打印列表中的元素__str__

Question

I've learned that __str__ can define an output of the string of the object.

Example:

class Person(object):
    def __init__(self, name):
        self.name = name
    def __str__(self):
        return self.name

p1 = Person('Steve Jobs')
p2 = Person('Bill Gates')
p3 = Person('Mark Zuckerberg')

print(p1)  # >>> Steve Jobs

it output Steve Jobs as I wished, instead of <__main__.Person object at 0x10410c588>

However, if I create a list:

lst = [p1, p2, p3]
print(lst)
# >>> [<__main__.Person object at 0x1045433c8>, <__main__.Person object at 0x1045434e0>, <__main__.Person object at 0x104543550>]

I have to :

print([i.__str__() for i in lst])
# >>> ['Steve Jobs', 'Bill Gates', 'Mark Zuckerberg']

to make it work??

This does not make sense much, right?

Answer 1

The list.__str__ uses the object's __repr__ to build the string. So, just delegate __repr__ to __str__ :

In [1]: class Person(object):
   ...:     def __init__(self, name):
   ...:         self.name = name
   ...:     def __str__(self):
   ...:         return self.name
   ...:     def __repr__(self):
   ...:         return str(self)
   ...:
   ...: p1 = Person('Steve Jobs')
   ...: p2 = Person('Bill Gates')
   ...: p3 = Person('Mark Zuckerberg')
   ...:

In [2]: print(p1)
Steve Jobs

In [3]: lst = [p1, p2, p3]
   ...:

In [4]: print(lst)
[Steve Jobs, Bill Gates, Mark Zuckerberg]

EDIT

If you want to stick to convention, do somthing like:

In [18]: class Person(object):
    ...:     def __init__(self, name):
    ...:         self.name = name
    ...:     def __str__(self):
    ...:         return self.name
    ...:     def __repr__(self):
    ...:         return f"{type(self).__name__}({self.name})"
    ...:

In [19]: p1 = Person('Steve Jobs')

In [20]: print([p1])
[Person(Steve Jobs)]

Answer 2

Consider implementing:

class Person(object):
    def __init__(self, name):
        self.name = name
    def __str__(self):
        return self.name
    def __repr__(self):
        return 'Person({!r})'.format(self.name)  # !r adds the quotes correctly

Which gives:

>>> lst
[Person('Steve Jobs'), Person('Bill Gates'), Person('Mark Zuckerberg')]

The reason you're seeing mismatching behavior is that print calls str() on its argument, but list str and repr are the same, and both call repr on each element.