python方法内的静态变量

Question

In a Python method, I would like to have a local variable whose value persists between calls to the method.

This question shows how to declare such "static variables" (c++ terminology) inside functions. I tried to do the same in an instance method, and failed.

Here's a working minimal example that reproduces the problem. You can copy-paste it into an interpreter.

class SomeClass(object):
    def some_method(self):
        if not hasattr(SomeClass.some_method, 'some_static_var'):
            SomeClass.some_method.some_static_var = 1  # breaks here

        for i in range(3):
            print SomeClass.some_method.some_static_var
            SomeClass.some_method.some_static_var += 1

if __name__ == '__main__':
    some_instance = SomeClass()
    some_instance.some_method()

On the line labeled "# breaks here", I get:

AttributeError: 'instancemethod' object has no attribute 'some_static_var'

I realize there's an easy workaround, where I make some_static_var a member variable of SomeClass . However, the variable really has no use outside of the method, so I'd much prefer to keep it from cluttering up SomeClass ' namespace if I could.

Answer 1

In python 2, you have to deal with bound and unbound methods. These do not have a __dict__ attribute, like functions do:

#python 2
'__dict__' in dir(SomeClass.some_method)
Out[9]: False

def stuff():
    pass

'__dict__' in dir(stuff)
Out[11]: True

In python 3, your code works fine! The concept of bound/unbound methods is gone, everything is a function.

#python 3
'__dict__' in dir(SomeClass.some_method)
Out[2]: True

Back to making your code work, you need to put the attribute on the thing which has a __dict__ : the actual function:

if not hasattr(SomeClass.some_method.__func__, 'some_static_var'):
    #etc

Read more on im_func and __func__ here

It is up to you to decide whether this makes your code more or less readable - for me, making these types of things class attributes is almost always the way to go; it doesn't matter that only one method is accessing said attribute, it's where I look for "static" type vars. I value readable code over clean namespaces.

This last paragraph was of course an editorial, everyone is entitled to their opinion :-)

Answer 2

You can't set attribute on method objects.

Creating class attributes instead (that is, SomeClass.some_var = 1 ) is the standard Python way. However, we might be able to suggest more appropriate fixes if you give us a high-level overview of your actual problem (what are you writing this code for?).

Answer 3

Use the global keyword to access file-level variables

my_static = None

class MyClass(object):
    def some_method(self):
        global my_static
        if my_static is None:
            my_static = 0
        else:
            my_static = my_static + 1
        print my_static

if __name__ == '__main__':
    instance = MyClass()
    instance.some_method()
    instance.some_method()

Outputs:

0
1

Although, as mentioned elsewhere, a class variable would be preferable