A question on python performance.

[chris.monsanto at gmail.com]

On Sep 26, 2:26 pm, "Joe Goldthwaite" <j... at goldthwaites.com> wrote:
> Hi everyone,
>
> I'm a developer who's been using python for a couple of years.  I wrote a
> fairly large application using it but I was learning the language at the
> same time so it most of the code kind of sucks.
>
> I've learned a lot since then and I've been going through my code trying to
> organize it better and make better use of Python's features.  I'm still  not
> an expert by any definition but I'm slowly getting better.
>
> I've been working on a trend class that takes twelve monthly numbers and
> returns a period to date, quarter to date, year to date and quarterly year
> to date numbers for a specific period. This worked but I ended up with a lot
> of code like this;
>
> def getValue(trend, param, per):
>    if param == 'Ptd':
>      return trend.Ptd(per)
>    elif param == 'Qtd':
>       return trend.Qtd(per)
>    elif param == 'Ytd':
>       return trend.Ytd(per)
>    elif param == 'YtdQ':
>       return trend.YtdQ(per)
>
> The code gets kind of wordy so I started trying to figure out how to call
> them dynamically since the param type is the same as the method the
> retrieves it.  I came up with this;
>
> def getValue(trend, param, per):
>    return trend.__class__.__dict__[param](trend, per)
>
> That worked but it seems like the above line would have to do lots more
> object look ups at runtime so I didn't think it would be very efficient.  I
> thought maybe I could add a caller method to the trend class and I came up
> with this;
>
> class trend:
>    ...
>    ...
>    ...
>    def caller(self, param, *args):
>       return self.__class__.__dict__[param](self, *args)
>
> This simplified the getValue function to this;
>
> def getValue(trend, param, per):
>         return trend.caller(param, per)
>
> Out of curiosity, I thought I'd do some benchmarking and see which one
> performs the best. I executed three multiple times;
>
> loop one.  Time=11.71 seconds;
>     trend.Ptd(per)
>     trend.Qtd(per)
>     trend.Ytd(per)
>     trend.YtdQ(per)
>
> loop two. 12.107 seconds;
>     trend.__class__.__dict__['Ptd'](trend, per)
>     trend.__class__.__dict__['Qtd'](trend, per)
>     trend.__class__.__dict__['Ytd'](trend, per)
>     trend.__class__.__dict__['YtdQ'](trend, per)
>
> loop three. 17.085 seconds;
>     trend.caller('Ptd', per)
>     trend.caller('Qtd', per)
>     trend.caller('Ytd', per)
>     trend.caller('YtdQ', per)
>
> The first surprise was how close the first and second loops were.  I would
> have thought the first loop would be much faster.  The second surprise was
> how much slower the third loop was.  I know it has an extra call in there
> but other than that, it's doing basically the same thing as loop two.  Is
> there that much overhead in making a class method call?
>
> Can anyone explain the differences?

Makes perfect sense to me! Think about it:

method 1: looks up the method directly from the object (fastest)
method 2: looks up __class__, then looks up __dict__, then gets the
element from __dict__
method 3: looks up caller, looks up __class__, looks up __dict__, gets
element from __dict__

To get the element directly from the object (method 1), Python has to
internally check __class__.__dict__[element], which shows why method 1
and method 2 are nearly the same speed. The last version has to look
up caller in addition to the process described by method 2.

The best way to do what you are doing:

getattr(self, param)(self, *args)