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static variables in python

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Join Date: Aug 2005

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Avner .H.

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static variables in python

Sep 27th, 2005

Hello fellows...

Is there something like static function variables in python?
I know there are static attributes of a class, but what if i want
to use a static variable inside a function?
somthing like this (stupid) code in c++ for example:

int static_num()
{
static int x = 0;
return ++x;
}

void main()
{
for (int i=0; i<10; i++)
{
cout << static_num() << endl;
}
}

If there aren't such static variables , is there a way for doing something similar to this c++ code?

thanks a lot!

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Join Date: Jul 2005

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CrazyDieter CrazyDieter is offline

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Re: static variables in python

Sep 27th, 2005

 Help with Code Tags 
 Python Syntax (Toggle Plain Text) 
x=0
def static_num() :
   global x
   x=x+1
   return x
 
for i in range(0,10) :
     print static_num
x=0
def static_num() :
   global x
   x=x+1
   return x

for i in range(0,10) :
     print static_num

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Join Date: Jul 2005

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bumsfeld

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Re: static variables in python

Sep 30th, 2005

I always learn that the use of global variables is to be discouraged. Is this the only way to copy static variable behaviour in Python?

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vegaseat

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Re: static variables in python

Oct 3rd, 2005

Bumsfeld you are right, avoid globals, they do lead to errors. Using a generator function avoids globals ...

 Help with Code Tags 
 python Syntax (Toggle Plain Text) 
def static_num2():
    k = 0
    while True:
       k += 1
       yield k
 
static = static_num2().next
 
for i in range(0,10) :
    print static()
def static_num2():
    k = 0
    while True:
       k += 1
       yield k

static = static_num2().next

for i in range(0,10) :
    print static()

Last edited by vegaseat; Dec 3rd, 2007 at 1:28 am. Reason: changed php tags

May 'the Google' be with you!

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Join Date: Jul 2005

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bumsfeld

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Re: static variables in python

Oct 3rd, 2005

I knew there had to be a way to avoid these global variables, I am still learning a lot, generator functions are somewhat confusing right now!

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Join Date: Dec 2007

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taw

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Re: static variables in python

Dec 2nd, 2007

Hmm. There are a couple of ways you could simulate the effect of static variables - but they aren't explicitly in the language.

One way is to abuse the fact that default parameters for functions are evaluated only once when the function is defined, and then kept for the rest of time - so that if one makes the default value a new list or dictionary then it can be used to share values between any running version of the function. For example:

 Help with Code Tags 
 python Syntax (Toggle Plain Text) 
def last_argument(n, arg_store = []):
    "Return the argument used when the function was last called"
    arg_store.append(n)
    if len(arg_store) == 1:
        return -1
    else:
        return arg_store[-2]
def last_argument(n, arg_store = []):
    "Return the argument used when the function was last called"
    arg_store.append(n)
    if len(arg_store) == 1:
        return -1
    else:
        return arg_store[-2]

I'm not sure whether this would be classed as a 'good solution' or not... I don't know whether it counts as confusing to use stored default arguments for sharing state - also you are never going to use this parameter.

As an alternative you could explicit give the function somewhere to store its state each time it's called. We could do this using an explicit paramter, a class or an instance.

For a parameter we'd effectively use the same as above, apart from we wouldn't specify a default value. We'd then call

 Help with Code Tags 
 python Syntax (Toggle Plain Text) 
state = []
last_argument(1, state)
last_argument(2, state)
state = []
last_argument(1, state)
last_argument(2, state)

The code for a class would use a class method for flair.

 Help with Code Tags 
 python Syntax (Toggle Plain Text) 
class A(object):
    last = -1
    @classmethod
    def last_argument(cls, n):
        temp = cls.last
        cls.last = n
        return temp
 
A.last_argument(1)
A.last_argument(17)...
class A(object):
    last = -1
    @classmethod
    def last_argument(cls, n):
        temp = cls.last
        cls.last = n
        return temp

A.last_argument(1)
A.last_argument(17)...

The code an instance of a class is much as you would expect.

The default argument approach is probably the approach closest to a static variable, the class method might feel the 'cleanest' - depending on one's bigotries.

Tom

Last edited by vegaseat; Dec 3rd, 2007 at 1:34 am. Reason: changed code tags from [\code] to [/code]