[Python] Python: replacing multiple list items by one

I want to write a calculator program. My goal is to replace multiple list items(with datatype str) by one(int). I've tried the .insert() method, but it creates a new inner list and places it at the end of the main list.

Something like this:

input_list = ['4','5','6','+','8','7','4']

#expected result
output_list = [456, '+', 874]

#actual result
input_list = ['4','5','6','+','8','7','4',['4','5','6']]


I also tried extend method and also without success.

My code:

num = ""
start = ""
for x in range(len(list)):
if list[x].isdigit() == True:
if start == "":
start = x
num += list[x]
continue
else:
num += list[x]
continue
else:
num = int(num)
list.insert(num,list[start:x])
num = ""
start = ""
continue

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