Question

Is there a big difference in speed in these two code fragments?

1.

x = set( i for i in data )

versus:

2.

x = set( [ i for i in data ] )

I’ve seen people recommending set() instead of set([]); is this just a matter of style?

Answer 1

The form

x = set(i for i in data)

is shorthand for:

x = set((i for i in data))

This creates a generator expression which evaluates lazily. Compared to:

x = set([i for i in data])

which creates an entire list before passing it to set

---

From a performance standpoint, generator expressions allow for short-circuiting in certain functions (all and any come to mind) and takes less memory as you don’t need to store the extra list — In some cases this can be very significant.

If you actually are going to iterate over the entire iterable data, and memory isn’t a problem for you, I’ve found that typically the list-comprehension is slightly faster then the equivalent generator expression*.

temp $ python -m timeit 'set(i for i in "xyzzfoobarbaz")'
100000 loops, best of 3: 3.55 usec per loop
temp $ python -m timeit 'set([i for i in "xyzzfoobarbaz"])'
100000 loops, best of 3: 3.42 usec per loop

Note that if you’re curious about speed — Your fastest bet will probably be just:

x = set(data)

proof:

temp $ python -m timeit 'set("xyzzfoobarbaz")'
1000000 loops, best of 3: 1.83 usec per loop

*Cpython only — I don’t know how Jython or pypy optimize this stuff.