Python- Accessing lists in dictionaries

Question

So, I am trying to solve this problem where I have to return a dictionary of all people who went on dates.

For example, this input:

[
 ["john", "amy"],
 ["abby", "john"],
 ["john", "michael"],
 ["michael", "chris"],
]

Should output:

{
 "john": set(["amy", "abby", "michael"]),
 "amy": set(["john"]),
 "abby": set(["john"]),
 "michael": set(["john", "chris"]),
 "chris": set(["michael"]),
}

but I have no idea how I can save the names and then insert it into a dictionary. I've tried looping through the list twice, but no luck.

Below, is the function that I've been using to return the necessary dictionary.

def return_all_dates(people_who_went_on_dates):
 pass

• Scott Hunter: Do you know how to create an empty dictionary? How to add an entry to a dictionary? How to add an item to a set?

Answer 1

you can do it like this:

dates = dict()
for row in people_who_went_on_dates:
 person1, person2 = row
 if person1 not in dates:
 dates[person1] = set()
 if person2 not in dates:
 dates[person2] = set()
 dates[person1].add(person2)
 dates[person2].add(person1)

• Алексей Р: can be simplified using dates.setdefault(row[0], set()).add(row[1])

Answer 2

Here is my approach:

data = [
 ["john", "amy"],
 ["abby", "john"],
 ["john", "michael"],
 ["michael", "chris"],
]
def return_dates(all_dates):
 #Create empty dictionary
 dates = {}
 #Iterate over each date
 for date in all_dates:
 #Add first person if not exists in the dictionary
 if date[0] not in dates:
 #Assign value with a list of 1 element
 dates[date[0]] = [date[1]]
 #Add second person too if not exists too
 if date[1] not in dates:
 #Assign value with a list of 1 element
 dates[date[1]] = [date[0]]
 
 #if one or both persons exists in the dictionary, we do different steps
 #Add second person to the first person list only if it does not exists
 if date[1] not in dates[date[0]]:
 #List method append to add a new element
 dates[date[0]].append(date[1])
 #Add first person to the second person list only if it does not exists
 if date[0] not in dates[date[1]]:
 dates[date[1]].append(date[0]) 
 
 #return dictionary
 return dates
 
result = return_dates(data)
print(result)

{'john': ['amy', 'abby', 'michael'], 'amy': ['john'], 'abby': ['john'], 'michael': ['john', 'chris'], 'chris': ['michael']}