reversing dictionary in python doesn't work

Question

This is a code for switching between the key and value in a dictionary:

for k in d:
    d[d[k]] = k
    d.pop(k)

The problem with this code that it makes more operations that it should make, and for some inputs return the wrong result

what is the reason of this problem?

Possible duplicate of [Modifying a Python dict while iterating over it](http://stackoverflow.com/questions/6777485/modifying-a-python-dict-while-iterating-over-it) — , Apr 14 '17 at 14:16

score 3 · Answer 1 · answered Apr 14 '17 at 14:10

3

You are adding values to a dictionary as you are iterating through it. Generally this is a bad idea. It is better to construct a new dictionary using dictionary comprehension.

new_d = {v:k for k,v in d.items()}

answered Apr 14 '17 at 14:10

James

32,991
4
47
70

score 3 · Accepted Answer · answered Apr 14 '17 at 14:11

The problem is that you're changing the dictionary while iterating over it, and who knows what problems that causes.

Note that generally speaking you can't reverse dictionaries: keys must be unique and hashable, but values don't have to be. I'm assuming your dictionaries have unique hashable values so that it's at least possible.

Then I would use a dictionary comprehension to create a new dictionary, and assign that:

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

reversing dictionary in python doesn't work

2 Answers2