use presence of activate script rather than sys.prefix to determine i…

…f a dir is a virtualenv

_pytest/main.py

@@ -169,6 +169,17 @@ def pytest_runtestloop(session):
     return True
 
 
+def _in_venv(path):
+    """Attempts to detect if ``path`` is the root of a Virtual Environment by
+    checking for the existence of the appropriate activate script"""
+    bindir = path.join('Scripts' if sys.platform.startswith('win') else 'bin')
+    if not bindir.exists():
+        return False
+    activates = ('activate', 'activate.csh', 'activate.fish',
+                 'Activate', 'Activate.bat', 'Activate.ps1')
+    return any([fname.basename in activates for fname in bindir.listdir()])
+
+
 def pytest_ignore_collect(path, config):
     ignore_paths = config._getconftest_pathlist("collect_ignore", path=path.dirpath())
     ignore_paths = ignore_paths or []
@@ -179,11 +190,10 @@ def pytest_ignore_collect(path, config):
     if py.path.local(path) in ignore_paths:
         return True
 
-    invenv =  py.path.local(sys.prefix) == path
-    allow_invenv = config.getoption("collect_in_virtualenv")
-    if invenv and not allow_invenv:
+    allow_in_venv = config.getoption("collect_in_virtualenv")
+    if _in_venv(path) and not allow_in_venv:
         config.warn(RuntimeWarning,
-            'Path "%s" appears to be a Python installation; skipping\n'
+            'Path "%s" appears to be a Python virtual installation; skipping\n'
             'Pass --collect-in-virtualenv to force collection of tests in "%s"\n'
             'Use --ignore="%s" to silence this warning' % (path, path, path)
         )