Initializing static member variable in a function

Question

Is it possible to have static variables within class functions(as in C++).
The following does not give me what is desired.
The motivation is to initialize(very expensive process) a lookup list inside a function - but only if and when it is called.
Subsequent invocations of the same function should not need the variable to be initialized again.
Is there an idiom to achieve this?
Its fine if the function is aligned to the class;so that the value of rules is then available to all instances of 'A'

>>> class A:
...     def func(self):
...         if not hasattr(self.func,"rules"):
...             print 'initialize'
...             rules=[8,7,6]
...         rules.append(4)
...         return rules
...         
>>> a=A()
>>> for x in range(5):
...     print a.func()
...     

initialize
[8, 7, 6, 4]
initialize
[8, 7, 6, 4]
initialize
[8, 7, 6, 4]
initialize
[8, 7, 6, 4]
initialize
[8, 7, 6, 4]

Do you want that variable "rules" to be associated with the class or with the instance? — BrenBarn
– BrenBarn, Commented May 28, 2013 at 5:29

jamylak · Accepted Answer · 2013-05-28 05:44:33Z

2

class A(object): # subclass object for newstyle class
    def func(self, rules=[]):
        if not rules:
            rules[:] = [8, 7, 6]        
        rules.append(4)
        return rules

>>> a = A()
>>> for x in range(5):
        print a.func()


[8, 7, 6, 4]
[8, 7, 6, 4, 4]
[8, 7, 6, 4, 4, 4]
[8, 7, 6, 4, 4, 4, 4]
[8, 7, 6, 4, 4, 4, 4, 4]

edited May 28, 2013 at 5:44

answered May 28, 2013 at 5:37

jamylak

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6 Comments

Thomas Fenzl Over a year ago

but that would calculate rules on parsing A, not on first use.

jamylak Over a year ago

@ThomasFenzl sorry missed that part

jamylak Over a year ago

@ThomasFenzl I've fixed that now

IUnknown Over a year ago

This is perfect - but please explain the principle behind this.What makes this behave differently from hasattr?

jamylak Over a year ago

@IUnknown rules is a member of the function itself. It begins empty, on the first run of func() it gets filled with [8, 7, 6]

|

BrenBarn · Accepted Answer · 2013-05-28 05:37:48Z

1

If you want the variable to be associated with the class, store it on the class:

class A(object):
    def func(self):
        if not hasattr(A,"rules"):
            print 'initialize'
            A.rules=[8,7,6]
        A.rules.append(4)
        return rules

answered May 28, 2013 at 5:37

BrenBarn

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