如何在带有 *args 和 **kwargs 的函数调用中使用默认参数？

Question

I have the following simple example code:

def wrapper(foo, para1=1, *args):
    print("para1: "+ str(para1))
    print(foo(*args))

def foo1(x):
    return 2*x

wrapper(foo1, 2, 3)

where I define a function wrapper that has one parameter para1 with default value 1 .

But in order to call the wrapper with function foo1 correctly, I have to set para1 all the time, because I have to pass the extra arguments to foo1 . That means the default value para1=1 does not make any sense, as I have to define it all the time anyway.

Or is there some other way to define the functions so I can easilz make use of this default value without to have to define it all the time?

For example,

wrapper(foo1, *args=(3,))
wrapper(foo=foo1, args=(3,))

does not work ...

Use case example:

def wrapper(foo, timeout=10, *args):
    time0 = time.time()
    while time0 < time.time() + timeout:
        if foo(*args):
             return True
        time.sleep(1)
    raise SomeTimeout Exception

Answer 1

You defined the foo1 function in a way that accepts a number, but you try to pass a tuple to it in the wrapper function. the following should work:

def wrapper(foo, para1=1, *args):
    print("para1: "+ str(para1))
    print(foo(*args))



def foo1(*x):
    return 2*x


wrapper(foo1, (2, 3))

in the definition of foo1 x changed to *x .

Answer 2

Edit: Since you have now made it clear it is for Python 2, def wrapper(foo, *args, timeout=10) would give you a syntax error. So use the below method instead.

With just *args alone I don't think it's very feasible. If possible, try making use of **kwargs instead:

def wrapper(foo, *args, **kwargs):
    time0 = time.time()
    # define a default timeout period if not explicitly passed in by keyword
    timeout = kwargs.get('timeout') if 'timeout' in kwargs else 10
    while time0 < time.time() + timeout:
        if foo(*args):
            return True
        time.sleep(1)
    raise SomeTimeOut Exception

Every time you want to explicitly provide a timeout period, call it as such:

wrapper(foo, 1, 2, 3, timeout=60)

Test code :

def wrapper(foo, *args, **kwargs):
    timeout = kwargs.get('timeout') if 'timeout' in kwargs else 10
    print 'Timeout = {0}'.format(timeout)
    foo(*args)

def foo(*args):
    print 'Args = {0}'.format([a for a in args])

wrapper(foo, 1, 2, 3, timeout=20)

# Results
# Timeout = 20
# Args = [1, 2, 3]

Answer 3

What about making it like this?

#/usr/bin/env python3

def f1(f, *args, **kwargs):
    if not 'b' in kwargs:
        kwargs['b'] = 1
    print('b =',kwargs['b'])
    f(args)

def fx(args):
    print(args)

f1(fx, 3, 4)
print()
f1(fx, 3, 4, b=2)

Output:

b = 1                                                                     
(3, 4)

b = 2                                                                     
(3, 4)