feat: match args leniently according to spec signature (#89)

Closes #78

Author: mcous

decoy/spy.py

@@ -5,10 +5,11 @@
 """
 from inspect import getattr_static, isclass, iscoroutinefunction, isfunction, signature
 from functools import partial
+from warnings import warn
 from typing import get_type_hints, Any, Callable, Dict, NamedTuple, Optional
 
 from .spy_calls import SpyCall
-
+from .warnings import IncorrectCallWarning
 
 CallHandler = Callable[[SpyCall], Any]
 
@@ -83,13 +84,25 @@ def __class__(self) -> Any:
 
         return type(self)
 
-    @property
-    def _call_name(self) -> str:
-        """Get the name of the spy for the call log."""
-        if self._name:
-            return self._name
-        else:
-            return f"{type(self).__module__}.{type(self).__qualname__}"
+    def _call(self, *args: Any, **kwargs: Any) -> Any:
+        spy_id = id(self)
+        spy_name = (
+            self._name
+            if self._name
+            else f"{type(self).__module__}.{type(self).__qualname__}"
+        )
+
+        if hasattr(self, "__signature__"):
+            try:
+                bound_args = self.__signature__.bind(*args, **kwargs)
+            except TypeError as e:
+                # stacklevel: 3 ensures warning is linked to call location
+                warn(IncorrectCallWarning(e), stacklevel=3)
+            else:
+                args = bound_args.args
+                kwargs = bound_args.kwargs
+
+        return self._handle_call(SpyCall(spy_id, spy_name, args, kwargs))
 
     def __repr__(self) -> str:
         """Get a helpful string representation of the spy."""
@@ -160,15 +173,15 @@ class Spy(BaseSpy):
 
     def __call__(self, *args: Any, **kwargs: Any) -> Any:
         """Handle a call to the spy."""
-        return self._handle_call(SpyCall(id(self), self._call_name, args, kwargs))
+        return self._call(*args, **kwargs)
 
 
 class AsyncSpy(BaseSpy):
     """An object that records all async. calls made to itself and its children."""
 
     async def __call__(self, *args: Any, **kwargs: Any) -> Any:
         """Handle a call to the spy asynchronously."""
-        return self._handle_call(SpyCall(id(self), self._call_name, args, kwargs))
+        return self._call(*args, **kwargs)
 
 
 SpyFactory = Callable[[SpyConfig], Any]

decoy/warnings.py

@@ -84,3 +84,15 @@ def __init__(self, rehearsal: VerifyRehearsal) -> None:
         )
         super().__init__(message)
         self.rehearsal = rehearsal
+
+
+class IncorrectCallWarning(DecoyWarning):
+    """A warning raised if a Decoy mock with a spec is called incorrectly.
+
+    If a call to a Decoy mock is incorrect according to `inspect.signature`,
+    this warning will be raised.
+
+    See the [IncorrectCallWarning guide][] for more details.
+
+    [IncorrectCallWarning guide]: ../usage/errors-and-warnings/#incorrectcallwarning
+    """

docs/usage/errors-and-warnings.md

@@ -151,6 +151,24 @@ Adding those `verify`s at the end may give you a feeling of "ok, good, now I'm c
 
 If Decoy detects a `verify` with the same configuration of a `when`, it will raise a `RedundantVerifyWarning` to encourage you to remove the redundant, over-constraining `verify` call.
 
+### IncorrectCallWarning
+
+If you provide a Decoy mock with a specification `cls` or `func`, any calls to that mock will be checked according to `inspect.signature`. If the call does not match the signature, Decoy will raise a `IncorrectCalWarning`.
+
+Decoy limits this to a warning, but in real life, this call would likely cause the Python engine to error at run time.
+
+```python
+def some_func(val: string) -> int:
+    ...
+
+spy = decoy.mock(func=some_func)
+
+spy("hello")                # ok
+spy(val="world")            # ok
+spy(wrong_name="ah!")       # triggers an IncorrectCallWarning
+spy("too", "many", "args")  # triggers an IncorrectCallWarning
+```
+
 [warnings system]: https://docs.python.org/3/library/warnings.html
 [warning filters]: https://docs.pytest.org/en/latest/how-to/capture-warnings.html
 [unittest.mock]: https://docs.python.org/3/library/unittest.mock.html

tests/common.py

@@ -13,7 +13,7 @@ def bar(self, a: int, b: float, c: str) -> bool:
         """Get the bar bool based on a few inputs."""
         ...
 
-    def do_the_thing(self, flag: bool) -> None:
+    def do_the_thing(self, *, flag: bool) -> None:
         """Perform a side-effect without a return value."""
         ...
 
@@ -42,7 +42,7 @@ async def bar(self, a: int, b: float, c: str) -> bool:
         """Get the bar bool based on a few inputs."""
         ...
 
-    async def do_the_thing(self, flag: bool) -> None:
+    async def do_the_thing(self, *, flag: bool) -> None:
         """Perform a side-effect without a return value."""
         ...
 

tests/test_decoy.py

@@ -67,6 +67,9 @@ def test_when_then_return(decoy: Decoy) -> None:
     result = subject("hello")
     assert result == "hello world"
 
+    result = subject(val="hello")
+    assert result == "hello world"
+
     result = subject("asdfghjkl")
     assert result is None
 
@@ -119,6 +122,7 @@ def test_verify(decoy: Decoy) -> None:
     subject("hello")
 
     decoy.verify(subject("hello"))
+    decoy.verify(subject(val="hello"))
 
     with pytest.raises(errors.VerifyError):
         decoy.verify(subject("goodbye"))