How to update a value in python dictionary if the key appears multiple times without importing?

Say that I have a list of tuples with multiple occurrences of the same key. I want to convert them into a dictionary but I don't want the keys to repeat or overwrite previous keys. I want to do it without importing packages.

[("John", 14), ("Bob", 5), ("John", 21)]

turn it into:

{"John": [14, 21],
"Bob": [5]}

Solution

Try using collections.defaultdict:

from collections import defaultdict
yourlist = [("John", 14), ("Bob", 5), ("John", 21)]
d = defaultdict(list)
for k, v in yourlist:
    d[k].append(v)
print(d)

Output:

defaultdict(<class 'list'>, {'John': [14, 21], 'Bob': [5]})

To make it an actual dictionary, add this line:

d = dict(d)

Then:

print(d)

Would give:

{'John': [14, 21], 'Bob': [5]}

Edit:

As OP mentioned in the comments, he doesn't want to use any imports. So try:

yourlist = [("John", 14), ("Bob", 5), ("John", 21)]
d = {}
for x, y in yourlist:
    if x in d:
        d[x].append(y)
    else:
        d[x] = [y]
print(d)

Output:

{'John': [14, 21], 'Bob': [5]}