Empty list as default parameter

[Jay O'Connor]

Paul Rubin wrote:

>bokr at oz.net (Bengt Richter) writes:
>  
>
>> changing the dynamic nature of the language.
>>    
>>
>
>I think some of that dynamicness should be toned down as Python matures.
>

One aspect of Python's dynamic nature that has always intrigued me is in 
the terms of how instance variables.  Python allows you to dynamically 
add instance variables as you need, which is pretty cool, but seems to 
require a lookup of every instance variable reference, which can be 
pretty slow.

Consider the following test case, using Python 2.3 on Win95 in IDLE:
=====================
    class A:
        def __init__(self, val=1):
            self.value= val

        def setval (self, v):
            self.value=v

        def getval (self):
            return self.value
       
    def test(self):
        for x in range (1,1000000):
            self.value = 1
            y = self.value

    import time
    a = A()
    print "start"
    t1 = time.time()
    a.test()
    t2 = time.time()
    print t2 - t1
    print "done"
=====================

This routinely return values of over 6.3.

Swithing the implementation of test() to use the accessor methods kicked 
the times up to over 20 seconds

By contrast the equvalent Smalltalk* code (VisualWorks 7,1) on the same 
machine. gave me consistant results of a little over 600 milliseconds 
when using direct variable access and roughly 1.5 seconds when using 
accessors. 

I think a portion of the difference in that in Smalltalk, instance 
variables are specified in the class metadata (in terms of ordering) 
when the class is compiled and thus an instance variable reference is 
just a pointer offset from the start of the object data in memory and 
thus the compiler can optimize instance variable reference by just 
compiling in the offset into the the mehod code.
 

This is a case where Python is more dynamic than Smalltalk in that 
Python allows easy addition of instance variables to objects, but it 
comes at a price in terms of performance.  The Smalltalk solution to 
adding instance variables dynamically is just to carry around a 
dictionary (similar to Python) but most experienced Smalltalkers know 
that "instance variables are much faster than dictionary lookups so 
consider if this is really the right design and consider the 
performance/flexibility tradeoff"

This is one case where I think Python's flexibility hurts it in the long 
run and perhaps as it goes forward, it will adapt to a more rigid style 
that will provide for better performance without too much tradeoff.  
After all, Python is still very young, Smalltalk has a twenty year head 
start on it.


*Smalltalk code
======================

A>>#test

    | y  |

    10000000 timesRepeat: [
        self value: 1.
        y :=  self value
    ]
-----------
| a |

a := A new.
Time millisecondsToRun: [a test].
======================