Generators vs. Functions?

[Steven D'Aprano]

On Sun, 05 Feb 2006 03:31:24 +0000, Neil Schemenauer wrote:

> Peter Hansen <peter at engcorp.com> wrote:
>> More precisely, the state of the function is *saved* when a yield 
>> occurs, so you certainly don't *recreate* it from scratch, but merely 
>> restore the state, and this should definitely be faster than creating it 
>> from scratch in the first place.
> 
> Right.  Resuming a generator is faster than calling a function.

Have you actually measured this, or are you just making a wild guess?

According to a short test performed by Magnus Lycka, resuming a generator
takes more time than calling a function. My own test agrees.

Here is my test, using Python 2.3. I've tried to make the test as fair as
possible, with the same number of name lookups in both pieces of test code.

# straight function, two name lookups

>>> import timeit
>>>
>>> t1 = timeit.Timer(stmt="func.next()", setup=
... """class K:
...   pass
...
... def next():
...   return 1
...
... func = K()
... func.next = next
... """)
>>>
>>> t1.timeit()
0.63980388641357422


# generator, two name lookups

>>> t2 = timeit.Timer(stmt="gen.next()", setup=
... """def g():
...   while 1: yield 1
...
... gen = g()
... """)
>>>
>>> t2.timeit()
0.82081794738769531


# straight function, one name lookup

>>> t3 = timeit.Timer(stmt="f()", setup=
... """def f():
...   return 1
... """)
>>> 
>>> t3.timeit()
0.47273492813110352


# generator, one name lookup 

>>> t4 = timeit.Timer(stmt="gnext()", setup=
... """def g():
...   while 1: yield 1
...
... gnext = g().next
... """)
>>>
>>> t4.timeit()
0.55085492134094238


So on the basis of my tests, there is a small, but significant speed
advantage to _calling_ a function versus _resuming_ a generator.

Of course the other advantages of generators often far outweigh the tiny
setup cost each time you call one. In addition, for any complex function
with significant execution time, the call/resume time may be an
insignificant fraction of the total execution time. There is little or no
point in avoiding generators due to a misplaced and foolish attempt to
optimise your code.


-- 
Steven.