Efficiently sum items by type

Question

I have a list of items with properties "Type" and "Time" that I want to quickly sum the time for each "Type" and append to another list. The list looks like this:

Items = [{'Name': A, 'Type': 'Run', 'Time': 5},  
          {'Name': B, 'Type': 'Walk', 'Time': 15},  
          {'Name': C, 'Type': 'Drive', 'Time': 2},
          {'Name': D, 'Type': 'Walk', 'Time': 17},  
          {'Name': E, 'Type': 'Run', 'Time': 5}]

I want to do something that works like this:

Travel_Times=[("Time_Running","Time_Walking","Time_Driving")]
Run=0
Walk=0
Drive=0    

for I in Items:
    if I['Type'] == 'Run':
       Run=Run+I['Time']
    elif I['Type'] == 'Walk': 
       Walk=Walk+I['Time']           
    elif I['Type'] == 'Drive': 
       Drive=Drive+I['Time']           

Travel_Times.append((Run,Walk,Drive))    

With Travel_Times finally looking like this:

print(Travel_Times)
[("Time_Running","Time_Walking","Time_Driving")
 (10,32,2)]

This seems like something that should be easy to do efficiently with either a list comprehension or something similar to collections.Counter, but I can't figure it out. The best way I have figured is to use a separate list comprehension for each "Type" but that requires iterating through the list repeatedly. I would appreciate any ideas on how to speed it up.

Thanks

Answer 1

Note that case is very important in Python :

• For isn't a valid statement
• Travel_times isn't the same as Travel_Times
• there's no : after elif
• Travel_Times.append(... has a leading space, which confuses Python
• items has one [ too many
• A isn't defined

Having said that, a Counter works just fine for your example :

from collections import Counter

time_counter = Counter()

items = [{'Name': 'A', 'Type': 'Run', 'Time': 5},  
          {'Name': 'B', 'Type': 'Walk', 'Time': 15},  
          {'Name': 'C', 'Type': 'Drive', 'Time': 2},
          {'Name': 'D', 'Type': 'Walk', 'Time': 17},  
          {'Name': 'E', 'Type': 'Run', 'Time': 5}]

for item in items:
    time_counter[item['Type']] += item['Time']

print(time_counter)
# Counter({'Walk': 32, 'Run': 10, 'Drive': 2})

To get a list of tuples :

[tuple(time_counter.keys()), tuple(time_counter.values())]
# [('Run', 'Drive', 'Walk'), (10, 2, 32)]

Answer 2

You can use a dict to keep track of the total times. Using the .get() method, you can tally up the total times. If the key for the activity doesn't already exist, set its tally to zero and count up from there.

items = [{'Name': 'A', 'Type': 'Run', 'Time': 5},  
          {'Name': 'B', 'Type': 'Walk', 'Time': 15},  
          {'Name': 'C', 'Type': 'Drive', 'Time': 2},
          {'Name': 'D', 'Type': 'Walk', 'Time': 17},  
          {'Name': 'E', 'Type': 'Run', 'Time': 5}]

totals = {}

for item in items:
    totals[item['Type']] = totals.get(item['Type'], 0) + item['Time']

for k, v in totals.items():
    print("Time {}ing:\t {} mins".format(k, v))