In Python, how can I refer to a return-value from a function called within a generator expression?

Question

Simplified, I want to do something like this:

({'publication': obj.pub_name, 'views': obj.views, } for obj = analyze_publication(p) for p in Publication.objects.all())

Of course, that doesn't work.

Right now, I'm using:

({'publication': obj.pub_name, 'views': obj.views, } for obj in (analyze_publication(p) for p in Publication.objects.all()))

I have no idea if the second code piece is how it's done or there's another syntax, or it's not efficient etc. I'm only 2 weeks into Python.

Answer 1

You have unbalanced parens, but other than that, you should have no (functional) problem nesting generator expressions. As with list comprehensions, they quickly become unwieldy when nested. I'd recommend moving it out to a named generator, for readability's sake.

If you are curious about performance, compare different approaches using the disassembler or profilers.

Answer 2

Just another version:

( dict(publication= obj.pub_name, views= obj.views) 
    for obj in map(analyze_publication, Publication.objects.all()) )

Answer 3

Rather than trying to contort the generator expression syntax to allow a name binding, I'd just use a function to produce the binding. Then it is simple:

def pubdict(pub):
    obj = analyze_publication(pub)
    return {"publication": obj.pub_name, "views": obj.views}

itertools.imap(pubdict, Publication.objects.all()