Handle annotated assignment in 3.6

In 3.6, annotated assignment was added, which takes the form of `a: int = 0`.
This would cause errors, since `:` is an operator, and its child would contain
nodes of type `annassign`.

We handle these the same way that we handle normal assignments, but by dropping
into the assignment logic at an offset in the expression we're handling.

mutmut/__init__.py

@@ -169,8 +169,7 @@ def and_or_test_mutation(children, node, **_):
 
 
 def expression_mutation(children, **_):
-    assert children[1].type == 'operator'
-    if children[1].value == '=':
+    def handle_assignment(children):
         if getattr(children[2], 'value', '---') != 'None':
             x = ' None'
         else:
@@ -179,6 +178,14 @@ def expression_mutation(children, **_):
         from parso.python.tree import Name
         children[2] = Name(value=x, start_pos=children[2].start_pos)
 
+        return children
+
+    if children[0].type == 'operator' and children[0].value == ':':
+        if children[2].value == '=':
+            children[1:] = handle_assignment(children[1:])
+    elif children[1].type == 'operator' and children[1].value == '=':
+        children = handle_assignment(children)
+
     return children
 
 
@@ -206,6 +213,7 @@ def decorator_mutation(children, **_):
     'lambdef': dict(children=lambda_mutation),
     'expr_stmt': dict(children=expression_mutation),
     'decorator': dict(children=decorator_mutation),
+    'annassign': dict(children=expression_mutation),
 }
 
 # TODO: detect regexes and mutate them in nasty ways? Maybe mutate all strings as if they are regexes

tests/test_main.py

@@ -14,6 +14,7 @@
     "e = 1",
     "f = 3",
     "d = dict(e=f)",
+    "g: int = 2",
 ]
 file_to_mutate_contents = '\n'.join(file_to_mutate_lines) + '\n'
 
@@ -27,6 +28,7 @@ def test_foo():
    assert e == 1
    assert f == 3
    assert d == dict(e=f)
+   assert g == 2
 '''
 
 
@@ -58,7 +60,7 @@ def test_simple_apply():
 def test_full_run_no_surviving_mutants():
     result = CliRunner().invoke(main, ['foo.py'], catch_exceptions=False)
     print(repr(result.output))
-    assert result.output == u'Running tests without mutations... Done\n--- starting mutation ---\n\r0 out of 7  (foo.py)\r1 out of 7  (foo.py    return a < b⤑0)\r2 out of 7  (foo.py e = 1⤑0)          \r3 out of 7  (foo.py e = 1⤑1)\r4 out of 7  (foo.py f = 3⤑0)\r5 out of 7  (foo.py f = 3⤑1)\r6 out of 7  (foo.py d = dict(e=f)⤑0)\r7 out of 7  (foo.py d = dict(e=f)⤑1)'
+    assert result.output == u'Running tests without mutations... Done\n--- starting mutation ---\n\r0 out of 8  (foo.py)\r1 out of 8  (foo.py    return a < b⤑0)\r2 out of 8  (foo.py e = 1⤑0)          \r3 out of 8  (foo.py e = 1⤑1)\r4 out of 8  (foo.py f = 3⤑0)\r5 out of 8  (foo.py f = 3⤑1)\r6 out of 8  (foo.py d = dict(e=f)⤑0)\r7 out of 8  (foo.py d = dict(e=f)⤑1)\r8 out of 8  (foo.py g: int = 2⤑0)   '
 
 
 @pytest.mark.usefixtures('filesystem')
@@ -68,7 +70,7 @@ def test_full_run_one_surviving_mutant():
 
     result = CliRunner().invoke(main, ['foo.py'], catch_exceptions=False)
     print(repr(result.output))
-    assert result.output == u'Running tests without mutations... Done\n--- starting mutation ---\n\r0 out of 7  (foo.py)                \r1 out of 7  (foo.py    return a < b⤑0)\r                                      \rFAILED: mutmut foo.py --apply --mutation "   return a < b⤑0"\n\r2 out of 7  (foo.py e = 1⤑0)\r3 out of 7  (foo.py e = 1⤑1)\r4 out of 7  (foo.py f = 3⤑0)\r5 out of 7  (foo.py f = 3⤑1)\r6 out of 7  (foo.py d = dict(e=f)⤑0)\r7 out of 7  (foo.py d = dict(e=f)⤑1)'
+    assert result.output == u'Running tests without mutations... Done\n--- starting mutation ---\n\r0 out of 8  (foo.py)             \r1 out of 8  (foo.py    return a < b⤑0)\r                                      \rFAILED: mutmut foo.py --apply --mutation "   return a < b⤑0"\n\r2 out of 8  (foo.py e = 1⤑0)\r3 out of 8  (foo.py e = 1⤑1)\r4 out of 8  (foo.py f = 3⤑0)\r5 out of 8  (foo.py f = 3⤑1)\r6 out of 8  (foo.py d = dict(e=f)⤑0)\r7 out of 8  (foo.py d = dict(e=f)⤑1)\r8 out of 8  (foo.py g: int = 2⤑0)   '
 
 
 @pytest.mark.usefixtures('filesystem')

tests/test_mutation.py

@@ -46,6 +46,8 @@
         ('a = {x for x in y}', 'a = None'),
         ('a = None', 'a = 7'),
         ('break', 'continue'),
+        ('a: int = 1', 'a: int = None'),
+        ('a: Optional[int] = None', 'a: Optional[int] = 7'),
     ]
 )
 def test_basic_mutations(original, expected):