pop method question

Steven D'Aprano steve at REMOVE.THIS.cybersource.com.au
Sat Mar 3 23:55:16 CET 2007


On Sat, 03 Mar 2007 15:56:39 -0500, Nicholas Parsons wrote:

> 
> On Mar 3, 2007, at 3:49 PM, Paul Rubin wrote:
> 
>> Nicholas Parsons <parsons.nicholas1 at gmail.com> writes:
>>> I was just playing around in IDLE at the interactive prompt and typed
>>> in dir({}) for the fun of it.  I was quite surprised to see a pop
>>> method defined there.  I mean is that a misnomer or what?  From the
>>> literature, pop is supposed to be an operation defined for a stack
>>> data structure.  A stack is defined to be an "ordered" list data
>>> structure.  Dictionaries in Python have no order but are sequences.
>>> Now, does anyone know why the python core has this pop method
>>> implemented for a dictionary type?
>>
>> Try typing:
>>
>>   help({}.pop)
>> --  
>> http://mail.python.org/mailman/listinfo/python-list
> 
> Thanks, that gives a more details explanation of what the behavior is  
> but doesn't answer my question above :(

Just because pop can be defined for an ordered stack doesn't mean pop
can't be generalized to other data types too.

I personally don't see that pop has any advantage, especially since the
most useful example

while some_dict:
    do_something_with(some_dict.pop()) 

doesn't work. Instead you have to write this:

for key in some_dict.keys(): 
    # can't iterate over the dictionary directly!
    do_something_with(some_dict.pop(key))

which is hardly any saving over:

for key in some_dict.keys(): 
    # can't iterate over the dictionary directly!
    do_something_with(some_dict[key])
    del some_dict[key]


To my mind, having to supply a key to dict.pop makes it rather pointless.


-- 
Steven.




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