@@ -69,7 +69,25 @@ class Key(AnyType):
6969
7070
7171class ConditionalTypeBinder :
72- """Keep track of conditional types of variables."""
72+ """Keep track of conditional types of variables.
73+
74+ NB: Variables are tracked by literal expression, so it is possible
75+ to confuse the binder; for example,
76+
77+ ```
78+ class A:
79+ a = None # type: Union[int, str]
80+ x = A()
81+ lst = [x]
82+ reveal_type(x.a) # Union[int, str]
83+ x.a = 1
84+ reveal_type(x.a) # int
85+ reveal_type(lst[0].a) # Union[int, str]
86+ lst[0].a = 'a'
87+ reveal_type(x.a) # int
88+ reveal_type(lst[0].a) # str
89+ ```
90+ """
7391
7492 def __init__ (self ) -> None :
7593 # The set of frames currently used. These map
@@ -88,10 +106,13 @@ def __init__(self) -> None:
88106 # Whenever a new key (e.g. x.a.b) is added, we update this
89107 self .dependencies = {} # type: Dict[Key, Set[Key]]
90108
91- # Set to True on return/break/raise, False on blocks that can block any of them
109+ # breaking_out is set to True on return/break/continue/raise
110+ # It is cleared on pop_frame() and placed in last_pop_breaking_out
111+ # Lines of code after breaking_out = True are unreachable and not
112+ # typechecked.
92113 self .breaking_out = False
93114
94- # Whether the last pop changed the top frame on exit
115+ # Whether the last pop changed the newly top frame on exit
95116 self .last_pop_changed = False
96117 # Whether the last pop was necessarily breaking out, and couldn't fall through
97118 self .last_pop_breaking_out = False
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