Fixed error message related to misuse of fixture_ref. Fixes #209

Author: Sylvain MARIE

pytest_cases/fixture_core1_unions.py

@@ -156,7 +156,8 @@ def __init__(self, params):
         self.params = params
 
     def __str__(self):
-        return "Invalid parameters list (`argvalues`) in pytest parametrize: %s" % self.params
+        return "Invalid parameters list (`argvalues`) in pytest parametrize. `list(argvalues)` returned an error. " \
+               "Please make sure that `argvalues` is a list, tuple or iterable : %r" % self.params
 
 
 def is_fixture_union_params(params):

pytest_cases/fixture_parametrize_plus.py

@@ -162,7 +162,10 @@ def __repr__(self):
     def _check_iterable(self):
         """Raise a TypeError if this fixture reference is not iterable, that is, it does not represent a tuple"""
         if self.theoretical_size is None:
-            raise TypeError("This fixture_ref has not yet been initialized")
+            raise TypeError("This `fixture_ref` has not yet been initialized, so it cannot be unpacked/iterated upon. "
+                            "This is not supposed to happen when a `fixture_ref` is used correctly, i.e. as an item in"
+                            " the `argvalues` of a `@parametrize` decorator. Please check the documentation for "
+                            "details.")
         if self.theoretical_size == 1:
             raise TypeError("This fixture_ref does not represent a tuple of arguments, it is not iterable")
 

pytest_cases/tests/pytest_extension/parametrize_plus/test_fixture_ref_basic1.py

@@ -3,6 +3,8 @@
 #
 # License: 3-clause BSD, <https://github.com/smarie/python-pytest-cases/blob/master/LICENSE>
 import pytest
+
+from pytest_cases.fixture_core1_unions import InvalidParamsList
 from pytest_cases import fixture, fixture_ref, parametrize
 from pytest_cases.tests.conftest import global_fixture
 
@@ -49,3 +51,11 @@ def c():
 def test_foo(a, b):
     """here the fixture is used for both parameters at the same time"""
     assert (a, b) == (3, 2)
+
+
+def test_invalid_argvalues_message():
+    """ Check that the error message is friendly when the fixture_ref is misused"""
+    with pytest.raises(InvalidParamsList) as exc_info:
+        parametrize("fixture_a", fixture_ref('a'))
+    assert str(exc_info.value).startswith("Invalid parameters list (`argvalues`) in pytest parametrize. "
+                                          "`list(argvalues)` returned an error.")