From 2a14bc7bb3e523d74c9abe5246303976c013d91a Mon Sep 17 00:00:00 2001
From: Yury Selivanov <yury@magic.io>
Date: Tue, 20 Nov 2018 13:15:08 -0500
Subject: Make MapMutation a context manager

---
 immutables/_map.c | 21 ++++++++++++++++++++-
 immutables/map.py |  7 +++++++
 tests/test_map.py | 21 +++++++++++++++++++++
 3 files changed, 48 insertions(+), 1 deletion(-)

diff --git a/immutables/_map.c b/immutables/_map.c
index 1abf730..9f57582 100644
--- a/immutables/_map.c
+++ b/immutables/_map.c
@@ -3850,7 +3850,6 @@ mapmut_tp_richcompare(PyObject *v, PyObject *w, int op)
     }
 }
 
-
 static PyObject *
 mapmut_py_finalize(MapMutationObject *self, PyObject *args)
 {
@@ -3868,6 +3867,24 @@ mapmut_py_finalize(MapMutationObject *self, PyObject *args)
     return (PyObject *)o;
 }
 
+static PyObject *
+mapmut_py_enter(MapMutationObject *self, PyObject *args)
+{
+    Py_INCREF(self);
+    return (PyObject *)self;
+}
+
+static PyObject *
+mapmut_py_exit(MapMutationObject *self, PyObject *args)
+{
+    PyObject *ret = mapmut_py_finalize(self, NULL);
+    if (ret == NULL) {
+        return NULL;
+    }
+    Py_DECREF(ret);
+    Py_RETURN_FALSE;
+}
+
 static int
 mapmut_tp_ass_sub(MapMutationObject *self, PyObject *key, PyObject *val)
 {
@@ -3951,6 +3968,8 @@ static PyMethodDef MapMutation_methods[] = {
     {"get", (PyCFunction)map_py_get, METH_VARARGS, NULL},
     {"pop", (PyCFunction)mapmut_py_pop, METH_VARARGS, NULL},
     {"finalize", (PyCFunction)mapmut_py_finalize, METH_NOARGS, NULL},
+    {"__enter__", (PyCFunction)mapmut_py_enter, METH_NOARGS, NULL},
+    {"__exit__", (PyCFunction)mapmut_py_exit, METH_VARARGS, NULL},
     {NULL, NULL}
 };
 
diff --git a/immutables/map.py b/immutables/map.py
index abfe9ed..bea7ad9 100644
--- a/immutables/map.py
+++ b/immutables/map.py
@@ -633,6 +633,13 @@ class MapMutation:
     def set(self, key, val):
         self[key] = val
 
+    def __enter__(self):
+        return self
+
+    def __exit__(self, *exc):
+        self.finalize()
+        return False
+
     def __delitem__(self, key):
         if self.__mutid == 0:
             raise ValueError(f'mutation {self!r} has been finalized')
diff --git a/tests/test_map.py b/tests/test_map.py
index 4fc87e3..d3b11fb 100644
--- a/tests/test_map.py
+++ b/tests/test_map.py
@@ -1174,6 +1174,27 @@ class BaseMapTest:
             with self.assertRaises(EqError):
                 mm.set(key2, 123)
 
+    def test_map_mut_14(self):
+        m = self.Map(a=1, b=2)
+
+        with m.mutate() as mm:
+            mm['z'] = 100
+            del mm['a']
+
+        self.assertEqual(mm.finalize(), self.Map(z=100, b=2))
+
+    def test_map_mut_15(self):
+        m = self.Map(a=1, b=2)
+
+        with self.assertRaises(ZeroDivisionError):
+            with m.mutate() as mm:
+                mm['z'] = 100
+                del mm['a']
+                1 / 0
+
+        self.assertEqual(mm.finalize(), self.Map(z=100, b=2))
+        self.assertEqual(m, self.Map(a=1, b=2))
+
     def test_map_mut_stress(self):
         COLLECTION_SIZE = 7000
         TEST_ITERS_EVERY = 647
-- 
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