Question

I’m trying to figure out the syntax for passing arguments from one list or dict to another in the for loop syntax.

The desired result I’m looking for is this:

for bean in beans:
  if bean.type == 'coffee':
    print bean

Only, instead of printing to stdout, I’d like to collect that string data and append it to another list. Eventually flattening the list.

The kicker, I want to perform this in a single line.

I know of the ''.join() method, I’m looking for this result so I can filter the results from the for-in loop.

Answer 1

[bean for bean in beans if bean.type == 'coffee']

List comprehensions are neat. Even neater, often you don’t need to produce a whole list – you just need an iterator that gives you the values the list would consist of. That’s a generator, and they can be expressed just as succinctly via generator expressions. Those are written in the same way as list comprehensions except that the square brackets become parens (and you can omit them if it’s the only argument in a function call) e.g. '\n'.join(str(bean) for bean in beans if bean.type == 'coffee'). The advantage is the mentioned laziness, i.e. you never generate values until they’re really needed (requested) and you don’t keep all of them in memory at the same time (unless of course the consumer does this).

You can use itertools.chain to chain multiple iterables (including lists) to one, or if you can’t change the fact you’re getting lists of lists, you can use (x for list in lists for x in list). For a generalized solution involving abritary deep nesting, you need a full function utilizing recursion.