[issue40809] list.Count() isn't working as expected for the series of same numbers in a list

[Steven D'Aprano]

Steven D'Aprano  added the comment:

Typo:

"on each step, you delete one item from the from and step forward"

Should be, you delete one item from the FRONT and step forward.

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[issue40809] list.Count() isn't working as expected for the series of same numbers in a list

[Steven D'Aprano]

Steven D'Aprano  added the comment:

Rémi is correct, this is not a bug.

The problem isn't with list.count. If you print list.count each time through 
the loop, you will see that it is working perfectly. 

The problem is that you are modifying the list as you are iterating over it. 
That means that you skip every second item. It isn't easy to see when all the 
items are the same, but we can alternate int and float values (which are equal) 
to see how every second value is skipped.


py> a = [1, 1.0, 1, 1.0, 1, 1.0, 1, 1.0]
py> for i in a:
... print(a, i, a.count(i))
... if a.count(i) > 1:
... a.remove(i)
...
[1, 1.0, 1, 1.0, 1, 1.0, 1, 1.0] 1 8
[1.0, 1, 1.0, 1, 1.0, 1, 1.0] 1 7
[1, 1.0, 1, 1.0, 1, 1.0] 1 6
[1.0, 1, 1.0, 1, 1.0] 1 5
py> a
[1, 1.0, 1, 1.0]


Notice that every one of the float 1.0 values gets skipped. That's because when 
you resize the list as you walk over it, the value which *would have been* the 
next value gets pushed back into the current position, and then you advance to 
the next position, and skip it.

So there's no bug in list.count, it is working perfectly each time. There's no 
bug in iteration either, it is walking the list correctly too. But there is a 
bug in your code: you are shrinking the list while walking over it, so on each 
step, you delete one item from the from and step forward, so you skip items.

The lesson here is not to delete items from a list while you are iterating over 
it.

Technically, it's okay to delete items so long as they are *ahead* of the 
current item, but not if they are *behind* it. But it is easiest to just use 
the rule, never delete items while iterating over the same list.

If you must delete items from a list, iterate over a copy.

--
nosy: +steven.daprano
resolution:  -> not a bug
stage:  -> resolved
status: open -> closed

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[issue40809] list.Count() isn't working as expected for the series of same numbers in a list

[chirag maliwal]

chirag maliwal  added the comment:

a = [0, 1, 2, 3, 4, 1, 2, 3, 5]

for i in a:
if a.count(i) > 1:
a.remove(i)

print(a)

"""
Output

[0, 4, 1, 2, 3, 5]
"""

It's working fine for the above code.
Yes I can use set for this but I tried with count and remove.

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[issue40809] list.Count() isn't working as expected for the series of same numbers in a list

[Rémi Lapeyre]

Rémi Lapeyre  added the comment:

Hi cmaliwal! You are mutating the list while iterating over it which can lead 
to all sorts of issues, to keep the unique elements of a list you can do 


>>> a = [1, 1, 1, 1]
>>> list(set(a))
[1]


If you think you found a bug in Python please check in the python-help mailing 
list or StackOverflow.

--
nosy: +remi.lapeyre
title: list.count() isn't working as expected for the series of same numbers in 
a list -> list.Count() isn't working as expected for the series of same numbers 
in a list

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[issue40809] list.count() isn't working as expected for the series of same numbers in a list

[chirag maliwal]

Change by chirag maliwal :


--
title: list.Count() isn't working as expected for the series of same numbers in 
a list -> list.count() isn't working as expected for the series of same numbers 
in a list

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[issue40809] list.Count() isn't working as expected for the series of same numbers in a list

[chirag maliwal]

New submission from chirag maliwal :

a = [1,1,1,1]

for i in a:
if a.count(i) > 1:
a.remove(i)
 
print(a)


"""
Output:
[1,1]


expected:
[1]
"""

--
files: test.py
messages: 370239
nosy: cmaliwal
priority: normal
severity: normal
status: open
title: list.Count() isn't working as expected for the series of same numbers in 
a list
type: behavior
versions: Python 3.6
Added file: https://bugs.python.org/file49200/test.py

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