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From: <avi.e.gross@gmail.com>
Newsgroups: comp.lang.python
Subject: RE: Popping key causes dict derived from object to revert to object
Date: Mon, 25 Mar 2024 12:04:48 -0400
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I am glad, Lori, you found a solution another way.

Actually, Lori, I think you were right in looking for a built-in method =
that complements pop() by returning everything else other than the item =
mentioned. There are philosophical and practical considerations that =
were no doubt considered and a reality that the functionality did exist =
albeit not in a pipelined format.

Consider a language like LISP which sort of processed lists of things in =
which a major concept was getting the first item or getting all the =
rest. Lots of LISP programs had complex structures using CAR() and CDR() =
nested various ways to say extract the third item as CAR(CDR(CDR(X))) to =
the point where some commonly used combos became functions with names =
like CADDR().

There are again many languages where functions or methods are available =
that include an exclusion variant from a collection perhaps by doing =
something as simple as using a minus sign to indicate what to remove, as =
is commonly done in R to remove a column in a data.frame while keeping =
the order of the remaining columns the same.

Lots of languages had similar concepts about ordered data structures but =
dictionaries may be a bit of something else and initially in Python were =
not guaranteed to have any kind of order. Python dicts are more like =
unordered sets.=20

So although there remains a concept of not first/rest but this/rest,  I =
suspect there was some thought about the process that ended in deciding =
not to supply some functionality. When you use pop() on something like a =
large dictionary, the original is left intact and is ignored and a copy =
of a single element is made and returned. To do  the opposite and return =
the rest has two choices. One is to make a big copy of the rest of the =
dictionary and the other is to use del internally and return the =
modified dict. The functions you could write do the latter.

So why not add one or more methods to do that? Who knows? But I think =
some may have considered it not needed including some who felt no need =
for a pipeline method when del would do. Another consideration was the =
common idiom for iterating on a collection. Besides pop() you can get =
lists of dictionary entries, keys or values that you can work with and =
you can even iterate with "for key in dict: ..."

Given how many ways common things can be done, and given that adding too =
many methods has costs including new users not understanding all the =
nuanced differences, this fairly useful functionality was left out.

Unfortunately, I think they were wrong here as instead we hear often =
from people like you who assume things would work other ways. I still =
think it would be simple enough to have had a .removekeys(keys) that =
would work in a pipeline to modify the dict by removing one or more =
items and perhaps another .removevalues(values) but at some point you =
may keep adding methods nobody ever uses. The reality is that many =
trivial one-liner comprehensions can easily do many such things using =
iteration.=20

But many simply do not work well in pipelined fashion and thus may need =
to be embedded in a method of your own by subclassing dict or rolling =
your own.



-----Original Message-----
From: Loris Bennett <loris.bennett@fu-berlin.de>=20
Sent: Monday, March 25, 2024 2:45 AM
To: avi.e.gross@gmail.com
Cc: python-list@python.org
Subject: Re: Popping key causes dict derived from object to revert to =
object

<avi.e.gross@gmail.com> writes:

> Loris wrote:
>
> "Yes, I was mistakenly thinking that the popping the element would =
leave
> me with the dict minus the popped key-value pair.  Seem like there is =
no
> such function."
>
> Others have tried to explain and pointed out you can del and then use =
the
> changed dict.
>
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