Method much slower than function?

[Gabriel Genellina]

En Wed, 13 Jun 2007 21:40:12 -0300, <idoerg at gmail.com> escribió:

> Hi all,
>
> I am running Python 2.5 on Feisty Ubuntu. I came across some code that
> is substantially slower when in a method than in a function.
>
> ################# START SOURCE #############
> # The function
>
> def readgenome(filehandle):
> 	s = ''
> 	for line in filehandle.xreadlines():
> 		if '>' in line:
> 			continue
> 		s += line.strip()
> 	return s
>
> # The method in a class
> class bar:
> 	def readgenome(self, filehandle):
> 		self.s = ''
> 		for line in filehandle.xreadlines():
> 			if '>' in line:
> 				continue
> 			self.s += line.strip()
>

In the function above, s is a local variable, and accessing local  
variables is very efficient (using an array of local variables, the  
compiler assigns statically an index for each one).
Using self.s, on the other hand, requires a name lookup for each access.  
The most obvious change would be to use a local variable s, and assign  
self.s = s only at the end. This should make both methods almost identical  
in performance.
In addition, += is rather inefficient for strings; the usual idiom is  
using ''.join(items)
And since you have Python 2.5, you can use the file as its own iterator;  
combining all this:

     return ''.join(line.strip() for line in filehandle if '>' not in line)

-- 
Gabriel Genellina