Add envcontext helper

pre_commit/envcontext.py

@@ -0,0 +1,54 @@
+from __future__ import absolute_import
+from __future__ import unicode_literals
+
+import collections
+import contextlib
+import os
+
+from pre_commit import five
+
+
+UNSET = collections.namedtuple('UNSET', ())()
+
+
+Var = collections.namedtuple('Var', ('name', 'default'))
+setattr(Var.__new__, five.defaults_attr, ('',))
+
+
+def format_env(parts, env):
+    return ''.join(
+        env.get(part.name, part.default)
+        if isinstance(part, Var)
+        else part
+        for part in parts
+    )
+
+
+@contextlib.contextmanager
+def envcontext(patch, _env=None):
+    """In this context, `os.environ` is modified according to `patch`.
+
+    `patch` is an iterable of 2-tuples (key, value):
+        `key`: string
+        `value`:
+            - string: `environ[key] == value` inside the context.
+            - UNSET: `key not in environ` inside the context.
+            - template: A template is a tuple of strings and Var which will be
+              replaced with the previous environment
+    """
+    env = os.environ if _env is None else _env
+    before = env.copy()
+
+    for k, v in patch:
+        if v is UNSET:
+            env.pop(k, None)
+        elif isinstance(v, tuple):
+            env[k] = format_env(v, before)
+        else:
+            env[k] = v
+
+    try:
+        yield
+    finally:
+        env.clear()
+        env.update(before)

pre_commit/five.py

@@ -12,6 +12,8 @@ if PY2:  # pragma: no cover (PY2 only)
             return s
         else:
             return s.encode('UTF-8')
+
+    defaults_attr = 'func_defaults'
 else:  # pragma: no cover (PY3 only)
     text = str
 
@@ -21,6 +23,8 @@ else:  # pragma: no cover (PY3 only)
         else:
             return s.decode('UTF-8')
 
+    defaults_attr = '__defaults__'
+
 
 def to_text(s):
     return s if isinstance(s, text) else s.decode('UTF-8')

tests/envcontext_test.py

@@ -0,0 +1,109 @@
+from __future__ import absolute_import
+from __future__ import unicode_literals
+
+import os
+
+import mock
+import pytest
+
+from pre_commit.envcontext import envcontext
+from pre_commit.envcontext import UNSET
+from pre_commit.envcontext import Var
+
+
+def _test(**kwargs):
+    before = kwargs.pop('before')
+    patch = kwargs.pop('patch')
+    expected = kwargs.pop('expected')
+    assert not kwargs
+
+    env = before.copy()
+    with envcontext(patch, _env=env):
+        assert env == expected
+    assert env == before
+
+
+def test_trivial():
+    _test(before={}, patch={}, expected={})
+
+
+def test_noop():
+    _test(before={'foo': 'bar'}, patch=(), expected={'foo': 'bar'})
+
+
+def test_adds():
+    _test(before={}, patch=[('foo', 'bar')], expected={'foo': 'bar'})
+
+
+def test_overrides():
+    _test(
+        before={'foo': 'baz'},
+        patch=[('foo', 'bar')],
+        expected={'foo': 'bar'},
+    )
+
+
+def test_unset_but_nothing_to_unset():
+    _test(before={}, patch=[('foo', UNSET)], expected={})
+
+
+def test_unset_things_to_remove():
+    _test(
+        before={'PYTHONHOME': ''},
+        patch=[('PYTHONHOME', UNSET)],
+        expected={},
+    )
+
+
+def test_templated_environment_variable_missing():
+    _test(
+        before={},
+        patch=[('PATH', ('~/bin:', Var('PATH')))],
+        expected={'PATH': '~/bin:'},
+    )
+
+
+def test_templated_environment_variable_defaults():
+    _test(
+        before={},
+        patch=[('PATH', ('~/bin:', Var('PATH', default='/bin')))],
+        expected={'PATH': '~/bin:/bin'},
+    )
+
+
+def test_templated_environment_variable_there():
+    _test(
+        before={'PATH': '/usr/local/bin:/usr/bin'},
+        patch=[('PATH', ('~/bin:', Var('PATH')))],
+        expected={'PATH': '~/bin:/usr/local/bin:/usr/bin'},
+    )
+
+
+def test_templated_environ_sources_from_previous():
+    _test(
+        before={'foo': 'bar'},
+        patch=(
+            ('foo', 'baz'),
+            ('herp', ('foo: ', Var('foo'))),
+        ),
+        expected={'foo': 'baz', 'herp': 'foo: bar'},
+    )
+
+
+def test_exception_safety():
+    class MyError(RuntimeError):
+        pass
+
+    env = {}
+    with pytest.raises(MyError):
+        with envcontext([('foo', 'bar')], _env=env):
+            raise MyError()
+    assert env == {}
+
+
+def test_integration_os_environ():
+    with mock.patch.dict(os.environ, {'FOO': 'bar'}, clear=True):
+        assert os.environ == {'FOO': 'bar'}
+        with envcontext([('HERP', 'derp')]):
+            assert os.environ == {'FOO': 'bar', 'HERP': 'derp'}
+        assert os.environ == {'FOO': 'bar'}