当参数默认为None时使用__repr__，但可能是一个str

Question

I have a class that can be initializes with several parameters. Some of these parameters have default values of None , but if a value is provided it should be str . I would like to have a nice pythonic output from __repr()__ , but can't figure out how to handle None as well as a possible str in one return statement. I would like to avoid having various return statements depending on whether a parameter is None or a str value.

A basic example:

class Demo:
    """Demo of how not to deal with None in __repr()__"""

    def __init__(self, w:int, x:int, y=None, z=None):
        self.w = w
        self.x = x
        self.y = y
        self.z = z

    def __repr__(self):
        return f"demo(w={self.w}, x={self.x}, y='{self.y}', z='{self.z}')"

Here are two examples of using this class:

>>> d1 = Demo(5, 4)
>>> d1
Demo(w=5, x=4, y='None', z='None')
>>> d2 = Demo(5, 4, 'me', 'you')
>>> d2
Demo(w=5, x=4, y='me', z='you')

Note how None is in single quotes, which is not what I want. But me and you are in quotes, which is appropriate given that they are str .

Answer 1

use repr in your __repr__ function

def __repr__(self):
    return f"demo(w={self.w}, x={self.x}, y={repr(self.y)}, z={repr(self.z)})"

test:

d = Demo(23,12,y='hello')
print(d)

demo(w=23, x=12, y='hello', z=None)