How do I control the title of instances of a class in Python?

Question

Purely a cosmetic thing, but when I create a class in python, how do I control its instances' titles? e.g. for

class Foo():
    def __init__(self, name):
         self.name = name

I can then create an instance:

>> f = Foo('bar')
>> f
<Foo instance at 0x10ab41a28>

If I wanted that to return bar, what would I do? (In Django, you use the __unicode__ method of the class, but I've tried setting both __unicode__ and __str__ functions to return self.name, and while these work on their own, they don't affect the value of the class.)

Yes, sorry. I mean to control the title for instances of the class. — futuraprime, Jul 04 '12 at 16:52
Do you just want to control the instance's string representation? — stelonix, Jul 04 '12 at 16:54
See also http://stackoverflow.com/questions/1436703/difference-between-str-and-repr-in-python — Kos, Jul 04 '12 at 16:55

score 4 · Accepted Answer · edited May 23 '17 at 11:56

I usually define __str__ to return a string representation, and then return the string in the __repr__ method, like this:

>>> class Foo():
...     def __init__(self, name):
...          self.name = name
...     def __str__(self):
...          return '<Foo name=%s>' % self.name
...     def __repr__(self):
...          return str(self)
... 
>>> f = Foo('bar')
>>> f
<Foo name=bar>

For more info, see Difference between str and repr in Python

score 2 · Answer 2 · edited Jul 04 '12 at 16:56

2

Define __repr__ instead - that will sort you out.

edited Jul 04 '12 at 16:56

KL-7

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answered Jul 04 '12 at 16:53

Jon Clements

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Thanks @kl-7 Using a touch device and it doesn't have the code escape symbol! – Jon Clements Jul 04 '12 at 17:04

score 1 · Answer 3 · answered Jul 04 '12 at 16:54

In this case, it's not the problem with your class.

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

>>> f = Foo()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __init__() takes exactly 2 arguments (1 given)
>>> f = Foo("bar")
>>> f
<__main__.Foo object at 0x7f2a9b9b7590>
>>> print(f)
bar

The problem is that by just typing f, you'll get the output of repr() instead of str(). If you want to change that, you'll need to define __repr__ instead of __str__ or __unicode__.

Usually, you'll be using the str() output anyways (print uses it, when coercing to string it gets used). repr() is really only used if you ask for it, like with:

>>> print("{0!r}".format(f))
<__main__.Foo object at 0x7f2a9b9b7590>

score 1 · Answer 4 · answered Jul 04 '12 at 16:54

You're mixing up a few things,

You set the name of the class when you used Class Foo:. This created a class whose name was Foo.

Now, when you do f = Foo('bar') and then f, the interpreter handily calls your instance's __repr__ method:

>>> class Foo():
...  def __repr__(self):
...   return 'REPR'
... 
>>> f = Foo()
>>> 
>>> f
REPR
>>>

Long story short, use the __repr__ method if you want to control what the interpreter returns when you write f.

If you don't, the interpreter will more-or-less do it for you:

>>> class F():
...  pass
... 
>>> f = F()
>>> f.__repr__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: F instance has no attribute '__repr__'
>>> repr(f)
'<__main__.F instance at 0x7f00381e2368>'

How do I control the title of instances of a class in Python?

4 Answers4