Invoke -mvenv with the original python if in a -mvirtualenv venv

pre_commit/languages/python_venv.py

@@ -1,14 +1,46 @@
 from __future__ import unicode_literals
 
+import os.path
+
 from pre_commit.languages import python
+from pre_commit.util import CalledProcessError
 from pre_commit.util import cmd_output
 
 
 ENVIRONMENT_DIR = 'py_venv'
 
 
+def orig_py_exe(exe):  # pragma: no cover (platform specific)
+    """A -mvenv virtualenv made from a -mvirtualenv virtualenv installs
+    packages to the incorrect location.  Attempt to find the _original_ exe
+    and invoke `-mvenv` from there.
+
+    See:
+    - https://github.com/pre-commit/pre-commit/issues/755
+    - https://github.com/pypa/virtualenv/issues/1095
+    - https://bugs.python.org/issue30811
+    """
+    try:
+        prefix_script = 'import sys; print(sys.real_prefix)'
+        _, prefix, _ = cmd_output(exe, '-c', prefix_script)
+        prefix = prefix.strip()
+    except CalledProcessError:
+        # not created from -mvirtualenv
+        return exe
+
+    if os.name == 'nt':
+        expected = os.path.join(prefix, 'python.exe')
+    else:
+        expected = os.path.join(prefix, 'bin', os.path.basename(exe))
+
+    if os.path.exists(expected):
+        return expected
+    else:
+        return exe
+
+
 def make_venv(envdir, python):
-    cmd_output(python, '-mvenv', envdir, cwd='/')
+    cmd_output(orig_py_exe(python), '-mvenv', envdir, cwd='/')
 
 
 get_default_version = python.get_default_version