在Python中,我可以指定一个函数参数在其他参数方面默认? [英] In Python, can I specify a function argument's default in terms of other arguments?
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问题描述
假设我有一个Python函数有两个参数,但我想第二个参数是可选的,默认是无论是作为第一个参数传递。所以,我希望做这样的事情:
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)
除了将不起作用。我能想到的唯一的解决方法是这样的:
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?
推荐答案
由于@Ignacio说,你不能做到这一点。在你后面的例子中,你可能会遇到这样的情况无
是 ARG2
的有效值。如果是这样的话,你可以使用一个哨兵值:
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'
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