Using .iteritems() to iterate over key, value in Python dictionary

Question

Note: I have read this post and Alex Martelli's response, but I don't really/fully understand his answer. It's a bit beyond my current understanding. I would like help understanding it better.

I understand that when you try the following for loop:

for key, value in dict:
    print key
    print value 

you get:

ValueError: too many values to unpack

Although you can loop over a dictionary and just get the keys with the following:

for key in dict:
    print key 

Can anyone provide a slightly less-advanced explanation for why you cannot iterate over a dictionary using key, value without using .iteritems() ?

Answer 1

Python has a feature called iterable unpacking. When you do

a, b = thing

Python assumes thing is a tuple or list or something with 2 items, and it assigns the first and second items to a and b. This also works in a for loop:

for a, b in thing:

is equivalent to

for c in thing:
    a, b = c

except it doesn't create that c variable.

This means that if you do

for k, v in d:

Python can't look at the fact that you've said k, v instead of k and give you items instead of keys, because maybe the keys are 2-tuples. It has to iterate over the keys and try to unpack each key into the k and v variables.

Answer 2

The other answer explains it well. But here are some further illustrations for how it behaves, by showing cases where it actually works without error (so you can see something):

>>> d = {(1,2): 3, (4,5): 6}
>>> for k, v in d:
        print k, v

1 2
4 5

The loop goes through the keys (1,2) and (4,5) and since those "happen to be" tuples of size 2, they can be assigned to k and v.

Works with strings as well, as long as they have exactly two characters:

>>> d = {"AB":3, "CD":6}
>>> for k, v in d:
        print k, v

A B
C D

I assume in your case it was something like this?

>>> d = {"ABC":3, "CD":6}
>>> for k, v in d:
        print k, v

Traceback (most recent call last):
  File "<pyshell#42>", line 1, in <module>
    for k, v in d:
ValueError: too many values to unpack

Here, the key "ABC" is a triple and thus Python complains about trying to unpack it into just two variables.