>>>
>>> a = []
>>> for i in range(2000):
...     a.append(set(range(i)))
...
>>> def f1(a):
...     s = set()
...     for aa in a : s |= aa
...
>>> def f2(a):
...     s = reduce(set.union, a)
...
>>> import cProfile
>>> cProfile.run('f1(a)')
         3 function calls in 0.420 CPU seconds

   Ordered by: standard name

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.420    0.420    0.420    0.420 <stdin>:1(f1)
        1    0.000    0.000    0.420    0.420 <string>:1(<module>)
        1    0.000    0.000    0.000    0.000 {method 'disable' of '_lsprof.Profiler' objects}


>>> cProfile.run('f2(a)')
         4 function calls in 1.150 CPU seconds

   Ordered by: standard name

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.000    0.000    1.150    1.150 <stdin>:1(f2)
        1    0.000    0.000    1.150    1.150 <string>:1(<module>)
        1    0.000    0.000    0.000    0.000 {method 'disable' of '_lsprof.Profiler' objects}
        1    1.150    1.150    1.150    1.150 {reduce}


>>>