Question

我正在尝试对类的成员函数返回的值使用@postcondition装饰器，如下所示：

def out_gt0(retval, inval):
    assert retval > 0, "Return value < 0"

class foo(object):
    def __init__(self, w, h):
        self.width = w
        self.height = h
    @postcondition(out_gt0)
    def bar(self):
        return -1

当我尝试调用成员函数'bar'时（因此激发@postcondition提供警告）我得到了这个：

>>> f = foo(2,3)
>>> f.bar()
Traceback (most recent call last):
  File "<pyshell#22>", line 1, in <module>
    f.bar()
  File "<pyshell#8>", line 106, in __call__
    result = self._func(*args, **kwargs)
TypeError: bar() takes exactly 1 argument (0 given)
>>>

我对@postcondition的定义是http://wiki.python.org/moin/PythonDecoratorLibrary#Pre-.2FPost-Conditions。

我认为错误的产生是因为作为@postcondition基础的函数不期望处理成员函数（当然我见过的所有例子都只是使用普通的旧函数）但是我不知道如何修复它所以我可以做到这一点？

对任何建议都会感激不尽。

Answer 1

您不需要做任何特别的事情：

import functools

def condition(pre_condition=None, post_condition=None):
    def decorator(func):
        @functools.wraps(func) # presever name, docstring, etc
        def wrapper(*args, **kwargs): #NOTE: no self
            if pre_condition is not None:
               assert pre_condition(*args, **kwargs)
            retval = func(*args, **kwargs) # call original function or method
            if post_condition is not None:
               assert post_condition(retval)
            return retval
        return wrapper
    return decorator

def pre_condition(check):
    return condition(pre_condition=check)

def post_condition(check):
    return condition(post_condition=check)

用法：

@pre_condition(lambda arg: arg > 0)
def function(arg): # ordinary function
    pass

class C(object):
    @post_condition(lambda ret: ret > 0)
    def method_fail(self):
        return 0
    @post_condition(lambda ret: ret > 0)
    def method_success(self):
        return 1

测试：

function(1)
try: function(0)
except AssertionError: pass
else: assert 0, "never happens"

c = C()
c.method_success()
try: c.method_fail()
except AssertionError: pass
else: assert 0, "never happens"

Answer 2

以下示例有效：

def out_gt0(retval):
    assert retval > 0, "Return value < 0"

def mypostfunc(callback):
    def mydecorator(func):
        def retFunc(self, *args, **kwargs):
            retval = func(self, *args, **kwargs)
            callback(retval)
            return retval
        return retFunc
    return mydecorator

class foo(object):
    def __init__(self, w, h):
        self.width = w
        self.height = h
    @mypostfunc(out_gt0)
    def bar1(self):
        return -1
    @mypostfunc(out_gt0)
    def bar2(self):
        return 1

f=foo(1,2)
print "bar2:", f.bar2()
print "bar1:", f.bar1()

，输出为：

bar2: 1
bar1:
Traceback (most recent call last):
  File "s.py", line 27, in <module>
    print "bar1:", f.bar1()
  File "s.py", line 9, in retFunc
    callback(retval)
  File "s.py", line 3, in out_gt0
    assert retval > 0, "Return value < 0"
AssertionError: Return value < 0

成员函数的Python @precondition / @postcondition - 如何？

2 个答案: