Enable re-writing of setuptools-installed plugins

Hook up the PEP 302 import hook very early in pytest startup so
that it gets installed before setuptools-installed plugins are
imported.  Also iterate over all installed plugins and mark them
for rewriting.  If an installed plugin is already imported then
a warning is issued, we can not break since that might break
existing plugins and the fallback will still be gracefull to
plain asserts.

Some existing tests are failing in this commit because of the new
warning triggered by inline pytest runs due to the hypothesis
plugin already being imported.  The tests will be fixed in the next
commit.

_pytest/config.py

@@ -5,6 +5,7 @@ import traceback
 import types
 import warnings
 
+import pkg_resources
 import py
 # DON't import pytest here because it causes import cycle troubles
 import sys, os
@@ -918,14 +919,63 @@ class Config(object):
         self._parser.addini('addopts', 'extra command line options', 'args')
         self._parser.addini('minversion', 'minimally required pytest version')
 
+    def _consider_importhook(self, args, entrypoint_name):
+        """Install the PEP 302 import hook if using assertion re-writing.
+
+        Needs to parse the --assert=<mode> option from the commandline
+        and find all the installed plugins to mark them for re-writing
+        by the importhook.
+        """
+        import _pytest.assertion
+        ns, unknown_args = self._parser.parse_known_and_unknown_args(args)
+        mode = ns.assertmode
+        if ns.noassert or ns.nomagic:
+            mode = "plain"
+        self._warn_about_missing_assertion(mode)
+        if mode != 'plain':
+            hook = _pytest.assertion.install_importhook(self, mode)
+            if hook:
+                for entrypoint in pkg_resources.iter_entry_points('pytest11'):
+                    for entry in entrypoint.dist._get_metadata('RECORD'):
+                        fn = entry.split(',')[0]
+                        is_simple_module = os.sep not in fn and fn.endswith('.py')
+                        is_package = fn.count(os.sep) == 1 and fn.endswith('__init__.py')
+                        if is_simple_module:
+                            module_name, ext = os.path.splitext(fn)
+                            hook.mark_rewrite(module_name)
+                        elif is_package:
+                            package_name = os.path.dirname(fn)
+                            hook.mark_rewrite(package_name)
+
+    def _warn_about_missing_assertion(self, mode):
+        try:
+            assert False
+        except AssertionError:
+            pass
+        else:
+            if mode == "rewrite":
+                specifically = ("assertions not in test modules or plugins"
+                                "will be ignored")
+            else:
+                specifically = "failing tests may report as passing"
+            sys.stderr.write("WARNING: " + specifically +
+                             " because assert statements are not executed "
+                             "by the underlying Python interpreter "
+                             "(are you using python -O?)\n")
+
     def _preparse(self, args, addopts=True):
         self._initini(args)
         if addopts:
             args[:] = shlex.split(os.environ.get('PYTEST_ADDOPTS', '')) + args
             args[:] = self.getini("addopts") + args
         self._checkversion()
+        entrypoint_name = 'pytest11'
+        self._consider_importhook(args, entrypoint_name)
         self.pluginmanager.consider_preparse(args)
-        self.pluginmanager.load_setuptools_entrypoints("pytest11")
+        try:
+            self.pluginmanager.load_setuptools_entrypoints(entrypoint_name)
+        except ImportError as e:
+            self.warn("I2", "could not load setuptools entry import: %s" % (e,))
         self.pluginmanager.consider_env()
         self.known_args_namespace = ns = self._parser.parse_known_args(args, namespace=self.option.copy())
         if self.known_args_namespace.confcutdir is None and self.inifile: