Dictionary Comprehension in Python 3

Question

I found the following stack overflow post about dict comprehensions in Python2.7 and Python 3+: Create a dictionary with list comprehension in Python stating that I can apply dictionary comprehensions like this:

d = {key: value for (key, value) in sequence} 

I tried it in Python 3. However, it raises an exception.

d = {'a':1, 'b':2, 'c':3, 'd':4} {key : value for (key, value) in d} {key : value for key, value in d} 

Both versions raise a ValueError saying that ValueError: need more than 1 value to unpack.

What is the easiest / the most direct way to make a dictionary comprehension in Python3?

Answer 1

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.

Answer 2

Well said above - you can drop items in Python3 if you do it this way:

{key: d[key] for key in d}

d = {'a':1, 'b':2, 'c':3, 'd':4} z = {x: d[x] for x in d} z >>>{'a': 1, 'b': 2, 'c': 3, 'd': 4} 

and this provides for the ability to use conditions as well

y = {x: d[x] for x in d if d[x] > 1} y >>>{'b': 2, 'c': 3, 'd': 4} 

Enjoy!