Dictionary Comprehension in Python 3

Looping over a dictionary only yields the keys. Use d.items() to loop over both keys and values:

{key: value for key, value in d.items()}

The ValueError exception you see is not a dict comprehension problem, nor is it limited to Python 3; you’d see the same problem in Python 2 or with a regular for loop:

>>> d = {'a':1, 'b':2, 'c':3, 'd':4}
>>> for key, value in d:
...     print key, value
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: need more than 1 value to unpack

because each iteration there is only one item being yielded.

Without a transformation, {k: v for k, v in d.items()} is just a verbose and costly d.copy(); use a dict comprehension only when you do a little more with the keys or values, or use conditions or a more complex loop construct.