Why is my dictionary value changing without me telling it to, and how do I stop it?

Question

This is my sample code:

dict1 = {'a': 5, 'b': 6, 'c': 7}
dict2 = dict1
for i in dict1:
    dict1[i] += 5
print dict1
print dict2

Output looks like this:

{'a': 10, 'c': 12, 'b': 11}
{'a': 10, 'c': 12, 'b': 11}

Why is dict2 changing without me telling it to?

Python 2.7.10 on GCC 4.8.2 Linux.

Also tried on 2.7.12 on GCC 5.4.0, same result.

dict2 is not a dictionary. It's a reference to the dictionary. It points to the same dictionary as dict1. — Sergio Tulentsev, May 09 '19 at 18:52
You will need to **copy** your dictionary to variable `dict2` otherwise you are just referencing the same spot in memory! — Reedinationer, May 09 '19 at 18:52
In Python the `=` works a bit different to many other programming languages: it will create a reference to the object on the right side and push that reference as a name into a namespace. No direct assignment is done. — Klaus D., May 09 '19 at 19:04

score 0 · Answer 1 · answered May 09 '19 at 18:53

When you write dict2 = dict1, you are not creating the copy of dict1, you just set dict2 as a pointer to the dict1 dictionary. So if you change dict1, dict2 will be changed too because it looks at the same dictionary in memory. If you want to copy dict1 to dict2, you should use deepcopy function from the copy module from the Python standard library.

Why is my dictionary value changing without me telling it to, and how do I stop it?

1 Answers1