Adding a value to a pre existing dictionary key when importing the data from a text file (Python 3)

Question

I'm trying to create a dictionary by pulling from a text file that looks something like this,

Fred beats Amy  
Fred beats Tom  
Tom beats Amy   
Amy beats Jordan

Right now I'm using

f = open("RockPaperScissors.txt", 'r')  
fwDict = {}  
for line in f:  
    k, v = line.strip().split('beats')  
    fwDict[k.strip()] = v.strip()  


f.close()

The problem I'm running into is that when "Fred" appears multiple times it just overwrites the previous value (Amy) and replaces it with the newest one (Tom), is there any way I can just add a new value if the key already exists, even when pulling from a txt file?

Thanks!

fedterzi · Accepted Answer · 2017-04-13T15:40:52.140

You could replace the value of the dictionary with a list:

f = open("RockPaperScissors.txt", 'r')  
fwDict = {}  
for line in f:  
    k, v = line.strip().split('beats')

    # If we already have the key in the dictionary, just append to the list
    if k.strip() in fwDict:
        fwDict[k.strip()].append(v.strip())

    # If we don't have the key in the dict, create the new key-value pair as a list
    else:
        fwDict[k.strip()] = [v.strip()]


f.close()

score 1 · Answer 2 · answered Apr 13 '17 at 15:43

This is a good use case for defaultdict, which upon accessing a key creates a default value.

If we use a list as a default in this case, your code can be simplified to the following:

from collections import defaultdict

fw_dict = defaultdict(list)

with open('RockPaperScissors.txt', 'r') as f:
    for line in f:
        k, v = line.strip().split('beats')
        fw_dict[k.strip()].append(v.strip())

That way you will have the result values for each key as you desire.

Note the with line, which ensures that file handle is closed at the end of the block. You don't have to do this, but it saves you having to do an f.close() later (or just rely on the process closing and dropping the handle).

use `split(' beats ')` and you won't have to `strip` k & v – Jean-François Fabre Apr 13 '17 at 15:53 — Jean-François Fabre, Apr 13 '17 at 15:53

score 0 · Answer 3 · answered Apr 13 '17 at 16:02

You can use set instead of list if you want to elements don't repeat selves:

f = open("RockPaperScissors.txt", 'r')  
fwDict = {}  
for line in f:  
    k, v = line.strip().split('beats')
    if k.strip() in fwDict:
        fwDict[k.strip()].add(v.strip())
    else:
        a = set()
        a.add(v.strip())
        fwDict[k.strip()] = a


f.close()

or with defaultdict:

from collections import defaultdict

f = open("RockPaperScissors.txt", 'r')  
fwDict = defaultdict(set)
for line in f:  
    k, v = line.strip().split('beats')
    fwDict[k.strip()].add(v.strip())


f.close()

Adding a value to a pre existing dictionary key when importing the data from a text file (Python 3)

3 Answers3