Fix error message for incorrect calls with unlock_instance (#1044)

* first commit

* added a test

pyccl/base.py

@@ -393,14 +393,14 @@ def __exit__(self, type, value, traceback):
             self.object_lock.lock()
 
     @classmethod
-    def unlock_instance(cls, func=None, *, argv=0, mutate=True):
+    def unlock_instance(cls, func=None, *, name=None, mutate=True):
         """Decorator that temporarily unlocks an instance of CCLObject.
 
         Arguments:
             func (``function``):
                 Function which changes one of its ``CCLObject`` arguments.
-            argv (``int``):
-                Which argument should be unlocked. Defaults to the first one.
+            name (``str``):
+                Name of the parameter to unlock. Defaults to the first one.
                 If not a ``CCLObject`` the decorator will do nothing.
             mutate (``bool``):
                 If after the function ``instance_old != instance_new``, the
@@ -409,17 +409,23 @@ def unlock_instance(cls, func=None, *, argv=0, mutate=True):
         """
         if func is None:
             # called with parentheses
-            return functools.partial(cls.unlock_instance, argv=argv,
+            return functools.partial(cls.unlock_instance, name=name,
                                      mutate=mutate)
 
+        if not hasattr(func, "__signature__"):
+            # store the function signature
+            func.__signature__ = signature(func)
+        names = list(func.__signature__.parameters.keys())
+        name = names[0] if name is None else name  # default name
+        if name not in names:
+            # ensure the name makes sense
+            raise NameError(f"{name} does not exist in {func.__name__}.")
+
         @functools.wraps(func)
         def wrapper(*args, **kwargs):
-            # Pick argument from list of `args` or `kwargs` as needed.
-            size = len(args)
-            arg = args[argv] if size > argv else list(kwargs.values())[argv-size]  # noqa
-            with UnlockInstance(arg, mutate=mutate):
-                out = func(*args, **kwargs)
-            return out
+            bound = func.__signature__.bind(*args, **kwargs)
+            with UnlockInstance(bound.arguments[name], mutate=mutate):
+                return func(*args, **kwargs)
         return wrapper
 
     @classmethod

pyccl/bcm.py

@@ -36,7 +36,7 @@ def bcm_model_fka(cosmo, k, a):
     return fka
 
 
-@unlock_instance(mutate=True, argv=1)
+@unlock_instance(mutate=True, name="pk2d")
 def bcm_correct_pk2d(cosmo, pk2d):
     """Apply the BCM model correction factor to a given power spectrum.
 

pyccl/tests/test_cclobject.py

@@ -4,6 +4,12 @@
 import functools
 
 
+def all_subclasses(cls):
+    """Get all subclasses of ``cls``. NOTE: Used in ``conftest.py``."""
+    return set(cls.__subclasses__()).union([s for c in cls.__subclasses__()
+                                            for s in all_subclasses(c)])
+
+
 def test_fancy_repr():
     # Test fancy-repr controls.
     cosmo1 = ccl.CosmologyVanillaLCDM()
@@ -211,7 +217,19 @@ def wrapper(self, *args, **kwargs):
     return wrapper
 
 
-def all_subclasses(cls):
-    """Get all subclasses of ``cls``. NOTE: Used in ``conftest.py``."""
-    return set(cls.__subclasses__()).union([s for c in cls.__subclasses__()
-                                            for s in all_subclasses(c)])
+def test_unlock_instance_errors():
+    # Test that unlock_instance gives the correct errors.
+
+    # 1. Developer error
+    with pytest.raises(NameError):
+        @ccl.unlock_instance(name="hello")
+        def func1(item, pk, a0=0, *, a1=None, a2):
+            return
+
+    # 2. User error
+    @ccl.unlock_instance(name="pk")
+    def func2(item, pk, a0=0, *, a1=None, a2):
+        return
+
+    with pytest.raises(TypeError):
+        func2()