Consider this class:
class foo(object):
pass
The default string representation looks something like this:
>>> str(foo)
"<class '__main__.foo'>"
How can I make this display a custom string?
5 Answers
Implement __str__() or __repr__() in the class's metaclass.
class MC(type):
def __repr__(self):
return 'Wahaha!'
class C(object):
__metaclass__ = MC
print(C)
Use __str__ if you mean a readable stringification, use __repr__ for unambiguous representations.
class foo(object):
def __str__(self):
return "representation"
def __unicode__(self):
return u"representation"
If you have to choose between __repr__ or __str__ go for the first one, as by default implementation __str__ calls __repr__ when it wasn't defined.
Custom Vector3 example:
class Vector3(object):
def __init__(self, args):
self.x = args[0]
self.y = args[1]
self.z = args[2]
def __repr__(self):
return "Vector3([{0},{1},{2}])".format(self.x, self.y, self.z)
def __str__(self):
return "x: {0}, y: {1}, z: {2}".format(self.x, self.y, self.z)
In this example, repr returns again a string that can be directly consumed/executed, whereas str is more useful as a debug output.
v = Vector3([1,2,3])
print repr(v) #Vector3([1,2,3])
print str(v) #x:1, y:2, z:3
Ignacio Vazquez-Abrams' approved answer is quite right. It is, however, from the Python 2 generation. An update for the now-current Python 3 would be:
class MC(type):
def __repr__(self):
return 'Wahaha!'
class C(object, metaclass=MC):
pass
print(C)
If you want code that runs across both Python 2 and Python 3, the six module has you covered:
from __future__ import print_function
from six import with_metaclass
class MC(type):
def __repr__(self):
return 'Wahaha!'
class C(with_metaclass(MC)):
pass
print(C)
Finally, if you have one class that you want to have a custom static repr, the class-based approach above works great. But if you have several, you'd have to generate a metaclass similar to MC for each, and that can get tiresome. In that case, taking your metaprogramming one step further and creating a metaclass factory makes things a bit cleaner:
from __future__ import print_function
from six import with_metaclass
def custom_class_repr(name):
"""
Factory that returns custom metaclass with a class ``__repr__`` that
returns ``name``.
"""
return type('whatever', (type,), {'__repr__': lambda self: name})
class C(with_metaclass(custom_class_repr('Wahaha!'))): pass
class D(with_metaclass(custom_class_repr('Booyah!'))): pass
class E(with_metaclass(custom_class_repr('Gotcha!'))): pass
print(C, D, E)
prints:
Wahaha! Booyah! Gotcha!
Metaprogramming isn't something you generally need everyday—but when you need it, it really hits the spot!
Just adding to all the fine answers, my version with decoration:
from __future__ import print_function
import six
def classrep(rep):
def decorate(cls):
class RepMetaclass(type):
def __repr__(self):
return rep
class Decorated(six.with_metaclass(RepMetaclass, cls)):
pass
return Decorated
return decorate
@classrep("Wahaha!")
class C(object):
pass
print(C)
stdout:
Wahaha!
The down sides:
- You can't declare
Cwithout a super class (noclass C:) Cinstances will be instances of some strange derivation, so it's probably a good idea to add a__repr__for the instances as well.
