Comparing lists in python gets weird

Question

The following piece of code removes an element of a list and than compares the two lists and should print the element that was removed (item#1)

old = generateList()  #same list
new = old.copy()      #same list

old.remove("item#1")  #remove one of the items

for item in new:
    if item not in old:
        print(item)

#Expecting: "item#1"
#Getting: Nothing

The problem is that the lists are big (1700+ items) and the code shown above doesn’t work
I tried slicing the list (Made sure the sliced version still had the item (item#1))
With 5 elements the code works.

old = generateList()[0:5]  #same list
new = old.copy()[0:5]      #same list

old.remove("item#1")  #remove one of the items

for item in new:
    if item not in old:
        print(item)

#Expecting: "item#1"
#Getting: "item#1"

Anybody knows what’s going on here?

Asked By: Guy zvi

||

Source

Answer 1

This is odd. It should be working. When making a question, try to provide the simplest working example and the expected output.

By the way, this is not a good way to do list comparison. For the reference, it’s best to use sets comparison, which is much faster.

result = list(set(new)-set(old))

gives you a list of items in new which are absent in old.

Answered By: tino

Answer 2

My guess: the item you are removing is duplicated.

list.remove removes only the first found item.

Example:

old = ['A', 'B', 'C', 'A']
new = old.copy()

old.remove('A')
# ['B', 'C', 'A']

'A' in old
# True

Answered By: mozway

Comparing lists in python gets weird

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