Sum of all arguments ever passed to counter

Question

Q4. Define a function make_counter that returns a counter function, which takes an immutable key and a numerical value argument and returns the sum of all arguments ever passed to counter with that same key.

Below is the solution:

def make_counter():
 """Return a counter function.
 >>> c = make_counter()
 >>> c('a', 3)
 3
 >>> c('a', 5)
 8
 >>> c('b', 7)
 7
 >>> c('a', 9)
 17
 >>> c2 = make_counter()
 >>> c2(1, 2)
 2
 >>> c2(3, 4)
 4
 >>> c2(3, c('b', 6))
 17 
 """
 dict = {} 
 def counter(key, value):
 if key in dict:
 dict[key].append(value)
 else:
 dict[key] = [value]
 lst = dict[key]
 total = 0
 length = len(lst)
 for index in range(length):
 total = total + lst[index]
 return total
 return counter
 """-m doctest gives some weird failure despite all above test case pass"""

Can we improve this solution?

• jonrsharpe: Rather than asking three questions in rapid succession, why not ask one, wait for feedback, apply it to the others then ask a follow-up? Also, there's no need to include all of the boiler-plate twice.
• jonrsharpe: Note that the reason for your doctest failure is trailing whitespace, after the 17 for the final test. It is quite picky about these things!

Answer 1

Choose another name for the dictionary besides dict. Don't ever hide the name of a built-in type.

Then...

d = {} 
def counter(key,value):
 d[key] = d.get(key,0) + value
 return d[key]

You're done. I see no need to build lists of all the passed values.