Make zope a semi-optional test dependency

For the admittedly small number of people who don't care about support
for `zope.interface`, it is unnecessary extra work to support even a
test-only dependency on `zope`.

The `zope`-specific tests are relatively contained, however, and with
judicious use of pytest fixtures, we can make it so that `zope` is not a
required dependency when skipping tests marked with `pytest.mark.zope`.

This allows users to test everything but the `zope`-specific tests with
no `zope` dependency with:

$ TOX_AP_TEST_EXTRAS="tests_no_zope" tox -e <env> -- -m 'not zope'

pyproject.toml

@@ -35,7 +35,7 @@ multi_line_output=3
 use_parentheses=true
 
 known_first_party="attr"
-known_third_party=["attr", "hypothesis", "pytest", "setuptools", "six", "zope"]
+known_third_party=["attr", "hypothesis", "pytest", "setuptools", "six"]
 
 
 [tool.towncrier]

setup.py

@@ -38,16 +38,16 @@
 INSTALL_REQUIRES = []
 EXTRAS_REQUIRE = {
     "docs": ["sphinx", "sphinx-rtd-theme", "zope.interface"],
-    "tests": [
+    "tests_no_zope": [
         # 5.0 introduced toml; parallel was broken until 5.0.2
         "coverage[toml]>=5.0.2",
         "hypothesis",
         "pympler",
         "pytest>=4.3.0",  # 4.3.0 dropped last use of `convert`
         "six",
-        "zope.interface",
     ],
 }
+EXTRAS_REQUIRE["tests"] = EXTRAS_REQUIRE["tests_no_zope"] + ["zope.interface"]
 EXTRAS_REQUIRE["dev"] = (
     EXTRAS_REQUIRE["tests"] + EXTRAS_REQUIRE["docs"] + ["pre-commit"]
 )

tests/test_validators.py

@@ -7,7 +7,6 @@
 import re
 
 import pytest
-import zope.interface
 
 import attr
 
@@ -29,6 +28,14 @@
 from .utils import simple_attr
 
 
+@pytest.fixture(scope="module")
+def zope():
+    import zope
+    import zope.interface
+
+    return zope
+
+
 class TestInstanceOf(object):
     """
     Tests for `instance_of`.
@@ -217,17 +224,22 @@ class C(object):
         assert C.__attrs_attrs__[0].validator == C.__attrs_attrs__[1].validator
 
 
-class IFoo(zope.interface.Interface):
-    """
-    An interface.
-    """
-
-    def f():
+@pytest.fixture(scope="module")
+def ifoo(zope):
+    class IFoo(zope.interface.Interface):
         """
-        A function called f.
+        An interface.
         """
 
+        def f():
+            """
+            A function called f.
+            """
 
+    return IFoo
+
+
+@pytest.mark.zope
 class TestProvides(object):
     """
     Tests for `provides`.
@@ -239,46 +251,46 @@ def test_in_all(self):
         """
         assert provides.__name__ in validator_module.__all__
 
-    def test_success(self):
+    def test_success(self, zope, ifoo):
         """
         Nothing happens if value provides requested interface.
         """
 
-        @zope.interface.implementer(IFoo)
+        @zope.interface.implementer(ifoo)
         class C(object):
             def f(self):
                 pass
 
-        v = provides(IFoo)
+        v = provides(ifoo)
         v(None, simple_attr("x"), C())
 
-    def test_fail(self):
+    def test_fail(self, ifoo):
         """
         Raises `TypeError` if interfaces isn't provided by value.
         """
         value = object()
         a = simple_attr("x")
 
-        v = provides(IFoo)
+        v = provides(ifoo)
         with pytest.raises(TypeError) as e:
             v(None, a, value)
         assert (
             "'x' must provide {interface!r} which {value!r} doesn't.".format(
-                interface=IFoo, value=value
+                interface=ifoo, value=value
             ),
             a,
-            IFoo,
+            ifoo,
             value,
         ) == e.value.args
 
-    def test_repr(self):
+    def test_repr(self, ifoo):
         """
         Returned validator has a useful `__repr__`.
         """
-        v = provides(IFoo)
+        v = provides(ifoo)
         assert (
             "<provides validator for interface {interface!r}>".format(
-                interface=IFoo
+                interface=ifoo
             )
         ) == repr(v)
 

tox.ini

@@ -1,6 +1,7 @@
 [pytest]
 addopts = -ra
 testpaths = tests
+markers = zope
 filterwarnings =
     once::Warning
     ignore:::pympler[.*]