在装饰器中解析args和kwargs

Question

我有一个带args和kwargs的函数,我需要在我的装饰器中根据函数中第二个 arg 的值做一些事情,如下面的代码所示:

def workaround_func():
    def decorator(fn):
        def case_decorator(*args, **kwargs):
            if args[1] == 2:
                print('The second argument is a 2!')
            return fn(*args, **kwargs)
        return case_decorator
    return decorator

@workaround_func()
def my_func(arg1, arg2, kwarg1=None):
    print('arg1: {} arg2: {}, kwargs: {}'.format(arg1, arg2, kwarg1))

问题是python允许用户使用第二个参数作为常规参数或关键字参数调用该函数,因此如果用户my_func使用arg2kwarg 调用它,它会引发一个IndexError,见下文:

In [8]: d.my_func(1, 2, kwarg1=3)
The second argument is a 2!
arg1: 1 arg2: 2, kwargs: 3

In [9]: d.my_func(1, arg2=2, kwarg1=3)
---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-9-87dc89222a9e> in <module>()
----> 1 d.my_func(1, arg2=2, kwarg1=3)

/home/camsparr/decoratorargs.py in case_decorator(*args, **kwargs)
      2     def decorator(fn):
      3         def case_decorator(*args, **kwargs):
----> 4             if args[1] == 2:
      5                 print('The second argument is a 2!')
      6             return fn(*args, **kwargs)

IndexError: tuple index out of range

有没有办法解决这个问题而不仅仅是做一个try/except并抓住了IndexError？

Answer 1

这是我能想到的最强大的方法来处理它......诀窍是检查第二个参数的名称.然后,在装饰器中,检查是否存在该名称kwargs.如果是,那么你使用它.如果不是,那么你使用args.

from inspect import getargspec    

def decorate(fn):
    argspec = getargspec(fn)
    second_argname = argspec[0][1]
    def inner(*args, **kwargs):
        special_value = (kwargs[second_argname] 
                         if second_argname in kwargs else args[1])
        if special_value == 2:
            print "foo"
        else:
            print "no foo for you"
        return fn(*args, **kwargs)
    return inner

@decorate
def foo(a, b, c=3):
    pass

foo(1,2,3)
foo(1,b=2,c=4)
foo(1,3,5)
foo(1,b=6,c=5)

运行此结果:

foo
foo
no foo for you
no foo for you

正如所料.

Answer 2

我使用python decorator包找到了答案。该软件包的一个功能是，无论用户如何传递，它都会保留位置/关键字参数。它具有减少大量代码的额外好处，因此我的原始代码为：

def workaround_func():
    def decorator(fn):
        def case_decorator(*args, **kwargs):
            if args[1] == 2:
                print('The second argument is a 2!')
            return fn(*args, **kwargs)
        return case_decorator
    return decorator

@workaround_func()
def my_func(arg1, arg2, kwarg1=None):
    print('arg1: {} arg2: {}, kwargs: {}'.format(arg1, arg2, kwarg1))

变成：

from decorator import decorator

@decorator
def workaround_decorator(f, *args, **kwargs):
    if args[1] == 2:
        print('The second argument is 2!')
    return f(*args, **kwargs)

@workaround_decorator
def my_func(arg1, arg2, kwarg1=None):
    print('arg1: {} arg2: {}, kwargs: {}'.format(arg1, arg2, kwarg1))