How complex is complex?

[Terry Reedy]

Vito De Tullio wrote:
> Tim Roberts wrote:
> 
>> bearophileHUGS at lycos.com wrote:
>>> In Python 3 those lines become shorter:
>>>
>>> for k, v in a.items():
>>> {k: v+1 for k, v in a.items()}

This is nonsensical.  It creates and discards a complete new dict for 
each item in the original dict.  The reuse of names 'k' and 'v' in the 
comprehension just confuse.

>> That's a syntax I have not seen in the 2-to-3 difference docs, so I'm not
>> familiar with it.  How does that cause "a" to be updated?

It does not.

> I think he would write
> 
>>>> a = { 'a': 4, 'c': 6, 'b': 5 }
>>>> a = { k:v+1 for k, v in a.items() }
>>>> a
> {'a': 5, 'c': 7, 'b': 6}

This *replaces* original dict a with a new dict rather than updating 
(its values) in place.  This is less efficient.  If there are other 
references to the original dict, the rebinding may or may not be correct.

I believe the in-place update was already given as:
for k,v in a.items():  a[k] = v+1 # or
for k in a.keys(): a[k] += 1


Terry Jan Reedy