How to handle a one line for loop without putting it in a List?

From my understanding, you have something like this:

class Cow(object):
    def __init__(self, legnames):
        self.legnames = legnames

listofcows = [Cow(['Red Leg', 'Blue Leg']), 
              Cow(['Green Leg', 'Red Leg'])]

It’s easiest extend a temporary list, like this:

legs = []

# As @delnan noted, its generally a bad idea to use a list
# comprehension to modify something else, so use a for loop
# instead.
for cow in listofcows:
    legs.extend(cow.legnames)


# Now use the temporary legs list...
print {'livestream':legs}

The name listofcows implies there may be, possibly in a distant future, several cows. Flattening a list of list with more than one item would be simply wrong.

But if the name is misleading (and why a one-lement list anyway, for that matter?), you have several options to flatten it.

Nested list comprehension: [legname for cow in listofcows cow.legnames for legname in cow.legnames]

Getting the first item: [your list comprehension][0]

And very likely some useful thingy from the standard library I don’t remember right now.

Is this what you’re looking for?

'livestream' : [cow.legnames[0] for cow in listofcows]

you can try reduce,
'livestream' : reduce(lambda a,b:a+b,[cow.legs for cow in listofcows])

if you need unique legs use set

'livestream' :list(set(reduce(lambda a,b:a+b,[cow.legs for cow in listofcows])))

You could also use chain.from_itterable from python itertools Consider the following example:

from itertools import chain

list(chain.from_iterable(cow.legnames for cow in listofcows))

See also chain.from_iterable from the python documentation.