How to say $a=$b->{"A"} ||={} in Python?

[Paul McGuire]

On Aug 16, 8:28 pm, Jonathan Gardner
<jgardner.jonathangardner.... at gmail.com> wrote:
> On Aug 16, 3:35 pm, beginner <zyzhu2... at gmail.com> wrote:
>
>
>
> > In perl it is just one line: $a=$b->{"A"} ||={}.
>
> a = b.setdefault('A', {})
>
> This combines all two actions together:
> - Sets b['A'] to {} if it is not already defined
> - Assigns b['A'] to a
>
> More info on dict methods here:
>
> http://docs.python.org/lib/typesmapping.html

No, this has already been proposed and discarded.  The OP does NOT
want this, because it always generates an empty {} whether it is
needed or not.  Not really a big hardship, but if the default value
were some expensive-to-construct container class, then you would be
creating one every time you wanted to reference a value, on the chance
that the key did not exist.

Carl Banks' post using defaultdict is the correct solution.  The
raison d'etre for defaultdict, and the reason that it is the solution
to the OP's question, is that instead of creating a just-in-case
default value every time, the defaultdict itself is constructed with a
factory method of some kind (in practice, it appears that this factory
method is usually the list or dict class constructor).  If a reference
to the defaultdict gives a not-yet-existing key, then the factory
method is called to construct the new value, that value is stored in
the dict with the given key, and the value is passed back to the
caller.  No instances are created unless they are truly required for
initializing an entry for a never-before-seen key.

-- Paul