Consider plugins loaded by PYTEST_PLUGINS for assertion rewrite

CHANGELOG.rst

@@ -6,6 +6,10 @@
 * pytest no longer recognizes coroutine functions as yield tests (`#2129`_).
   Thanks to `@malinoff`_ for the PR.
 
+* Plugins loaded by the ``PYTEST_PLUGINS`` environment variable are now automatically
+  considered for assertion rewriting (`#2185`_).
+  Thanks `@nicoddemus`_ for the PR.
+
 * Improve error message when pytest.warns fails (`#2150`_). The type(s) of the
   expected warnings and the list of caught warnings is added to the
   error message. Thanks `@lesteve`_ for the PR.
@@ -23,6 +27,7 @@
 .. _#2129: https://github.com/pytest-dev/pytest/issues/2129
 .. _#2148: https://github.com/pytest-dev/pytest/issues/2148
 .. _#2150: https://github.com/pytest-dev/pytest/issues/2150
+.. _#2185: https://github.com/pytest-dev/pytest/issues/2185
 
 
 3.0.5 (2016-12-05)

_pytest/config.py

@@ -402,31 +402,26 @@ def consider_env(self):
         self._import_plugin_specs(os.environ.get("PYTEST_PLUGINS"))
 
     def consider_module(self, mod):
-        plugins = getattr(mod, 'pytest_plugins', [])
-        if isinstance(plugins, str):
-            plugins = [plugins]
-        self.rewrite_hook.mark_rewrite(*plugins)
-        self._import_plugin_specs(plugins)
+        self._import_plugin_specs(getattr(mod, 'pytest_plugins', []))
 
     def _import_plugin_specs(self, spec):
-        if spec:
-            if isinstance(spec, str):
-                spec = spec.split(",")
-            for import_spec in spec:
-                self.import_plugin(import_spec)
+        plugins = _get_plugin_specs_as_list(spec)
+        for import_spec in plugins:
+            self.import_plugin(import_spec)
 
     def import_plugin(self, modname):
         # most often modname refers to builtin modules, e.g. "pytester",
         # "terminal" or "capture".  Those plugins are registered under their
         # basename for historic purposes but must be imported with the
         # _pytest prefix.
-        assert isinstance(modname, str)
+        assert isinstance(modname, str), "module name as string required, got %r" % modname
         if self.get_plugin(modname) is not None:
             return
         if modname in builtin_plugins:
             importspec = "_pytest." + modname
         else:
             importspec = modname
+        self.rewrite_hook.mark_rewrite(modname)
         try:
             __import__(importspec)
         except ImportError as e:
@@ -447,6 +442,24 @@ def import_plugin(self, modname):
             self.consider_module(mod)
 
 
+def _get_plugin_specs_as_list(specs):
+    """
+    Parses a list of "plugin specs" and returns a list of plugin names.
+
+    Plugin specs can be given as a list of strings separated by "," or already as a list/tuple in
+    which case it is returned as a list. Specs can also be `None` in which case an
+    empty list is returned.
+    """
+    if specs is not None:
+        if isinstance(specs, str):
+            specs = specs.split(',') if specs else []
+        if not isinstance(specs, (list, tuple)):
+            raise UsageError("Plugin specs must be a ','-separated string or a "
+                             "list/tuple of strings for plugin names. Given: %r" % specs)
+        return list(specs)
+    return []
+
+
 class Parser:
     """ Parser for command line arguments and ini-file values.
 

testing/test_assertrewrite.py

@@ -682,7 +682,7 @@ def test_remember_rewritten_modules(self, pytestconfig, testdir, monkeypatch):
         hook.mark_rewrite('test_remember_rewritten_modules')
         assert warnings == []
 
-    def test_rewrite_warning_using_pytest_plugins(self, testdir, monkeypatch):
+    def test_rewrite_warning_using_pytest_plugins(self, testdir):
         testdir.makepyfile(**{
             'conftest.py': "pytest_plugins = ['core', 'gui', 'sci']",
             'core.py': "",
@@ -695,6 +695,22 @@ def test_rewrite_warning_using_pytest_plugins(self, testdir, monkeypatch):
         result.stdout.fnmatch_lines(['*= 1 passed in *=*'])
         assert 'pytest-warning summary' not in result.stdout.str()
 
+    def test_rewrite_warning_using_pytest_plugins_env_var(self, testdir, monkeypatch):
+        monkeypatch.setenv('PYTEST_PLUGINS', 'plugin')
+        testdir.makepyfile(**{
+            'plugin.py': "",
+            'test_rewrite_warning_using_pytest_plugins_env_var.py': """
+                import plugin
+                pytest_plugins = ['plugin']
+                def test():
+                    pass
+            """,
+        })
+        testdir.chdir()
+        result = testdir.runpytest_subprocess()
+        result.stdout.fnmatch_lines(['*= 1 passed in *=*'])
+        assert 'pytest-warning summary' not in result.stdout.str()
+
 
 class TestAssertionRewriteHookDetails(object):
     def test_loader_is_package_false_for_module(self, testdir):

testing/test_config.py

@@ -587,6 +587,21 @@ def pytest_load_initial_conftests(self):
     assert [x.function.__module__ for x in l] == expected
 
 
+def test_get_plugin_specs_as_list():
+    from _pytest.config import _get_plugin_specs_as_list
+    with pytest.raises(pytest.UsageError):
+        _get_plugin_specs_as_list(set(['foo']))
+    with pytest.raises(pytest.UsageError):
+        _get_plugin_specs_as_list(dict())
+
+    assert _get_plugin_specs_as_list(None) == []
+    assert _get_plugin_specs_as_list('') == []
+    assert _get_plugin_specs_as_list('foo') == ['foo']
+    assert _get_plugin_specs_as_list('foo,bar') == ['foo', 'bar']
+    assert _get_plugin_specs_as_list(['foo', 'bar']) == ['foo', 'bar']
+    assert _get_plugin_specs_as_list(('foo', 'bar')) == ['foo', 'bar']
+
+
 class TestWarning:
     def test_warn_config(self, testdir):
         testdir.makeconftest("""