CODEWITHSUNDEEP
pythonlistcompare
user3582887
user3582887

Reputation: 413

List comparison failing

For some reason I am getting the lists to always be equal, even though when printed, you can see that they are not always equal.

It is returning True when the previous_list = [1,0,0,0,0] and the current_list = [0,0,0,0,0]

There are many questions regarding comparing lists, but I have tried many different answers like the cmp(), but it returns 0 as well. So why does it say its the same but when printed, you can see they are different?

while True:
    previous_status_list = current_status_list
    print str(previous_status_list) + " PREVIOUS"
    if door_status == 'closed' and status_for_loop == 'closed':
        overall_status_list[0] = 0
    else:
        overall_status_list[0] = 1
    if door_locked_status == False:
        overall_status_list[1] = 0
    else:
        overall_status_list[1] = 1
    if door_breached_status == False:
        overall_status_list[2] = 0
    else:
        overall_status_list[2] = 1
    if door_propped_status == False:
        overall_status_list[3] = 0
    else:
        overall_status_list[3] = 1
    if egress_status == False:
        overall_status_list[4] = 0
    else:
        overall_status_list[4] = 1
    current_status_list = overall_status_list
    if current_status_list == previous_status_list:
        print 'TRUE'
    else:
        print 'False'
    print str(current_status_list)  + " CURRENT"
    sleeptime.sleep(3)

Upvotes: 1

Views: 65

Answers (1)

Martijn Pieters
Martijn Pieters

Reputation: 1121834

You are not comparing two different lists, as you didn't make a copy:

previous_status_list = current_status_list

previous_status_list and current_status_list reference the same object. Altering the list through one reference alters the other:

Create a (shallow) copy instead:

previous_status_list = current_status_list[:]

Demo:

>>> current_status_list = ['foo', 'bar', 'baz']
>>> previous_status_list = current_status_list
>>> current_status_list is previous_status_list
True
>>> current_status_list.append('spam')
>>> previous_status_list
['foo', 'bar', 'baz', 'spam']
>>> previous_status_list = current_status_list[:]  # shallow copy
>>> current_status_list is previous_status_list
False
>>> current_status_list.append('ham')
>>> current_status_list
['foo', 'bar', 'baz', 'spam', 'ham']
>>> previous_status_list
['foo', 'bar', 'baz', 'spam']

Note how the is identity operator shows that the two names refer to the same object, until a copy is created. Also, changes to the list through current_status_list show up in previous_status_list until such time that a copy is created.

Upvotes: 4

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