[Tutor] Combining two Dictionaries

[Ryan Strunk]

Hello everyone,

I had an interesting thing come up earlier in my programming, and I'm trying
to wrap my mind around why it occurred.
I wanted to take two dictionaries with the same keys and combine their
values to make one, big super dictionary.

def combine(d1, d2):
    for key in d1:
        if key in d2:
            d2[key] += d1[key]

When I assign values to each dictionary, this works perfectly.
d1 = {'a': 1, 'b': 2, 'c': 3}
d2 = {'a': 10, 'b': 20, 'c': 30}
combine(d1, d2)
d1 = {'a': 1, 'b': 2, 'c': 3}
d2 = {'a': 11, 'b': 22, 'c': 33}

When I initialize the class which holds these dictionaries, though, I need
to make sure that all the keys contained in d2 match the keys of d1. Thus I
tried:
d1 = {'a': 0, 'b': 0, 'c': 0}
d2 = d1
My understanding was that d2 looked at d1 once, grabbed its keys and values,
and went off to do its own thing.  Just as if you typed:
x = 3
y = x
x = 6
y still holds the value 3.
This turns out not to be the case with dictionaries, and I'm not sure why
this is so. Why when you change a dictionary's keys in place does a copied
dictionary take on the new values?
Thanks for any help you can provide.

Best,
Ryan