Refactor plugin specs handling into an isolated function

_pytest/config.py

@@ -399,30 +399,22 @@ def consider_conftest(self, conftestmodule):
             self.consider_module(conftestmodule)
 
     def consider_env(self):
-        specs = os.environ.get("PYTEST_PLUGINS")
-        if specs:
-            plugins = specs.split(',')
-            self._import_plugin_specs(plugins)
+        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._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_plugins_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:
@@ -450,6 +442,24 @@ def import_plugin(self, modname):
             self.consider_module(mod)
 
 
+def _get_plugins_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:
+        if isinstance(specs, str):
+            specs = specs.split(',')
+        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_config.py

@@ -587,6 +587,19 @@ def pytest_load_initial_conftests(self):
     assert [x.function.__module__ for x in l] == expected
 
 
+def test_get_plugins_as_list():
+    from _pytest.config import _get_plugins_as_list
+    with pytest.raises(pytest.UsageError):
+        _get_plugins_as_list(set(['foo']))
+
+    assert _get_plugins_as_list(None) == []
+    assert _get_plugins_as_list('') == []
+    assert _get_plugins_as_list('foo') == ['foo']
+    assert _get_plugins_as_list('foo,bar') == ['foo', 'bar']
+    assert _get_plugins_as_list(['foo', 'bar']) == ['foo', 'bar']
+    assert _get_plugins_as_list(('foo', 'bar')) == ['foo', 'bar']
+
+
 class TestWarning:
     def test_warn_config(self, testdir):
         testdir.makeconftest("""