How to construct nested dictionary comprehension in Python with correct ordering?

dictionary, dictionary-comprehension, nested, python

Solution

Note that there is a better way of doing this without a dict comprehension; see below. I’ll first address the issues with your approach.

You need to use nesting order in comprehensions. List your loops in the same order they would be in when nesting a regular loop.

The `line.split()` expression returns a sequence of two items, but each of those items is not a tuple of a key and a value; instead there is only ever one element being iterated over. Wrap the split in a tuple so you have a 'sequence' of `(key, value)` items being yielded to assign the two results to two items:

dict2 = {key:value for line in ["1:One", "2:Two", "4:Four"]
                   for key, value in (line.split(":"),)}

This is the equivalent of:

dict2 = {}
for line in ["1:One", "2:Two", "4:Four"]:
    for key, value in (line.split(":"),):
        dict2[key] = value

where the nested loop is only needed because you cannot do:

dict2 = {}
for line in ["1:One", "2:Two", "4:Four"]:
    key, value = line.split(":")
    dict2[key] = value

However, in this case, instead of a dictionary comprehension, you should use the `dict()` constructor. It wants two-element sequences, simplifying the whole operation:

dict2 = dict(line.split(":") for line in ["1:One", "2:Two", "4:Four"])

Problem

I was trying to shorten the code for this problem when I encountered the problem. Basically, I was trying a nested dictionary comprehension & was unsuccessful in the attempt. Here is what I tried. ``` dict2 = {key:value for key, value in line.split(":") for line in ["1:One", "2:Two", "4:Four"]} print dict2 ``` When I run this, it gives me ``` NameError: name 'line' is not defined ``` And, when I reverse the `for` statements like this ``` dict2 = {key:value for line in ["1:One", "2:Two", "4:Four"] for key, value in line.split(":")} print dict2 ``` It results in ``` ValueError: need more than 1 value to unpack ``` I need help on the nesting structure of `dictionary` (or `list`) comprehension. An example would be help a lot.

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