Force serial hook runs during tests

pre_commit/languages/helpers.py

@@ -50,7 +50,7 @@ def no_install(prefix, version, additional_dependencies):
 
 
 def target_concurrency(hook):
-    if hook['require_serial']:
+    if hook['require_serial'] or 'PRE_COMMIT_NO_CONCURRENCY' in os.environ:
         return 1
     else:
         # Travis appears to have a bunch of CPUs, but we can't use them all.

tests/languages/helpers_test.py

@@ -40,7 +40,15 @@ def test_target_concurrency_normal():
 
 
 def test_target_concurrency_cpu_count_require_serial_true():
-    assert helpers.target_concurrency({'require_serial': True}) == 1
+    with mock.patch.dict(os.environ, {}, clear=True):
+        assert helpers.target_concurrency({'require_serial': True}) == 1
+
+
+def test_target_concurrency_testing_env_var():
+    with mock.patch.dict(
+            os.environ, {'PRE_COMMIT_NO_CONCURRENCY': '1'}, clear=True,
+    ):
+        assert helpers.target_concurrency({'require_serial': False}) == 1
 
 
 def test_target_concurrency_on_travis():
@@ -52,4 +60,5 @@ def test_target_concurrency_cpu_count_not_implemented():
     with mock.patch.object(
             multiprocessing, 'cpu_count', side_effect=NotImplementedError,
     ):
-        assert helpers.target_concurrency({'require_serial': False}) == 1
+        with mock.patch.dict(os.environ, {}, clear=True):
+            assert helpers.target_concurrency({'require_serial': False}) == 1

tox.ini

@@ -27,3 +27,4 @@ env =
     GIT_AUTHOR_EMAIL=test@example.com
     GIT_COMMITTER_EMAIL=test@example.com
     VIRTUALENV_NO_DOWNLOAD=1
+    PRE_COMMIT_NO_CONCURRENCY=1