add warning all elements to sampled_from are strategies, suggestive o…

…f one_of; close HypothesisWorks#3819

AUTHORS.rst

@@ -181,6 +181,7 @@ their individual contributions.
 * `Tyler Gibbons <https://www.github.com/kavec>`_ ([email protected])
 * `Tyler Nickerson <https://www.github.com/nmbrgts>`_
 * `Vidya Rani <https://www.github.com/vidyarani-dg>`_ ([email protected])
+* `Vince Reuter <https://github.com/vreuter>`_ ([email protected])
 * `Vincent Michel <https://www.github.com/vxgmichel>`_ ([email protected])
 * `Viorel Pluta <https://github.com/viopl>`_ ([email protected])
 * `Vytautas Strimaitis <https://www.github.com/vstrimaitis>`_

hypothesis-python/src/hypothesis/strategies/_internal/core.py

@@ -214,6 +214,10 @@ def sampled_from(
                 "so maybe you tried to write an enum as if it was a dataclass?"
             )
         raise InvalidArgument("Cannot sample from a length-zero sequence.")
+    elif all(isinstance(x, SearchStrategy) for x in values):
+        warnings.warn(
+            "sample_from was given a collection of strategies; was one_of intended?"
+        )
     if len(values) == 1:
         return just(values[0])
     try:

hypothesis-python/tests/cover/test_lookup.py

@@ -311,21 +311,29 @@ class Baz(Foo):
 
 
 @pytest.mark.parametrize(
-    "var,expected",
+    "var,expected,exp_warn",
     [
-        (typing.TypeVar("V"), object),
-        (typing.TypeVar("V", bound=int), int),
-        (typing.TypeVar("V", bound=Foo), (Bar, Baz)),
-        (typing.TypeVar("V", bound=typing.Union[int, str]), (int, str)),
-        (typing.TypeVar("V", int, str), (int, str)),
+        # Expect a warning exactly when the type constraint/bound should trigger
+        # passing a strategy to sampled_from.
+        (typing.TypeVar("V"), object, False),
+        (typing.TypeVar("V", bound=int), int, False),
+        (typing.TypeVar("V", bound=Foo), (Bar, Baz), True),
+        (typing.TypeVar("V", bound=typing.Union[int, str]), (int, str), True),
+        (typing.TypeVar("V", int, str), (int, str), False),
     ],
 )
 @settings(suppress_health_check=[HealthCheck.too_slow])
 @given(data=st.data())
-def test_typevar_type_is_consistent(data, var, expected):
+def test_typevar_type_is_consistent(data, var, expected, exp_warn):
     strat = st.from_type(var)
-    v1 = data.draw(strat)
-    v2 = data.draw(strat)
+    if exp_warn:
+        with warnings.catch_warnings:
+            warnings.filterwarnings(action="ignore", category=UserWarning)
+            v1 = data.draw(strat)
+            v2 = data.draw(strat)
+    else:
+        v1 = data.draw(strat)
+        v2 = data.draw(strat)
     assume(v1 != v2)  # Values may vary, just not types
     assert type(v1) == type(v2)
     assert isinstance(v1, expected)

hypothesis-python/tests/cover/test_sampled_from.py

@@ -190,3 +190,13 @@ class AnnotationsInsteadOfElements(enum.Enum):
 def test_suggests_elements_instead_of_annotations():
     with pytest.raises(InvalidArgument, match="Cannot sample.*annotations.*dataclass"):
         st.sampled_from(AnnotationsInsteadOfElements).example()
+
+
+@pytest.mark.parametrize("wrap", [list, tuple])
+def test_warns_when_given_entirely_strategies_as_elements(wrap):
+    elements = wrap([st.booleans(), st.decimals(), st.integers(), st.text()])
+    with pytest.warns(
+        UserWarning,
+        match="sample_from was given a collection of strategies; was one_of intended?",
+    ):
+        st.sampled_from(elements)