Python unintentionally deleting keys in other dictionaries

Question

When using del or .pop() on x_copy, the operation is magically applied to x as well. i.e. keys in both dictionaries are deleted.

Example code:

x = {
    "key1": 0,
    "key2": 0,
}

x_copy = x
x_copy.pop("key1")

print(f"x =      {x}")
print(f"x copy = {x_copy}")

This code should only remove the key from x, but the key is lost from both dictionaries.

Actual output:

>>> x =      {'key2': 0}
>>> x copy = {'key2': 0}

Expected output

>>> x =      {'key1': 0, 'key2': 0}
>>> x copy = {'key2': 0}

Same happens if I replace .pop() with del:

x_copy = x
x_copy.pop("key1")

Why is this happening?

Python 3.10

Because you didn't copy the dictionary, you have just given another name to it. https://moonbooks.org/Articles/How-to-copy-a-dictionary-in-python-/ — matszwecja, Feb 10 '22 at 09:34
because you set `x_copy = x`, in memory both variables have the same value because you assign them to the same memory — Linh Nguyen, Feb 10 '22 at 09:35

score 1 · Accepted Answer · answered Feb 10 '22 at 09:41

x_copy = x gives you 2 variables pointing at the same place in memory and all the operations that modify one will be reflected in another. You need to actually create another object in memory (e.g. with x.copy())and then you can treat it as an indepentant copy. https://moonbooks.org/Articles/How-to-copy-a-dictionary-in-python-/

score 0 · Answer 2 · answered Feb 10 '22 at 09:37

0

Didn't make shallow copy

x.copy()

Answered by @matszwecja

answered Feb 10 '22 at 09:37

Freddy Mcloughlan

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Python unintentionally deleting keys in other dictionaries

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