Make dict comprehension on one line

[jasongrout]

I had to stare at this for a long time, while my brain was parsing the statement one line at a time, trying to figure out what in the world it was doing. I then realized it was a dict comprehension. Personally, I think this comprehension makes a lot more sense when scanning through the code when it is on one line.

I realize this is a stylistic issue, so if I'm outvoted, fine. This comprehension took me enough time to understand (or even syntactically recognize as a dict comprehension) that I thought it was worth a PR.

[ssanderson]

As the person who wrote this expression, I'm -1 on this change.

I prefer multi-line comprehensions over one giant line because I think it clearly differentiates the three operations that are happening in the comprehension:

{
     <expression>
     <iteration>
     <filter>
}

Maybe I'm just used to reading my own style, but I find this separation much clearer than having to parse the logical sections in a long line by hand.

[minrk]

I don't have a strong feeling. I don't personally find a one-line comprehension easier to follow than a multi-line one, if anything it's slightly the other way around. I'm not especially fond of filtered comprehensions in general.

[ssanderson]

The multiple bindings of the name key in the expressions here make this unusually hard to parse I think. A totally sane alternative is also to not use a comprehension at all:

errors = {}
for key in maybe_none_keys:
    if model[key] is not None:
        errors[key] = model[key]

This feels verbose to me, but it's almost certainly the most straightforward spelling.

[minrk]

I'd be AOK with removing the comprehension if that's helpful to people. I don't think reshaping the comprehension improves it, though.

[jasongrout]

I think why I struggled to understand it were:

1. not used to dict comprehensions
2. having for and if on separate lines helped me parse them as for and if statements, which threw my brain for a loop when it just didn't make sense
3. The list comprehension a few lines above is almost the same thing, but on a single line

In general, I think comprehensions are more efficient than explicit loops, so I prefer them, so I'll close this as a style issue.

[jasongrout]

P.S. It wasn't just me. I showed it to @SylvainCorlay as well, and he stared at it blankly for a few seconds before I explained what it was.

[minrk]

@jasongrout if you wanted to keep this PR and turn it into the for-loop, that would be fine. I don't think this is a case where any efficiency benefits of a comprehension will be noticeable, since it's also talking to a filesystem or external data store.