Added some more tests for strange behavious of lambdas in hashing

tests/test_hash.py

@@ -93,17 +93,69 @@ def func(a):
 
     obj1 = FloatAggregator(func)
 
-    @functools.wraps(func)
-    def _func(a):
-        return func(a)
+    def decorator(func):
+        @functools.wraps(func)
+        def _func(a):
+            return func(a)
 
-    obj2 = FloatAggregator(_func)
+        return _func
+
+    obj2 = FloatAggregator(decorator(func))
 
     assert custom_hash(obj1) != custom_hash(obj2)
 
 
+def test_hash_lambdas_same():
+    def func(a, b):
+        return np.mean(a) + b
+
+    def func2():
+        return FloatAggregator(lambda a: func(a, 1))
+
+    obj1 = func2()
+    obj2 = func2()
+
+    assert custom_hash(obj1) == custom_hash(obj2)
+
+
 def test_hash_lambdas_different():
+    # This is quite interesting, these two lambdas are different, as they have different names, as they are
+    # defined in the same scope. in the pevious test, where there was only on lambda defined, the names were the same
+    # hence the hash the same.
     obj1 = FloatAggregator(lambda a: np.mean(a))
-    obj2 = FloatAggregator(lambda a: np.mean(a))
+    obj2 = obj1
+    obj1 = FloatAggregator(lambda a: np.mean(a))
+    assert custom_hash(obj1) != custom_hash(obj2)
+
+
+def test_hash_partials_same():
+    def func(a, b):
+        return np.mean(a) + b
+
+    obj1 = FloatAggregator(functools.partial(func, b=1))
+    obj2 = FloatAggregator(functools.partial(func, b=1))
+
+    assert custom_hash(obj1) == custom_hash(obj2)
+
+
+def test_hash_partials_different():
+    def func(a, b):
+        return np.mean(a) + b
+
+    obj1 = FloatAggregator(functools.partial(func, b=1))
+    obj2 = FloatAggregator(functools.partial(func, b=2))
+
+    assert custom_hash(obj1) != custom_hash(obj2)
+
+
+def test_hash_partials_different2():
+    def func(a, b):
+        return np.mean(a) + b
+
+    def func2(a, b):
+        return np.mean(a) + b
+
+    obj1 = FloatAggregator(functools.partial(func, b=1))
+    obj2 = FloatAggregator(functools.partial(func2, b=1))
 
     assert custom_hash(obj1) != custom_hash(obj2)

tpcp/_hash.py

@@ -4,6 +4,7 @@
 import pickle
 import sys
 import types
+import warnings
 from pathlib import Path
 
 from joblib.func_inspect import get_func_code
@@ -64,6 +65,17 @@ def save(self, obj):
             # However, in the context of tpcp, that is not really a concern. In most possible cases, this just means
             # that some (likely obscure) guardrail will not trigger for you.
             if isinstance(obj, types.FunctionType):
+                if "<lambda>" in obj.__qualname__:
+                    warnings.warn(
+                        "You are attempting to hash a lambda defined within a closure, likely because you used it as a "
+                        "parameter to a tpcp object (e.g. an Aggregator). "
+                        "While this works most of the time, it can to lead to some unexpected false positive hash "
+                        "equalities, depending on how you define the lambdas. "
+                        "We highly recommend to use a named function or a `functools.partial` instead.",
+                        stacklevel=1,
+                    )
+                # Note, that for lambdas this actully hashes the entire definition line.
+                # This means potentially more of the surrounding code than the lambda itself is hashed.
                 obj = ("F", obj.__qualname__, get_func_code(obj), vars(obj))
 
             if isinstance(obj, type):