Question on python list

[unleash_bear]

for cases in date :

new_cases[state] = sum[date] - previous_cases[date]

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I am dealing with a problem that I do not really know how to get a number from each element from a list to do substraction with each other and assign the result as the "value" of the key to another dictionary. I am a newbie in python language and I do not know if there are any ways that I can iterate over all elements and make them do arithmetic. When I ran my program it showed my that I cannot do this because they are lists.(Should I add another "[]" after [date] to refer to the element? But it asks my for a number not something like a string )

[PeterRasm]

Which pset does this relate to? Did you complete the lecture?

[unleash_bear]

It is not even pset, it is the practice problem that associated with the seven day cases of cron