Static (local) Method Variables (was: can someone explain?)

Jp Calderone exarkun at intarweb.us
Tue Feb 18 07:57:58 CET 2003


On Tue, Feb 18, 2003 at 12:49:18AM -0500, Mongryong wrote:
> Some people have asked about how to do static (local) method variables
> (like in C/C++).  Apparently, there is a simple way to do it via default
> function parameters.  Consider the following example:
> 
> def append(x, list=[]):
> 	list += x
> 	return list
> 
> >>> append(1)
> [1]
> >>> append(2)
> [1,2]
> 
> Now, the above isn't really a 'true' immitation of C/C++'s static method
> variable implementation.  A true static method variable is created at
> first call.  In Python, default parameters are created at 'import
> time'.  Hence, you're not saving any 'memory space' like you would in
> C/C++.

  Default parameters are *evaluated* at function definition time.  This is
ocassionally the same as "import time", but not always.

  Since we're discussing free functions, there is no memory saved in either
Python or C.  If we were discussing static class members, C++ style, then a
far more appropriate comparison is Python class attributes, which are
"shared" amongst all instances of a class, just as C++'s static class
members are.

> 
> Note, the above only works for referencable objects and not built in
> types like 'ints'.
> 

  This is wrong.  All objects in Python are "referencable".  What do you
think "x = 5" does?  I believe you are suffering from the common confusion
about mutable vs immutable objects.

  Jp

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