Question

This is more a programming exercise than a real-world problem: I am looking for a generator expression that resembles the behavior of append.

Consider:

def combine(sequence, obj):
    for item in sequence:
        yield item
    yield obj

s = ''.join(combine(sequence, obj))

This generator basically resembles append. In the workflow of my program the above is as fast as

sequence.append(obj)
s = ''.join(sequence)

I am now wondering if there is a neat generator expression genexpr with

s = ''.join(genexpr)

that resembles the append behavior above without performance caveats.

s = ''.join(_ for a in [sequence, [obj]] for _ in a)

performs bad.

Answer 1

Try using chain from itertools module:

''.join(chain(sequence, [obj]))

If you don’t want to create a new list for obj, then you may try this:

''.join(chain(sequence, repeat(obj,1)))

I would use [obj] as it’s more readable and I doubt that repeat iterator has a less overhead than list creation.