[Chicago] I don't get why this works

[Scott Sexton]

All these are equivalent: a['a'] = a[a['a']] = 'b'
So that gives you two equalities:
    1: a['a'] = 'b'
    2: a[a['a']] = 'b'.
It's clear that for a['a'] = 'b', you'd expect {'a':'b'}.
Now in a[a['a']] = 'b', you can substitute the inside a['a'] (equality 1
above) for its value 'b'.
This gives you a['b'] = b
So that's why you get {'a':'b', 'b':'b'}




On Mon, Apr 1, 2013 at 10:24 PM, Brantley Harris <deadwisdom at gmail.com>wrote:

> > a = {}
> > a['a'] = a[a['a']] = 'b'
> {'a': 'b', 'b': 'b'}
>
> What the what?  I would think that should KeyError, but it runs fine.
>  Someone splain.
>
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