Exceptions, assigning a tuple

[Erik Max Francis]

Derek Fountain wrote:

> OK, that makes a lot of sense. However, the thing being unpacked here
> isn't
> a tuple - it's an object of type IOError. How does an object know how
> to
> unpack itself when used in an assignment like this? More specifically,
> how
> does the IOError object know to return its errno and strerror values
> when
> it gets assigned to a 2 value tuple?

Because the Exception class, which IOError is derived from, works that
way:

>>> e = Exception(1, 2, 3)
>>> tuple(e)
(1, 2, 3)
>>> x, y, z = e
>>> x 
1
>>> y
2
>>> z
3

The "exception parameter" is an exception object, which knows how to
behave like a tuple.

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