[Python-Dev] [UPDATE] PEP 274, Dict Comprehensions

[Guido van Rossum]

>     - There is one further shortcut we could adopt.  Suppose we wanted
>       to create a set of items, such as in the "list_of_email_addrs"
>       example above.  Here, we're simply taking the target of the for
>       loop and turning that into the key for the dict comprehension.

Hm, I don't like this.  I think it's confusing: you really have to
think about whether

    {x for x in <whatever>}

produces a dictionary whose keys are in <whatever> or one whose values
are in <whatever>.

>     - Should nested for loops be allowed?  The following example,
>       taken from an earlier revision of this PEP illustrates the
>       problem:
> 
>       >>> print {k, v for k in range(4) for v in range(-4, 0, 1)}

Don't you mean {k: v for ...}?  The second range() also seems like you
meant range(0, -4, -1).

>       The intent of this example was to produce a mapping from a
>       number to its negative, but this code doesn't work because -- as
>       in list comprehensions -- the for loops are nested, not in
>       parallel!  So the value of this expression is actually
> 
>       {0: -1, 1: -1, 2: -1, 3: -1}
> 
>       which seems of dubious value.  For symmetry with list
>       comprehensions, perhaps this should be allowed, but it might be
>       better to disallow this syntax.

Nested for loops can be useful when used properly:

    {(k, v): k+v for k in range(4) for v in range(4)}

Your example should have been expressed using:

    {k: v for k, v in zip(range(4), range(0, -4, -1))}

--Guido van Rossum (home page: http://www.python.org/~guido/)