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How do I change the string representation of a Python class?

Posted by: admin November 1, 2017 Leave a comment

Questions:

In Java, I can override the toString() method of my class. Then Java’s print function prints the string representation of the object defined by its toString(). Is there a Python equivalent to Java’s toString()?

For example, I have a PlayCard class. I have an instance c of PlayCard. Now:

>>> print(c)
<__main__.Card object at 0x01FD5D30>

But what I want is something like:

>>> print(c)
A♣

How do I customize the string representation of my class instances?

I’m using Python 3.x

Answers:

The closest equivalent to Java’s toString is to implement __str__ for your class. Put this in your class definition:

def __str__(self):
     return "foo"

You may also want to implement __repr__ to aid in debugging.

See here for more information:

Questions:
Answers:

This is not as easy as it seems, some core library functions don’t work when only str is overwritten (checked with Python 2.7), see this thread for examples
How to make a class JSON serializable
Also, try this

import json

class A(unicode):
    def __str__(self):
        return 'a'
    def __unicode__(self):
        return u'a'
    def __repr__(self):
        return 'a'

a = A()
json.dumps(a)

produces

'""'

and not

'"a"'

as would be expected.

EDIT: answering mchicago’s comment:

unicode does not have any attributes — it is an immutable string, the value of which is hidden and not available from high-level Python code. The json module uses re for generating the string representation which seems to have access to this internal attribute. Here’s a simple example to justify this:

b = A('b')
print b

produces

'a'

while

json.dumps({'b': b})

produces

{"b": "b"}

so you see that the internal representation is used by some native libraries, probably for performance reasons.

See also this for more details: http://www.laurentluce.com/posts/python-string-objects-implementation/