object._ _ str _ _(self) Purpose[Readability]: return a string which is a more convenient or concise representation can be used.

object._ _ repr _ _(self) Purpose[Unambiguous]: return a string which should look like a valid Python expression that could be used to recreate an object with the same value (given an appropriate environemnt).

• If _ _ repr _ _ is defined, and _ _ str _ _ is not, the object will behave as though _ _ str _ _= _ _ repr _ _.

• Almost every object you implement should have a functional _ _ repr _ _ that’s usable for understanding the object.

• Implementing _ _ str _ _ is optional: do that if you need a “pretty print” functionality. (for example, used by a report generator)

Why need to implement str and repr?

object._ _ str _ _ (self)

Called by str(object) and the built-in functions format() and print() to compute the “informal” or nicely printable string representation of an object. The return value must be a string object.

This method differs from object._ _ repr _ _() in that there is no expectation that _ _ str _ _ () return a valid Python expression: a more convenient or concise representation can be used.

The default implementation defined by the built-in type object calls object._ _ repr _ _ ().

class Car:
    def __init__(self, color, mileage):
        self.color = color
        self.mileage = mileage

    def __str__(self):
        return 'The color of car is {} and mileage is {}.'.format(self.color, self.mileage)

my_Car = Car('red', 1234)
print(my_Car)
'{}'.format(my_Car)
str(my_Car)

The goal of _ _ str _ _ is to be readable

The goal is to represent it in a way that a user, not a programmer, would want to read it.

object._ _ repr _ _ (self)

Called by the repr() built-in function to compute the “official” string representation of an object. If at all possible, this should look like a valid Python expression that could be used to recreate an object with the same value (given an appropriate environment). If this is not possible, a string of the form <…some useful description…> should be returned. The return value must be a string object.

If a class defines _ _ repr _ _ () but not _ _ str _ _ (), then _ _ repr _ _ () is also used when an “informal” string representation of instances of that class is required.

This is typically used for debugging, so it is important that the representation is information-rich and unambiguous.

class Car:
    def __init__(self, color, mileage):
        self.color = color
        self.mileage = mileage

    def __repr__(self):
        return 'Car(color=%r, mileage=%r)'%(self.color, self.mileage)

my_Car = Car('red', 1234)
print(my_Car)
'{}'.format(my_Car)
str(my_Car)
my_Car

Better:

class Car:
    def __init__(self, color, mileage):
        self.color = color
        self.mileage = mileage

    def __repr__(self):
        return f'Car(color={self.color!r}, mileage={self.mileage!r})'

Good _ _ repr _ _ template:

class Car:
    def __init__(self, color, mileage):
        self.color = color
        self.mileage = mileage

    def __repr__(self):
        return f'{self.__class__.__name__}(color={self.color!r}, mileage={self.mileage!r})'

Reference

https://www.oschina.net/translate/difference-between-str-and-repr-in-python?cmp

https://baijiahao.baidu.com/s?id=1596817611604972751&wfr=spider&for=pc

https://docs.python.org/3.7/reference/datamodel.html?highlight=__repr__#object.__repr__