Popping key causes dict derived from object to revert to object

[avi.e.gross at gmail.com]

Lori,

The list comprehension you are thinking of does work if you change things a
bit. But it is not a great idea as a main purpose of a dict is that using a
hash means things are found in linear time.  A comprehension iterates on all
values. If you wanted to select just some items to keep in a list, your code
could be modified from:

dict_list = [d.pop('a') for d in dict_list]

to have an IF clause that would specify something like comparing it to the
item you do not want to keep.

But your idiom might be better done to make another dictionaly, not list
with something like:

New_dict = {key:value for key in dict if key != "whatever"}

Or variants on that. It builds a new dictionary, at nontrivial expense, as
compared to using del on an existing dictionary.

-----Original Message-----
From: Python-list <python-list-bounces+avi.e.gross=gmail.com at python.org> On
Behalf Of Loris Bennett via Python-list
Sent: Monday, March 25, 2024 2:56 AM
To: python-list at python.org
Subject: Re: Popping key causes dict derived from object to revert to object

Grant Edwards <grant.b.edwards at gmail.com> writes:

> On 2024-03-22, Loris Bennett via Python-list <python-list at python.org>
wrote:
>
>> Yes, I was mistakenly thinking that the popping the element would
>> leave me with the dict minus the popped key-value pair.
>
> It does.

Indeed, but I was thinking in the context of 

  dict_list = [d.pop('a') for d in dict_list]

and incorrectly expecting to get a list of 'd' without key 'a', instead
of a list of the 'd['a]'.

>> Seem like there is no such function.
>
> Yes, there is. You can do that with either pop or del:
>
>     >>> d = {'a':1, 'b':2, 'c':3}
>     >>> d
>     {'a': 1, 'b': 2, 'c': 3}
>     >>> d.pop('b')
>     2
>     >>> d
>     {'a': 1, 'c': 3}
>
>
>     >>> d = {'a':1, 'b':2, 'c':3}
>     >>> del d['b']
>     >>> d
>     {'a': 1, 'c': 3}
>
> In both cases, you're left with the dict minus the key/value pair.
>
> In the first case, the deleted value printed by the REPL because it
> was returned by the expression "d.pop('b')" (a method call).
>
> In the second case is no value shown by the REPL because "del d['b']"
> is a statement not an expression.

Thanks for pointing out 'del'.  My main problem, however, was failing to
realise that the list comprehension is populated by the return value of
the 'pop', not the popped dict.

Cheers,

Loris

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