Interpret strings to "plugins" arg in pytest.main() as module names

See #855

CHANGELOG

@@ -4,6 +4,11 @@
 - fix issue856: consider --color parameter in all outputs (for example
   --fixtures). Thanks Barney Gale for the report and Bruno Oliveira for the PR.
 
+- fix issue855: passing str objects as `plugins` argument to pytest.main
+  is now interpreted as a module name to be imported and registered as a
+  plugin, instead of silently having no effect.
+  Thanks xmo-odoo for the report and Bruno Oliveira for the PR.
+
 - fix issue744: fix for ast.Call changes in Python 3.5+.  Thanks
   Guido van Rossum, Matthias Bussonnier, Stefan Zimmermann and
   Thomas Kluyver.

_pytest/config.py

@@ -80,7 +80,10 @@ def _prepareconfig(args=None, plugins=None):
     try:
         if plugins:
             for plugin in plugins:
-                pluginmanager.register(plugin)
+                if isinstance(plugin, py.builtin._basestring):
+                    pluginmanager.consider_pluginarg(plugin)
+                else:
+                    pluginmanager.register(plugin)
         return pluginmanager.hook.pytest_cmdline_parse(
                 pluginmanager=pluginmanager, args=args)
     except Exception:

testing/acceptance_test.py

@@ -1,3 +1,4 @@
+import sys
 import py, pytest
 
 class TestGeneralUsage:
@@ -370,6 +371,21 @@ class TestGeneralUsage:
             "*fixture 'invalid_fixture' not found",
         ])
 
+    def test_plugins_given_as_strings(self, tmpdir, monkeypatch):
+        """test that str values passed to main() as `plugins` arg
+        are interpreted as module names to be imported and registered.
+        #855.
+        """
+        with pytest.raises(ImportError) as excinfo:
+            pytest.main([str(tmpdir)], plugins=['invalid.module'])
+        assert 'invalid' in str(excinfo.value)
+
+        p = tmpdir.join('test_test_plugins_given_as_strings.py')
+        p.write('def test_foo(): pass')
+        mod = py.std.types.ModuleType("myplugin")
+        monkeypatch.setitem(sys.modules, 'myplugin', mod)
+        assert pytest.main(args=[str(tmpdir)], plugins=['myplugin']) == 0
+
 
 class TestInvocationVariants:
     def test_earlyinit(self, testdir):