Python——函数的可选参数

Question

Say I want to build a function that either adds two numbers a and b , or subtracts them and adds a third number c and subtracts the fourth number d . I intend to specify which of the two operations is to be performed by an argument sum ; if this is True , the first operation is performed, if False , then the second operations is performed. I would write this as:

def f(a, b, c, d, sum=True):
    if sum: return a+b
    else: return a-b+c-d

For example, f(1,2,3,4) returns 3 , while f(1,2,3,4,sum=False) returns -2 , as expected. Clearly, though, c and d only need to be defined when sum=False . How do I do this? I've tried setting c and d as *args, but I keep getting errors of the type "unsupported operand" or "positional argument follows keyword argument".

Answer 1

Use a default value of None for c and d:

def f(a, b, c=None, d=None, sum=True):
    if sum:
        return a+b
    else: 
        return a-b+c-d

This also allows you to add error-checking logic to your function - check whether c and d are present when sum is False:

def f(a, b, c=None, d=None, sum=True):
    if sum: 
        return a+b
    else: 
        if None in (c, d):
            raise TypeError('f requires c and d args with parameter sum=False')
        return a-b+c-d

Answer 2

Others have given the way you can do it, but at a more fundamental level I'd ask why this is even the same method? Seems to me you have two different methods here, f_sum and f (or whatever), one takes two parameters and the other 4.

Or if it's a more complex operation and c and d are just additional parameters / attributes, you could default them to whatever the null value is eg for addition just default them to 0:

 
 
 
 
  
  
  def f(a, b, c=0, d=0)
 
 
 

a+b-0+0 won't do anything so if these parameters are not provided the result will be identical to a+b without even needing a conditional or magical flag. sorry missed that the second case was a - b and misread it as a +

Answer 3

The answer to your question is:

def f(a, b, c=None, d=None, sum=True):
  if sum: return a + b
  else: return a - b + c - d

However, you could further simplify it to this:

def f(a, b, c=0, d=0):
  return a - b + c - d

As the values of c and d depend on whether sum is true or false.

Answer 4

This solves your problem:

def f(a, b, c=0, d=0, sum=True):
    if sum: return a+b
    else: return a-b+c-d