如何使用装饰器为函数参数设置默认值？

Question

I'd like to define, for the bar function, the default value to the parameter param2 :

def foo(default_value):
    def bar(param1, param2=default_value)
        """bar does some stuff"""
    return bar
bar = foo(default_value)

How would this be possible with decorators ?

More globally, if someone has some documentation about how can we manipulate args and kwargs when decorating a function with argument, I would be happy to take a look.

Answer 1

The syntax for decorators with arguments is different. It should return a function that will take a function and return another function. For example:

def foo(default_value):
    def wrapped(f):
        def bar(param1, param2=default_value):
            print "inside bar"
            print "param1=%s param%s"%(param1, param2)
        return bar
    return wrapped

Now when you decorate a function:

@foo(default_value="test param2")
def test_decorator():
    pass

Calling it will result in:

test_decorator("test param1")

inside bar
param1=test param1 param2=test param2

Alternatively with not accepting the default value:

test_decorator("test param1", "other test param2")

inside bar
param1=test param1 param2=other test param2

You can read more on the subject here

Answer 2

I'm not sure I understand what your sample code means, but the following seems to do what what the title of your question asks:

PLACE_HOLDER = object()

def default_arg(default_value):
    def decorator(f):
        if f.func_defaults and f.func_defaults[0] is PLACE_HOLDER:
            f.func_defaults = (default_value,)
        return f
    return decorator

@default_arg(42)
def bar(param1, param2=PLACE_HOLDER):
    """bar does some stuff"""
    print('bar({}, {}) called'.format(param1, param2))


bar(1)

Output:

bar(1, 42) called