Mystery of Dictionaries and For loop in Python 3

Question

I have one dictionary:

dog = {'color': 'black'}

I created 3 similar dogs inside one list:

dogs = []
dog = {'color': 'black'}
for num in range(3):
   dogs.append(dog)

When I try to modify first dog of my dogs list:

dogs[0]['color'] = 'white'

It retrieves me whole list been modified:

[{'color': 'white'}, {'color': 'white'}, {'color': 'white'}]

But when I declare my dog dictionary inside for loop and try to change single dog dictionary:

for num in range(3):
 dog = {'color': 'black'}
 dogs.append (dog)

 dogs[0]['color'] = 'white'

I get the desired output:

[{'color': 'white'}, {'color': 'black'}, {'color': 'black'}]

You say it yourself, in the first case you have *one dictionary*. In the second case you create three separate dictionaries. You didn't create *"3 similar dogs"*, you added three references to the exact same object. — jonrsharpe, Jul 05 '20 at 09:36
And similarly for lists - in your first case if `dog` were a list, `dogs` would be a list of references to the single same list. — DisappointedByUnaccountableMod, Jul 05 '20 at 09:40
Looks like your dog should be a class, which can be instantiated. — Jan Christoph Terasa, Jul 05 '20 at 09:46
Read the following: https://nedbatchelder.com/text/names.html — juanpa.arrivillaga, Jul 05 '20 at 10:17
@juanpa.arrivillaga Thank you very much, that one helps me out! :) — ojacomarket, Jul 05 '20 at 18:10

score 0 · Answer 1 · answered Jul 05 '20 at 09:40

In your first example, you are always appending the same object/same dog.

In the second example, you are appending three different objects, as you are creating them in the loop.

If you'd like to append copies of your dog, just do:

dogs = []
dog = {'color': 'black'}
for num in range(3):
   dogs.append(dog.copy())

Be aware that in this case, the original dog is not a part of your dogs list, only copies of it.

edit: if your dict is nested, you'll need .deepcopy() --> https://stackoverflow.com/a/2465951/13450078

score 0 · Accepted Answer · answered Jul 05 '20 at 09:42

The difference is that in the first example

dogs = []
dog = {'color': 'black'}
for num in range(3):
   dogs.append(dog)
dogs[0]['color'] = 'white'

each dog in the dogs list refer to the same dog dictionary and by changing the color of the dictionary it's reflecting on the same object.
In the second code

for num in range(3):
    dog = {'color': 'black'}
    dogs.append (dog)

dogs[0]['color'] = 'white'

On each iteration, a new dictionary object is created so by adding each object to the list the reference of the list item to a dictionary object is different and by that, if you are changing the color of a specific item from the list it's changing only his value.

Mystery of Dictionaries and For loop in Python 3

2 Answers2