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
在尝试了几种方法后,我最终使用了@warvariuc的答案的改进版本:
import types
def func(_static=types.SimpleNamespace(counter=0)):
_static.counter += 1
print(_static.counter)
其他回答
使用generator函数生成迭代器。
def foo_gen():
n = 0
while True:
n+=1
yield n
然后像这样使用它
foo = foo_gen().next
for i in range(0,10):
print foo()
如果你想要一个上限:
def foo_gen(limit=100000):
n = 0
while n < limit:
n+=1
yield n
如果迭代器终止(如上面的例子),您也可以直接遍历它,如
for i in foo_gen(20):
print i
当然,在这些简单的情况下,最好使用xrange:)
这是yield statement的文档。
鉴于这个问题,我可以提出另一个可能更好用的替代方案,对方法和函数看起来都是一样的:
@static_var2('seed',0)
def funccounter(statics, add=1):
statics.seed += add
return statics.seed
print funccounter() #1
print funccounter(add=2) #3
print funccounter() #4
class ACircle(object):
@static_var2('seed',0)
def counter(statics, self, add=1):
statics.seed += add
return statics.seed
c = ACircle()
print c.counter() #1
print c.counter(add=2) #3
print c.counter() #4
d = ACircle()
print d.counter() #5
print d.counter(add=2) #7
print d.counter() #8
如果你喜欢这种用法,下面是它的实现:
class StaticMan(object):
def __init__(self):
self.__dict__['_d'] = {}
def __getattr__(self, name):
return self.__dict__['_d'][name]
def __getitem__(self, name):
return self.__dict__['_d'][name]
def __setattr__(self, name, val):
self.__dict__['_d'][name] = val
def __setitem__(self, name, val):
self.__dict__['_d'][name] = val
def static_var2(name, val):
def decorator(original):
if not hasattr(original, ':staticman'):
def wrapped(*args, **kwargs):
return original(getattr(wrapped, ':staticman'), *args, **kwargs)
setattr(wrapped, ':staticman', StaticMan())
f = wrapped
else:
f = original #already wrapped
getattr(f, ':staticman')[name] = val
return f
return decorator
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)
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
惯用的方法是使用类,类可以有属性。如果您需要实例不分离,请使用单例。
有许多方法可以将“静态”变量伪造或蒙骗到Python中(到目前为止没有提到的一种方法是使用可变的默认参数),但这不是Python的惯用方法。只需要使用一个类。
如果您的使用模式合适,也可以使用生成器。