如何测试负数并引发 ValueError？

Question

I'm working through this tutorial . I'm working through this iteratively. At this point I have the following Binary class:

class Binary:
    def __init__(self,value):
        self.value = str(value)
        if self.value[:2] == '0b':
            print('a binary!')
            self.value= int(self.value, base=2)
        elif self.value[:2] == '0x':
            print('a hex!')
            self.value= int(self.value, base=16)
        else:
            print(self.value)

    def __int__(self):
        if self.value[:1] == '-':
            return ValueError
        return int(self.value)

I'm running through a suite of tests using pytest, including:

 def __int__(self):
>       if self.value[:1] == '-':
E       TypeError: 'int' object is not subscriptable

I understand the error since I saved the parameter as an int. What is a good strategy to test for a negative number and raise a valueerror exception in this case?

Answer 1

If value is an int, then value < 0 is the test that value is negative. You should already know this. Perhaps you should review the tutorial in the doc set.

A separate problem is that your init method is confused. It turns the input into a string and sometimes turns the string (back?) into an int. By the time init is finished, self.value should have an allowed value of the desired type.