[Tutor] map() and lambda to change class instance attribute (fwd)

[Bernard Lebel]

Well, that was a nice explanation. Thanks once again Kent!

Bernard


On 5/16/05, Kent Johnson <kent37 at tds.net> wrote:
> Bernard Lebel wrote:
> > Hi Kent,
> >
> > So if I undestand you right, mapping a function with map() when it is
> > a built-in function will/may be faster than a for loop, but if it's a
> > custom function (ie. a def one), it will most likely be slower?
> 
> I guess I didn't proofread that last mail...what I meant is
> 
> - Write the code for clarity first
> - If you find a performance bottleneck then test different solutions to see which is fastest for
> your actual data and usage.
> - Mapping a builtin function over a list is relatively fast in Python and it is worth trying if it
> meets your needs.
> - Calling a function is relatively expensive in Python so if using map() requires you to define a
> helper that may wipe out its advantage.
> - Testing is crucial! Guessing is only good for helping to come up with ideas to test.
> 
> Here is a program that tests six ways to apply a function to elements of a list. The functions don't
> create new lists except as side effects since that was the original problem. They use map(), list
> comprehension and an explicit for loop to apply str() or a Python function returning str() to the
> elements of a list. (The list already contains strings so the actual function call should be fast.)
> 
> ####
> 
> import timeit
> 
> data = [str(n) for n in range(100)]
> 
> def strByMap():
>      map(str, data)
> 
> def strByListComp():
>      [str(n) for n in data]
> 
> def strByLoop():
>      for n in data:
>          str(n)
> 
> 
> def func(x):
>      return str(x)
> 
> 
> def funcByMap():
>      map(func, data)
> 
> def funcByListComp():
>      [func(n) for n in data]
> 
> def funcByLoop():
>      for n in data:
>          func(n)
> 
> 
> def timeOne(fn):
>      setup = "from __main__ import " + fn.__name__
>      stmt = '%s()' % (fn.__name__)
> 
>      t = timeit.Timer(stmt, setup)
>      secs = min(t.repeat(number=1000))
>      print '%15s %s' % (fn.__name__, secs)
> 
> for fn in [ strByMap, strByListComp, strByLoop, funcByMap, funcByListComp, funcByLoop ]:
>      timeOne(fn)
> 
> ###
> 
> Here are the results on my computer:
> 
>         strByMap 0.0359623918682
>    strByListComp 0.0581065470611
>        strByLoop 0.0481150537289
>        funcByMap 0.0810943849009
>   funcByListComp 0.0891375859222
>       funcByLoop 0.0806144356336
> 
> So for this test, map is fastest when calling a builtin and explicit looping is fastest when calling
> a function. With the explicit loop you might be able to inline the function, in which case it would
> be much faster than either map or list comp.
> 
> Modifying the program slightly so that each function actually creates a list changes the results
> dramatically:
>         strByMap 0.0365733633744
>    strByListComp 0.0579948010152
>        strByLoop 0.0764722890758
>        funcByMap 0.0811885309446
>   funcByListComp 0.0883995032888
>       funcByLoop 0.10586876265
> 
> Now map() is fastest in both cases, though a for loop with inlined code beats map() with an external
> function.
> 
> Kent
> >
> >
> > Thanks
> > Bernard
> >
> >
> > On 5/13/05, Kent Johnson <kent37 at tds.net> wrote:
> >
> >>Bernard Lebel wrote:
> >>
> >>>The authors even go as far as saysing, on page 228 (first paragraph)
> >>>that map() used that way has a performance benefit and is faster than
> >>>a for loop.
> >>
> >>That may well be correct, at least in the case where the function passed to map is a builtin.
> >>Mapping a builtin to over a list is extremely fast. So write the code with a for loop so it is
> >>clear. When you identify a performance bottleneck you can try rewriting your loop using map or list
> >>comprehension, which is also fast. Until then it is premature optimization. For typical loops over a
> >>small number of items I can't imagine you would notice the difference.
> >>
> >>Kent
> >>
> >>_______________________________________________
> >>Tutor maillist  -  Tutor at python.org
> >>http://mail.python.org/mailman/listinfo/tutor
> >>
> >
> > _______________________________________________
> > Tutor maillist  -  Tutor at python.org
> > http://mail.python.org/mailman/listinfo/tutor
> >
> 
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