Why list.append() in Python changes every item in the original list?

I have this piece of code:

import json

a = '''
{
    "c": {
        "d": [
            {
                "e": "f"
            }
        ]
    }
}
'''
j = json.loads(a)
l = []

for n in range(20):
    if n % 2 == 0:
        j['c']['d'][0]['e'] = n
        l.append(j)
        
print(l)

The output of this code:

[{'c': {'d': [{'e': 18}]}}, {'c': {'d': [{'e': 18}]}}, {'c': {'d': [{'e': 18}]}}, {'c': {'d': [{'e': 18}]}}, {'c': {'d': [{'e': 18}]}}, {'c': {'d': [{'e': 18}]}}, {'c': {'d': [{'e': 18}]}}, {'c': {'d': [{'e': 18}]}}, {'c': {'d': [{'e': 18}]}}, {'c': {'d': [{'e': 18}]}}]

The output I was expected:

[{'c': {'d': [{'e': 0}]}}, {'c': {'d': [{'e': 2}]}}, {'c': {'d': [{'e': 4}]}}, {'c': {'d': [{'e': 6}]}}, {'c': {'d': [{'e': 8}]}}, {'c': {'d': [{'e': 10}]}}, {'c': {'d': [{'e': 12}]}}, {'c': {'d': [{'e': 14}]}}, {'c': {'d': [{'e': 16}]}}, {'c': {'d': [{'e': 18}]}}]

How to get the expected output?

Answer

When you modify j and subsequently append it to the l, it’s a reference to the same dict that you append. Here is one way to operate on copies:

import copy
import json

a = '''
{
    "c": {
        "d": [
            {
                "e": "f"
            }
        ]
    }
}
'''
j = json.loads(a)
l = []
for n in range(10):
    j2 = copy.deepcopy(j)
    j2['c']['d'][0]['e'] = 2 * n
    l.append(j2)
        
print(l)