Implement a simplified xargs in python

tests/xargs_test.py

@@ -0,0 +1,47 @@
+from __future__ import absolute_import
+from __future__ import unicode_literals
+
+import pytest
+
+from pre_commit import xargs
+
+
+def test_partition_trivial():
+    assert xargs.partition(('cmd',), ()) == (('cmd',),)
+
+
+def test_partition_simple():
+    assert xargs.partition(('cmd',), ('foo',)) == (('cmd', 'foo'),)
+
+
+def test_partition_limits():
+    ret = xargs.partition(
+        ('ninechars',), (
+            # Just match the end (with spaces)
+            '.' * 5, '.' * 4,
+            # Just match the end (single arg)
+            '.' * 10,
+            # Goes over the end
+            '.' * 5,
+            '.' * 6,
+        ),
+        _max_length=20,
+    )
+    assert ret == (
+        ('ninechars', '.' * 5, '.' * 4),
+        ('ninechars', '.' * 10),
+        ('ninechars', '.' * 5),
+        ('ninechars', '.' * 6),
+    )
+
+
+def test_argument_too_long():
+    with pytest.raises(xargs.ArgumentTooLongError):
+        xargs.partition(('a' * 5,), ('a' * 5,), _max_length=10)
+
+
+def test_xargs_smoke():
+    ret, out, err = xargs.xargs(('echo',), ('hello', 'world'))
+    assert ret == 0
+    assert out == b'hello world\n'
+    assert err == b''