Python - Dictionary from unsorted text, list comprehension?

Question

I'm hoping somebody a little more versed in list comprehension can offer some suggestions.

Consider the following data set:

Onions,copper,manganese,magnesium,phosphorus
Tomatoes,copper,manganese,potassium
Garlic,manganese
Celery,manganese,potassium,sodium,salt
Bell Peppers,copper,manganese
Butter,sodium,salt
Eggplant,copper,manganese
Grapes,copper,manganese,potassium

I need to formulate a dictionary, such that the key is the mineral, and the value is a set of foods which contain that mineral- like this:

{'copper': {'Tomatoes', 'Onions', 'Bell Peppers', 'Eggplant'}, 'maganeese': {'Onions', 'Tomatoes', 'Garlic', 'Celery', 'Bell Peppers', 'Eggplant', 'Grapes'}...  etc.}

You'll notice that the food is in the first positioin, followed by the minerals it contains.

I figure that I'll probably need to separate the foods and minerals into two lists, list of foods and list of minerals. Logically, I am stuck with how one could accomplish this task at all.

with open ('file.txt', 'r') as fp:
    D = dict()
    food_list = []
    mineral_list = []
    for line in fp:
        line = line.strip().split(",")
        line = [x for x in line if x]
        food_list.append(line[0])
    print(food_list)

Can anybody provide a push in the right direction here?

Answer 1

You could do something like this:

import pprint

mineral_table = {}
with open("ip.txt") as infile:
    for line in infile:
        # split the line into vegetable and minerals
        vegetable, *minerals = line.strip().split(',')

        # for each mineral add the vegetable to the mineral list
        for mineral in minerals:
            mineral_table.setdefault(mineral, []).append(vegetable)

pprint.pprint(mineral_table)

Output

{'copper': ['Onions', 'Tomatoes', 'Bell Peppers'],
 'magnesium': ['Onions'],
 'manganese': ['Onions', 'Tomatoes', 'Garlic', 'Celery', 'Bell Peppers'],
 'phosphorus': ['Onions'],
 'potassium': ['Tomatoes', 'Celery'],
 'salt': ['Celery'],
 'sodium': ['Celery']}

The line:

# split the line into vegetable and minerals
vegetable, *minerals = line.strip().split(',')

uses extended iterable unpacking . The for loop usessetdefault , from the documentation:

If key is in the dictionary, return its value. If not, insert key with a value of default and return default. default defaults to None.