缺少一个参数时如何引发自己的错误？

Question

Hi I'm pretty new to Python and I've just started to learn about errors and exceptions.I have this function in a class that inserts a line at a given index called num.I know python will raise an error if no num is given but I want to raise my own error.How do I do that?This is what I tried. But the error raised is still the default error?

 def insertNum(self, num, line):
    if num== None:
        raise Exception("Num not specified.")
    else:
        self.list.insert(num, line)
    return self.list

Answer 1

You can set the default value of num to None and then check if the value is None .

def insertNum(self, line, num=None):
    if num is None:
        raise Exception("Num not specified.")
    else:
        self.list.insert(num, line)
    return self.list

If you pass only one parameter to the insertNum method, num will be set the None (the default value) and will raise the exception.

If you don't want to change the order of the arguments, you can use this:

def insertNum(self, num, line=None):
    if line is None:
        raise Exception("Num not specified.")
    else:
        self.list.insert(num, line)
    return self.list

A simple demonstration for how default arguments work:

>>> def foo(bar, baz=None):
...     print(bar, baz)
...
>>> foo(1, 2)
1 2
>>> foo(2)
2 None

Answer 2

I suggest you read about exceptions and errors
But the main idea is that you catch errors and then you handle them the way you like.

try: 
 #do something 
except Exception as e:
 # error occured
 print("A wild error appeared")

Answer 3

You can use try...except statement.

def insertNum(num, line):
   try:
     list.insert(num, line)
     return list
   except:
     print('custom error')

Answer 4

wrap your function with another function that will have try and except` and there you could raise what ever error/exception you want.

def wrapper_func(self, num, line):
    try:
        self.insertNum(num, line)
    except Exception as e:
        raise Exception("whatever you want")