allow opt-in for using venv over virtualenv

pre_commit/languages/python.py

@@ -114,23 +114,29 @@ def healthy(prefix, language_version):
 
 
 def norm_version(version):
+    if version.endswith('+venv'):
+        use_venv = True
+        version = version[:-5]
+    else:
+        use_venv = False
     if os.name == 'nt':  # pragma: no cover (windows)
         # Try looking up by name
         version_exec = find_executable(version)
         if version_exec and version_exec != version:
-            return version_exec
+            return version_exec, use_venv
 
         version_exec = _find_by_py_launcher(version)
         if version_exec:
-            return version_exec
+            return version_exec, use_venv
 
         # If it is in the form pythonx.x search in the default
         # place on windows
         if version.startswith('python'):
-            return r'C:\{}\python.exe'.format(version.replace('.', ''))
+            path = r'C:\{}\python.exe'.format(version.replace('.', ''))
+            return path, use_venv
 
         # Otherwise assume it is a path
-    return os.path.expanduser(version)
+    return os.path.expanduser(version), use_venv
 
 
 def install_environment(prefix, version, additional_dependencies):
@@ -141,25 +147,22 @@ def install_environment(prefix, version, additional_dependencies):
     env_dir = prefix.path(directory)
     with clean_path_on_failure(env_dir):
         if version != 'default':
-            target_python = norm_version(version)
+            target_python, use_venv = norm_version(version)
         else:
             target_python = os.path.realpath(sys.executable)
+            use_venv = False
 
-        try:
-            subprocess.check_call(
-                [target_python, '-c', 'import venv'],
-                stderr=devnull,
-                stdout=devnull,
-            )
-            venv_python = target_python
-            venv_module = 'venv'
-            extra_cmd = []
-        except subprocess.CalledProcessError:
-            venv_python = sys.executable
-            venv_module = 'virtualenv'
-            extra_cmd = ['-p', target_python]
-
-        venv_cmd = [venv_python, '-m', venv_module, env_dir] + extra_cmd
+        if use_venv:
+            venv_cmd = [target_python, '-m', 'venv', env_dir]
+        else:
+            venv_cmd = [
+                sys.executable,
+                '-m',
+                'virtualenv',
+                '-p',
+                target_python,
+                env_dir,
+            ]
         venv_env = dict(os.environ, VIRTUALENV_NO_DOWNLOAD='1')
         cmd_output(*venv_cmd, cwd='/', env=venv_env)
         with in_env(prefix, version):

tests/languages/python_test.py

@@ -3,6 +3,8 @@ from __future__ import unicode_literals
 
 import os.path
 
+import pytest
+
 from pre_commit.languages import python
 
 
@@ -15,4 +17,14 @@ def test_norm_version_expanduser():
         path = '~/.pyenv/versions/3.4.3/bin/python'
         expected_path = home + '/.pyenv/versions/3.4.3/bin/python'
     result = python.norm_version(path)
-    assert result == expected_path
+    assert result == (expected_path, False)
+
+
+@pytest.mark.parametrize(
+    'version,use_venv', [
+        ('python3.6', False),
+        ('python3.6+venv', True),
+    ],
+)
+def test_norm_version_venv(version, use_venv):
+    assert python.norm_version(version)[1] == use_venv