[issue17973] '+=' on a list inside tuple both succeeds and raises an exception

[Mark Dickinson]

Mark Dickinson added the comment:

@andy.chugunov: tuples are immutable in the sense that you can't put a new object into a tuple or remove objects from a tuple.  That doesn't mean that tuples can't contain mutable objects, or prevent you from mutating the objects inside a tuple.

So the append method call is fine:  it's not modifying the tuple itself (the tuple still has references to exactly the same objects both before and after the append call);  it's merely mutating one the objects inside the tuple.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue17973>
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