对象列表的自定义列表表示

Question

I know that representation of an object can customized be using the __repr__ magic method.

class A:

    ...

    def __repr__(self):
        return 'A(id={})'.format(self.id)

Is there a way to customize the representation when it is a list of these objects, such that it doesn't just print a list of the object's representation [A(id=1), A(id=10), A(id=11)] .

For example:

class AGroup:
    def __init__(self, lst):
        self.lst = lst

>>> lst = [A(1), A(10), A(11)]
>>> grp = AGroup(lst)
>>> grp.lst
<Group of 3 A's>  # instead of [A(id=1), A(id=10), A(id=11)]

I was thinking of wrapping the list into another object before assigning to self.lst so that I can modify its repr. Any other idea?

Answer 1

You can store the list passed to the AGroup constructor in an instance of a sub-class of list , so that you can override the __repr__ method of the list sub-class to return an output in your desired format:

class A:
    def __init__(self, id):
        self.id = id
    def __repr__(self):
        return 'A(id={})'.format(self.id)

class AGroup:
    class AList(list):
        def __repr__(self):
            return "<Group of %d A's>" % len(self)

    def __init__(self, lst):
        self.lst = self.AList(lst)

lst = [A(1), A(10), A(11)]
grp = AGroup(lst)
print(grp.lst)

This outputs:

<Group of 3 A's>

Answer 2

to group class instance we need to overwrite eq, ne, and hash function. by giving the static hash value will treat all instances of this class as one object.

from collections import Counter
class A:
    def __init__(self , id):
        self.id = id
    def __hash__(self): return hash('A')
    def __eq__(self, x): return type(x) is A
    def __ne__(self, x): return type(x) is not A

    def __repr__(self):
        return 'A'


class B:
    def __init__(self , id):
        self.id = id
    def __hash__(self): return hash('B')
    def __eq__(self, x): return type(x) is B
    def __ne__(self, x): return type(x) is not B

    def __repr__(self):
        return 'B'


class AGroup:
    def __init__(self, lst):
        count = Counter(lst)
        self.lst = []
        for cl in Counter(lst):
            self.lst.append('<Group of {} {}\'s>'.format(count[cl] , cl))



lst = [A(1), B(10), A(11)]
grp = AGroup(lst)
grp.lst


["<Group of 2 A's>", "<Group of 1 B's>"]