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?
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
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。
其他回答
另一个(不推荐!)对https://stackoverflow.com/a/279598/916373这样的可调用对象的扭曲,如果您不介意使用一个时髦的调用签名的话
class foo(object):
counter = 0;
@staticmethod
def __call__():
foo.counter += 1
print "counter is %i" % foo.counter
>>> foo()()
counter is 1
>>> foo()()
counter is 2
其他答案已经演示了您应该如何做到这一点。这里有一种方法你不应该:
>>> def foo(counter=[0]):
... counter[0] += 1
... print("Counter is %i." % counter[0]);
...
>>> foo()
Counter is 1.
>>> foo()
Counter is 2.
>>>
默认值仅在函数第一次求值时初始化,而不是每次执行时初始化,因此可以使用列表或任何其他可变对象来存储静态值。
许多人已经建议测试“hasattr”,但有一个更简单的答案:
def func():
func.counter = getattr(func, 'counter', 0) + 1
没有try/except,没有测试hasattr,只有默认的getattr。
有点相反,但这应该是有效的:
def foo():
foo.counter += 1
print "Counter is %d" % foo.counter
foo.counter = 0
如果你想让计数器初始化代码在顶部而不是底部,你可以创建一个装饰器:
def static_vars(**kwargs):
def decorate(func):
for k in kwargs:
setattr(func, k, kwargs[k])
return func
return decorate
然后像这样使用代码:
@static_vars(counter=0)
def foo():
foo.counter += 1
print "Counter is %d" % foo.counter
它仍然需要你使用foo。前缀,不幸的是。
(图片来源:@ony)
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)