[Tutor] removing consecutive duplicates from list

[Peter Otten]

On 21/04/2021 01:10, dn via Tutor wrote:
> On 21/04/2021 05.27, Peter Otten wrote:
>> On 20/04/2021 18:48, Manprit Singh wrote:
>>
>>> Consider a list given below:
>>> lst = [2, 3, 3, 4, 5, 5, 3, 7, 9, 9, 4]
>>> i need to remove consecutive duplicates from list lst:
>>> the answer must be :
>>>
>>> [2, 3, 4, 5, 3, 7, 9, 4]
>>>
>>> The code that i have written to solve it, is written below:
>>> lst = [2, 3, 3, 4, 5, 5, 3, 7, 9, 9, 4]
>>> ls = lst[1:]+[object()]
>>> [x for x, y in zip(lst, ls) if x != y]
>>>
>>> The list comprehension gives the desired result. just need to know if
>>> this
>>>
>>> program can be done in a more readable and less complex way.
> 
> 
>> The itertools module has many tools that you can use to deal with
>> problems like the above:
> ...
> 
>>>>> from itertools import groupby
>>>>> wanted == [k for k, g in groupby(lst)]
>> True
>>
>> Pick your favourite ;)
> 
> 
> Whilst, admiring your (@Manprit) one-liner and @Peter's grasp of the
> itertools library; am imagining yellow, if not red, flags.
> 
> Taking the term "less complex", let's ask: how many programmers
> (including yourself in six months' time) will be able to scan any of
> these and conclude?deduce that their objective is the removal of
> consecutive duplicates?
> 
> At the very least, please enclose your 'brilliance' within a function,
> which will give it a (better than a comment) label/name, eg:
> 
> def remove_consecutive_duplicates( source ):
>      return [k for k, g in groupby(lst)]
> 
> 
> Ultimately, I favor @Alan's "explicit form" approach (shhhh! Don't tell
> him, else he'll fall off his chair...).
> 
> Here's me dusting-off an old solution to this problem, which we used to
> give ComSc students - updated to (I think) v3.9+:
> 
> 
> 
> from typing import Iterable
> 
> lst = [2, 3, 3, 4, 5, 5, 3, 7, 9, 9, 4]
> result = [2, 3, 4, 5, 3, 7, 9, 4]
> 
> def remove_consecutive_duplicates( source:Iterable )->list:
>      """Clean input iterable (containing any data-types),
>         removing consecutive duplicates.
>      """
>      cleaned = list()
>      current_value = None
>      for this_element in source:
>          if this_element != current_value:
>              cleaned.append( this_element )
>              current_value = this_element
>      return cleaned
> 
> print( "Correct?", result == remove_consecutive_duplicates( lst ) )
> 
> 
> Plus:
> 
> remove_consecutive_duplicates( "abba" ) == ['a', 'b', 'a']

I am going to suggest you change that into a generator, like

def remove_consecutive_duplicates(items):
     """Remove consecutive duplicates.

     >>> list(remove_consecutive_duplicates(""))
     []
     >>> list(remove_consecutive_duplicates("abba"))
     ['a', 'b', 'a']
     >>> list(remove_consecutive_duplicates(
     ... [1, 1.0, "x", "x", "x", "y", 2.0, 2])
     ... )
     [1, 'x', 'y', 2.0]
     >>> list(remove_consecutive_duplicates(iter("aabbbc")))
     ['a', 'b', 'c']
     >>> list(remove_consecutive_duplicates([None]*3))
     [None]
     """
     it = iter(items)
     try:
         prev = next(it)
     except StopIteration: I hate you, Chris A. ;)
         return
     yield prev
     for item in it:
         if prev != item:
             yield item
             prev = item