这个C/ c++代码在Python中的等效代码是什么?

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

具体来说,如何在函数级别实现静态成员,而不是在类级别?将函数放入类中会改变什么吗?


当前回答

你可以创建一个所谓的“函数对象”,并给它一个标准的(非静态的)成员变量,而不是创建一个具有静态局部变量的函数。

既然你给出了一个c++编写的例子,我将首先解释什么是c++中的“函数对象”。“函数对象”就是任何带有重载操作符()的类。类实例的行为类似于函数。例如,你可以写int x = square(5);即使square是一个对象(带有重载操作符()),从技术上讲也不是一个“函数”。你可以给一个函数对象任何你可以给一个类对象的特性。

# 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;

在Python中,我们也可以重载operator(),除非该方法被命名为__call__:

下面是一个类定义:

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

下面是一个使用这个类的例子:

from foo import foo

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

打印到控制台的输出是:

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

如果你想让你的函数接受输入参数,你也可以将它们添加到__call__:

# 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

其他回答

当然,这是一个老问题,但我想我可以提供一些更新。

看来性能论点已经过时了。 对于siInt_try和isInt_re2,相同的测试套件似乎给出了类似的结果。 当然,结果会有所不同,但这是在我的计算机上使用python 3.4.4的一次会话,使用Xeon W3550的内核4.3.01。 我已经运行了几次,结果似乎相似。 我将全局正则表达式移动到函数静态,但性能差异可以忽略不计。

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

考虑到性能问题,try/catch似乎可以生成最适合未来和墙角情况的代码,所以可能只是将其包装在函数中

_counter = 0
def foo():
   global _counter
   _counter += 1
   print 'counter is', _counter

Python习惯上使用下划线来表示私有变量。在C语言中,在函数内部声明静态变量的唯一原因是将它隐藏在函数之外,这并不是真正的Python惯用方法。

我个人更喜欢下面的装饰。各有各的。

def staticize(name, factory):
    """Makes a pseudo-static variable in calling function.

    If name `name` exists in calling function, return it. 
    Otherwise, saves return value of `factory()` in 
    name `name` of calling function and return it.

    :param name: name to use to store static object 
    in calling function
    :type name: String
    :param factory: used to initialize name `name` 
    in calling function
    :type factory: function
    :rtype: `type(factory())`

    >>> def steveholt(z):
    ...     a = staticize('a', list)
    ...     a.append(z)
    >>> steveholt.a
    Traceback (most recent call last):
    ...
    AttributeError: 'function' object has no attribute 'a'
    >>> steveholt(1)
    >>> steveholt.a
    [1]
    >>> steveholt('a')
    >>> steveholt.a
    [1, 'a']
    >>> steveholt.a = []
    >>> steveholt.a
    []
    >>> steveholt('zzz')
    >>> steveholt.a
    ['zzz']

    """
    from inspect import stack
    # get scope enclosing calling function
    calling_fn_scope = stack()[2][0]
    # get calling function
    calling_fn_name = stack()[1][3]
    calling_fn = calling_fn_scope.f_locals[calling_fn_name]
    if not hasattr(calling_fn, name):
        setattr(calling_fn, name, factory())
    return getattr(calling_fn, name)

米格尔·安吉洛的自我重新定义解决方案甚至可以不需要任何装饰:

def fun(increment=1):
    global fun
    counter = 0
    def fun(increment=1):
        nonlocal counter
        counter += increment
        print(counter)
    fun(increment)

fun()    #=> 1
fun()    #=> 2
fun(10)  #=> 12

第二行必须进行调整,以获得有限的范围:

def outerfun():
    def innerfun(increment=1):
        nonlocal innerfun
        counter = 0
        def innerfun(increment=1):
            nonlocal counter
            counter += increment
            print(counter)
        innerfun(increment)

    innerfun()    #=> 1
    innerfun()    #=> 2
    innerfun(10)  #=> 12

outerfun()

装饰器的优点是,你不必额外注意你的施工范围。

我写了一个简单的函数来使用静态变量:

def Static():
    ### get the func object by which Static() is called.
    from inspect import currentframe, getframeinfo
    caller = currentframe().f_back
    func_name = getframeinfo(caller)[2]
    # print(func_name)
    caller = caller.f_back
    func = caller.f_locals.get(
        func_name, caller.f_globals.get(
            func_name
        )
    )
    
    class StaticVars:
        def has(self, varName):
            return hasattr(self, varName)
        def declare(self, varName, value):
            if not self.has(varName):
                setattr(self, varName, value)

    if hasattr(func, "staticVars"):
        return func.staticVars
    else:
        # add an attribute to func
        func.staticVars = StaticVars()
        return func.staticVars

使用方法:

def myfunc(arg):
    if Static().has('test1'):
        Static().test += 1
    else:
        Static().test = 1
    print(Static().test)

    # declare() only takes effect in the first time for each static variable.
    Static().declare('test2', 1)
    print(Static().test2)
    Static().test2 += 1