What is the Python equivalent of static variables

2019-09-10 07:16发布

What is the idiomatic Python equivalent of this C/C++ code?

void foo()
{
    static int counter = 0;
    counter++;
    printf("counter is %d\n", counter);
}

specifically, how does one implement the static member at the function level, as opposed to the class level? And does placing the function into a class change anything?

标签: python
27条回答
干净又极端
2楼-- · 2019-09-10 07:22

A bit reversed, but this should work:

def foo():
    foo.counter += 1
    print "Counter is %d" % foo.counter
foo.counter = 0

If you want the counter initialization code at the top instead of the bottom, you can create a decorator:

def static_var(varname, value):
    def decorate(func):
        setattr(func, varname, value)
        return func
    return decorate

Then use the code like this:

@static_var("counter", 0)
def foo():
    foo.counter += 1
    print "Counter is %d" % foo.counter

It'll still require you to use the foo. prefix, unfortunately.


EDIT (thanks to ony): This looks even nicer:

def static_vars(**kwargs):
    def decorate(func):
        for k in kwargs:
            setattr(func, k, kwargs[k])
        return func
    return decorate

@static_vars(counter=0)
def foo():
    foo.counter += 1
    print "Counter is %d" % foo.counter
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3楼-- · 2019-09-10 07:23

A little bit more readable, but more verbose (Zen of Python: explicit is better than implicit):

>>> def func(_static={'counter': 0}):
...     _static['counter'] += 1
...     print _static['counter']
...
>>> func()
1
>>> func()
2
>>>

See here for an explanation of how this works.

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我想做一个坏孩纸
4楼-- · 2019-09-10 07:26

Using an attribute of a function as static variable has some potential drawbacks:

  • Every time you want to access the variable, you have to write out the full name of the function.
  • Outside code can access the variable easily and mess with the value.

Idiomatic python for the second issue would probably be naming the variable with a leading underscore to signal that it is not meant to be accessed, while keeping it accessible after the fact.

An alternative would be a pattern using lexical closures, which are supported with the nonlocal keyword in python 3.

def make_counter():
    i = 0
    def counter():
        nonlocal i
        i = i + 1
        return i
    return counter
counter = make_counter()

Sadly I know no way to encapsulate this solution into a decorator.

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不美不萌又怎样
5楼-- · 2019-09-10 07:26

Sure this is an old question but I think I might provide some update.

It seems that the performance argument is obsolete. The same test suite appears to give similar results for siInt_try and isInt_re2. Of course results vary, but this is one session on my computer with python 3.4.4 on kernel 4.3.01 with Xeon W3550. I have run it several times and the results seem to be similar. I moved the global regex into function static, but the performance difference is negligible.

isInt_try: 0.3690
isInt_str: 0.3981
isInt_re: 0.5870
isInt_re2: 0.3632

With performance issue out of the way, it seems that try/catch would produce the most future- and cornercase- proof code so maybe just wrap it in function

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来,给爷笑一个
6楼-- · 2019-09-10 07:27
def staticvariables(**variables):
    def decorate(function):
        for variable in variables:
            setattr(function, variable, variables[variable])
        return function
    return decorate

@staticvariables(counter=0, bar=1)
def foo():
    print(foo.counter)
    print(foo.bar)

Much like vincent's code above, this would be used as a function decorator and static variables must be accessed with the function name as a prefix. The advantage of this code (although admittedly anyone might be smart enough to figure it out) is that you can have multiple static variables and initialise them in a more conventional manner.

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冷血范
7楼-- · 2019-09-10 07:27

Instead of creating a function having a static local variable, you can always create what is called a "function object" and give it a standard (non-static) member variable.

Since you gave an example written C++, I will first explain what a "function object" is in C++. A "function object" is simply any class with an overloaded operator(). Instances of the class will behave like functions. For example, you can write int x = square(5); even if square is an object (with overloaded operator()) and not technically not a "function." You can give a function-object any of the features that you could give a class object.

# C++ function object
class Foo_class {
    private:
        int counter;     
    public:
        Foo_class() {
             counter = 0;
        }
        void operator() () {  
            counter++;
            printf("counter is %d\n", counter);
        }     
   };
   Foo_class foo;

In Python, we can also overload operator() except that the method is instead named __call__:

Here is a class definition:

class Foo_class:
    def __init__(self): # __init__ is similair to a C++ class constructor
        self.counter = 0
        # self.counter is like a static member
        # variable of a function named "foo"
    def __call__(self): # overload operator()
        self.counter += 1
        print("counter is %d" % self.counter);
foo = Foo_class() # call the constructor

Here is an example of the class being used:

from foo import foo

for i in range(0, 5):
    foo() # function call

The output printed to the console is:

counter is 1
counter is 2
counter is 3
counter is 4
counter is 5

If you want your function to take input arguments, you can add those to __call__ as well:

# FILE: foo.py - - - - - - - - - - - - - - - - - - - - - - - - -

class Foo_class:
    def __init__(self):
        self.counter = 0
    def __call__(self, x, y, z): # overload operator()
        self.counter += 1
        print("counter is %d" % self.counter);
        print("x, y, z, are %d, %d, %d" % (x, y, z));
foo = Foo_class() # call the constructor

# FILE: main.py - - - - - - - - - - - - - - - - - - - - - - - - - - - - 

from foo import foo

for i in range(0, 5):
    foo(7, 8, 9) # function call

# Console Output - - - - - - - - - - - - - - - - - - - - - - - - - - 

counter is 1
x, y, z, are 7, 8, 9
counter is 2
x, y, z, are 7, 8, 9
counter is 3
x, y, z, are 7, 8, 9
counter is 4
x, y, z, are 7, 8, 9
counter is 5
x, y, z, are 7, 8, 9
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