在 Python 中，我可以根据其他参数指定函数参数的默认值吗?

问题描述：

假设我有一个带有两个参数的 python 函数，但我希望第二个参数是可选的，默认值是作为第一个参数传递的任何参数.所以，我想做这样的事情:

Suppose I have a python function that takes two arguments, but I want the second arg to be optional, with the default being whatever was passed as the first argument. So, I want to do something like this:

def myfunc(arg1, arg2=arg1):
    print (arg1, arg2)

除非那不起作用.我能想到的唯一解决方法是:

Except that doesn't work. The only workaround I can think of is this:

def myfunc(arg1, arg2=None):
    if arg2 is None:
        arg2 = arg1
    print (arg1, arg2)

有没有更好的方法来做到这一点?

Is there a better way to do this?

答

正如@Ignacio 所说，你不能这样做.在后一个示例中，您可能会遇到 None 是 arg2 的有效值的情况.如果是这种情况，您可以使用标记值:

As @Ignacio says, you can't do this. In your latter example, you might have a situation where None is a valid value for arg2. If this is the case, you can use a sentinel value:

sentinel = object()
def myfunc(arg1, arg2=sentinel):
    if arg2 is sentinel:
        arg2 = arg1
    print (arg1, arg2)

myfunc("foo")           # Prints 'foo foo'
myfunc("foo", None)     # Prints 'foo None'

在 Python 中，我可以根据其他参数指定函数参数的默认值吗?

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