[Python] Why is the sorting result of Python lists so strange? [duplicate]

The code is as follows, using key function to count the number of times each element appears in the list:

input_list = list('aaddccdc')
input_list.sort(key=lambda i: (-input_list.count(i), ord(i)))
print(input_list)


We get the result in unexpected order

['a', 'a', 'c', 'c', 'c', 'd', 'd', 'd']


If it is modified so that we use a copy in the key function

input_list = list('aaddccdc')

snapshot = input_list.copy()
input_list.sort(key=lambda i: (-snapshot.count(i), ord(i)))

print(input_list)


We get the different, expected order

['c', 'c', 'c', 'd', 'd', 'd', 'a', 'a']


Why is that?

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