在列表上执行sum（）时如何替换python列表中的元素

Question

I have a python list containing integers and string data types. All I need to do is to perform a replacement of the element when performing sum() function on the list.

For ex: The list is lis=[ 1, 4, 'Jan', 8] , I need to replace Jan with integer 1 when performing sum function on it. So the final output of sum function - sum(lis) becomes 1 + 4 + 1 + 8 which results to 14 .

There may be multiple such strings in the list and all those strings needs to be replaced when performing sum() on list. Suppose Feb=2, Mar=3, Apr=4 , Feb replaced by 2 when sum() is called etc.

I don't want the elements in the list to be replaced. They should be replaced only when the sum is called. Is there any inbuilt function or something to do that?

Answer 1

You want to map the elements of your list to values during summation. You will need to create a function that does that (there could not be a built-in one for that, only you know the values). Something like

def f(value):
   if isinstance(value, int):
      return value
   return months[value]

where months is a dict like {'jan': 1, 'feb': 2...}

Then

sum([f(value) for value in my_list])

or

sum(map(f, my_list))

Answer 2

You can use a dictionary that maps months to month number and then a list comprehension as follows

months = { 'Jan': 1, 'Feb': 2, 'Mar': 3, 'Apr': 4, 'May': 5
          'Jun': 6, 'Jul': 7, 'Aug': 8, 'Sep':9, 'Oct':10
          'Nov':11, 'Dec':12 }
sum([months[x] if isinstance(x, str) else x for x in lis])