I recently compared the processing speeds of [] and list() and was surprised to discover that [] runs more than three times faster than list(). I ran the same test with {} and dict() and the results were practically identical: [] and {} both took around 0.128sec / million cycles, while list() and dict() took roughly 0.428sec / million cycles each.
Why is this? Do [] and {} (and probably () and '', too) immediately pass back a copies of some empty stock literal while their explicitly-named counterparts (list(), dict(), tuple(), str()) fully go about creating an object, whether or not they actually have elements?
I have no idea how these two methods differ but I’d love to find out.
I couldn’t find an answer in the docs or on SO, and searching for empty brackets turned out to be more problematic than I’d expected.
I got my timing results by calling timeit.timeit("[]") and timeit.timeit("list()"), and timeit.timeit("{}") and timeit.timeit("dict()"), to compare lists and dictionaries, respectively. I’m running Python 2.7.9.
I recently discovered “Why is if True slower than if 1?” that compares the performance of if True to if 1 and seems to touch on a similar literal-versus-global scenario; perhaps it’s worth considering as well.