the actual parameter declaration, thus
+  hindering readability
+
+`NoInject` solves both of those concerns, for example (those two declarations are equivalent)::
+
+    @inject
+    @noninjectable('b')
+    def fun(a: TypeA, b: TypeB) -> None:
+        pass
+
+    @inject
+    def fun(a: TypeA, b: NoInject[TypeB]) -> None:
+        pass
+
+.. seealso::
+
+    Function :func:`get_bindings`
+        A way to inspect how various injection declarations interact with each other.
+
+.. versionadded:: 0.18.0
+
+.. _PEP 593: https://www.python.org/dev/peps/pep-0593/
+.. _typing_extensions: https://pypi.org/project/typing-extensions/
+"""
+
+
 def reraise(original: Exception, exception: Exception, maximum_frames: int = 1) -> None:
     prev_cls, prev, tb = sys.exc_info()
     frames = inspect.getinnerframes(cast(types.TracebackType, tb))
@@ -511,6 +575,13 @@ def _is_specialization(cls, generic_class):
         # issubclass(SomeGeneric[X], SomeGeneric) so we need some other way to
         # determine whether a particular object is a generic class with type parameters
         # provided. Fortunately there seems to be __origin__ attribute that's useful here.
+
+        # We need to special-case Annotated as its __origin__ behaves differently than
+        # other typing generic classes. See https://github.com/python/typing/pull/635
+        # for some details.
+        if generic_class is Annotated and isinstance(cls, _AnnotatedAlias):
+            return True
+
         if not hasattr(cls, '__origin__'):
             return False
         origin = cls.__origin__
@@ -866,22 +937,26 @@ def repr_key(k):
 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
+    If the callable is not decorated with :func:`inject` and does not have any of its
+    parameters declared as injectable using :data:`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::
+    excluded from dependency injection (either with :func:`noninjectable`, :data:`NoInject`
+    or only explicit injection with :data:`Inject` being used). 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:
@@ -890,14 +965,55 @@ def get_bindings(callable: Callable) -> Dict[str, type]:
         >>> get_bindings(function3)
         {'a': int}
 
+        >>> # The simple case of no @inject but injection requested with Inject[...]
+        >>> def function4(a: Inject[int], b: str) -> None:
+        ... pass
+        ...
+        >>> get_bindings(function4)
+        {'a': int}
+
+        >>> # Using @inject with Inject is redundant but it should not break anything
+        >>> @inject
+        ... def function5(a: Inject[int], b: str) -> None:
+        ...     pass
+        ...
+        >>> get_bindings(function5)
+        {'a': int, 'b': str}
+
+        >>> # We need to be able to exclude a parameter from injection with NoInject
+        >>> @inject
+        ... def function6(a: int, b: NoInject[str]) -> None:
+        ...     pass
+        ...
+        >>> get_bindings(function6)
+        {'a': int}
+
+        >>> # The presence of NoInject should not trigger anything on its own
+        >>> def function7(a: int, b: NoInject[str]) -> None:
+        ...     pass
+        ...
+        >>> get_bindings(function7)
+        {}
+
     This function is used internally so by calling it you can learn what exactly
     Injector is going to try to provide to a callable.
     """
+    look_for_explicit_bindings = False
     if not hasattr(callable, '__bindings__'):
-        return {}
+        type_hints = get_type_hints(callable, include_extras=True)
+        has_injectable_parameters = any(
+            _is_specialization(v, Annotated) and _inject_marker in v.__metadata__ for v in type_hints.values()
+        )
+
+        if not has_injectable_parameters:
+            return {}
+        else:
+            look_for_explicit_bindings = True
 
-    if cast(Any, callable).__bindings__ == 'deferred':
-        read_and_store_bindings(callable, _infer_injected_bindings(callable))
+    if look_for_explicit_bindings or cast(Any, callable).__bindings__ == 'deferred':
+        read_and_store_bindings(
+            callable, _infer_injected_bindings(callable, only_explicit_bindings=look_for_explicit_bindings)
+        )
     noninjectables = getattr(callable, '__noninjectables__', set())
     return {k: v for k, v in cast(Any, callable).__bindings__.items() if k not in noninjectables}
 
