object.__str__ 和 object.__repr__ 是做什么的？ 警告：它与 Class.__str__ 和 Class.__repr__ 不同

Question

class TestClass:
    def __init__(self):
        pass
    def __str__(self):
        return 'You have called __str__'
    def __repr__(self):
        return 'You have called __repr__'

a = TestClass()
print(object.__repr__(a))
print(object.__str__(a))

Output:

<__main__.TestClass object at 0x7fe175c547c0>
You have called __repr__

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What does those two functions do?

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My understanding is that calling str(a) returns a.__str__() and calling repr(a) returns a.__repr__() . print(a) also prints the string, a.__str__() since there is an implicit str(a) conversion going on.

Note that my question is not a duplicate to another popular question; see my first comment below.

The behaviour is COUNTERINTUITIVE; doing print(object.__str__(a)) prints the repr string instead of the str string.

Answer 1

The object class provides default implementations for the __repr__ and __str__ methods.

• object.__repr__ displays the type of the object and its id (the address of the object in CPython)

• object.__str__(a) calls repr(a) . The rationale is that if __str__ is not overriden in a class str(a) will automatically call repr(a) , using a possible override of __repr__ . More exactly, the official documentation of the Python Language Reference / Data Model / Special method names / Basic customization says:

  object.__str__(self)

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

  ... which in turn calls the overriden __repr__ method like repr(object) would have done.

This is exactly what happens here because you are directly calling object.__str__ while this is not expected.