python: append dict into list issue

Question

I'm actually new on python. With exam code, I found something strange

Here's my code.

cat_log = []

container = {}

for index in range(10) : 

    container["idx"] = index

    cat_log.append(container)

print(cat_log)

I want the result like this: [{'idx' : 0 } , { 'idx' : 1 } , { 'idx' : 2 } , { 'idx' : 3 }].
But my code doesn't output this result.
Something like this: [{ 'idx' : 3 } , { 'idx' : 3 } , { 'idx' : 3 } , { 'idx' : 3 }]
I've googled why this happens, but I didn't find any right idea.
What's wrong and why?

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Answer 1

When you append container you are not making a copy, so you are just appending a reference to the same dict over and over. You are mutating the dict as well and you cannot have multiple values of the same key in a dictionary, it is just overwriting idx with the current index each time.

Instead, append a new dictionary each iteration. Eg:

cat_log = []
for index in range(10) : 
    cat_log.append({"idx": index})
print(cat_log)

python: append dict into list issue

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