@@ -906,10 +1022,10 @@ class _BindingNotYetAvailable(Exception):
     pass
 
 
-def _infer_injected_bindings(callable):
+def _infer_injected_bindings(callable, only_explicit_bindings: bool):
     spec = inspect.getfullargspec(callable)
     try:
-        bindings = get_type_hints(callable)
+        bindings = get_type_hints(callable, include_extras=True)
     except NameError as e:
         raise _BindingNotYetAvailable(e)
 
@@ -929,14 +1045,26 @@ def _infer_injected_bindings(callable):
     bindings.pop(spec.varkw, None)
 
     for k, v in list(bindings.items()):
+        if _is_specialization(v, Annotated):
+            v, metadata = v.__origin__, v.__metadata__
+            bindings[k] = v
+        else:
+            metadata = tuple()
+
+        if only_explicit_bindings and _inject_marker not in metadata or _noinject_marker in metadata:
+            del bindings[k]
+            break
+
         if _is_specialization(v, Union):
             # We don't treat Optional parameters in any special way at the moment.
             if TYPING353:
                 union_members = v.__args__
             else:
                 union_members = v.__union_params__
             new_members = tuple(set(union_members) - {type(None)})
-            new_union = Union[new_members]
+            # mypy stared complaining about this line for some reason:
+            #     error: Variable "new_members" is not valid as a type
+            new_union = Union[new_members]  # type: ignore
             # mypy complains about this construct:
             #     error: The type alias is invalid in runtime context
             # See: https://github.com/python/mypy/issues/5354
@@ -1051,6 +1179,14 @@ def inject(constructor_or_class):
         Third party libraries may, however, provide support for injecting dependencies
         into non-constructor methods or free functions in one form or another.
 
+    .. seealso::
+
+        Generic type :data:`Inject`
+            A more explicit way to declare parameters as injectable.
+
+        Function :func:`get_bindings`
+            A way to inspect how various injection declarations interact with each other.
+
     .. versionchanged:: 0.16.2
 
         (Re)added support for decorating classes with @inject.
@@ -1060,7 +1196,7 @@ def inject(constructor_or_class):
     else:
         function = constructor_or_class
         try:
-            bindings = _infer_injected_bindings(function)
+            bindings = _infer_injected_bindings(function, only_explicit_bindings=False)
             read_and_store_bindings(function, bindings)
         except _BindingNotYetAvailable:
             function.__bindings__ = 'deferred'
@@ -1089,6 +1225,15 @@ def noninjectable(*args):
     each other and the order in which a function is decorated with
     :func:`inject` and :func:`noninjectable`
     doesn't matter.
+
+    .. seealso::
+
+        Generic type :data:`NoInject`
+            A nicer way to declare parameters as noninjectable.
+
+        Function :func:`get_bindings`
+            A way to inspect how various injection declarations interact with each other.
+
     """
 
     def decorator(function):

injector_test.py

@@ -46,6 +46,8 @@
     ClassAssistedBuilder,
     Error,
     UnknownArgument,
+    Inject,
+    NoInject,
 )
 
 
@@ -1438,3 +1440,29 @@ def function3(a: int, b: str) -> None:
         pass
 
     assert get_bindings(function3) == {'a': int}
+
+    # The simple case of no @inject but injection requested with Inject[...]
+    def function4(a: Inject[int], b: str) -> None:
+        pass
+
+    assert get_bindings(function4) == {'a': int}
+
+    # Using @inject with Inject is redundant but it should not break anything
+    @inject
+    def function5(a: Inject[int], b: str) -> None:
+        pass
+
+    assert get_bindings(function5) == {'a': int, 'b': str}
+
+    # We need to be able to exclude a parameter from injection with NoInject
+    @inject
+    def function6(a: int, b: NoInject[str]) -> None:
+        pass
+
+    assert get_bindings(function6) == {'a': int}
+
+    # The presence of NoInject should not trigger anything on its own
+    def function7(a: int, b: NoInject[str]) -> None:
+        pass
+
+    assert get_bindings(function7) == {}

setup.py

@@ -69,4 +69,5 @@ def read_injector_variable(name):
         'IoC',
         'Inversion of Control container',
     ],
+    install_requires=['typing_extensions>=3.7.4'],
 )