Question

I am reading DiveIntoPython.

I’m wondering why I can directly print the instance of a UserDict as a dictionary.
In detail, these codes

import UserDict;

d = UserDict.UserDict({1:1, 'a':2});
print d;
print d.data;

will have output

{'a': 2, 1: 1}
{'a': 2, 1: 1}

And these codes

class MyDict:
    def __init__(self, dictData=None):
        self.data = dictData;

d = MyDict({1:1, 'a':2});
print d;
print d.data;

will have output (on my machine)

<__main__.MyDict instance at 0x10049ef80>
{'a': 2, 1: 1}

In other words, How I can define my class, and print its instances as a built-in datatype?
Thank you!

Answer 1

How an object is printed comes down to it’s repr – when you inherit from a mixin, it already provides the repr function. Also note, these days you can just inherit from dict directly.

In your case, you can define

def __repr__(self):
    return repr(self.data)

The difference between __str__ and __repr__ is that mostly __str__ should be readable and understable. __repr__ where it’s possible can be used to provide an eval‘d string constructing the original object (although not necessary) – see the great answer here for the difference: Difference between __str__ and __repr__ in Python