CODEWITHSUNDEEP
pythonpython-3.x
mrCarnivore
mrCarnivore

Reputation: 5068

Printing a class instance results in None if I change the class member values

I have run into a problem I can't explain:

class A:
    def __init__(self, rid, title):
        self.rid = rid
        self.title = title
        self.b = []

    def __add__(self, other):
        if type(other) == B:
            self.b += [other]

    def __str__(self):
        return self.rid + ' - ' + self.title

    def __repr__(self):
        return str(self)

class B:
    def __init__(self, rid, title):
        self.rid = rid
        self.title = title

b = B('123', 'abc')
a = A('345', 'cde')

print(a)

a += b

print(a)

The first print results in the expected output:

345 - cde

However, the second print (after the adding of b) results in:

None

Why is that? I am not changing the rid or the title of a nor do I create new and uninitialized instance called a, or am I?

Upvotes: 0

Views: 38

Answers (1)

Duncan
Duncan

Reputation: 95622

The expression a += b is a shorthand for: a = a.__add__(b)

As your __add__() method returns None that means you will assign None to a.

Upvotes: 2

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