substitution __str__ method of an instance

[netimen]

I couldn't substitute __str__ method of an instance. Though I managed
to substitute ordinary method of an instance:

from types import MethodType

class Foo(object):
pass

class Printer(object) :

def __call__(self, obj_self):
return 'printed'

f = Foo()

f.printer = MethodType(Prin ter(), f, Foo)
print f.printer() # works fine - I get: 'printed'

print f # get: <__main__.Foo object at 0x00D69C10>
f.__str__ = MethodType(Prin ter(), f, Foo)
print f # still get: <__main__.Foo object at 0x00D69C10>. Why?
Foo.__str__ = MethodType(Prin ter(), None, Foo)
print f # works fine - I get: 'printed'


How can I substitute __str__ method of an instance?

[Diez B. Roggisch]

Re: substitution __str__ method of an instance

netimen wrote:

I couldn't substitute __str__ method of an instance. Though I managed
to substitute ordinary method of an instance:
>
from types import MethodType
>
class Foo(object):
pass
>
class Printer(object) :
>
def __call__(self, obj_self):
return 'printed'
>
f = Foo()
>
f.printer = MethodType(Prin ter(), f, Foo)
print f.printer() # works fine - I get: 'printed'
>
print f # get: <__main__.Foo object at 0x00D69C10>
f.__str__ = MethodType(Prin ter(), f, Foo)
print f # still get: <__main__.Foo object at 0x00D69C10>. Why?
Foo.__str__ = MethodType(Prin ter(), None, Foo)
print f # works fine - I get: 'printed'
>
>
How can I substitute __str__ method of an instance?

You can't. Special methods are only looked up on classes.

Diez

[Christian Heimes]

Re: substitution __str__ method of an instance

netimen wrote:

How can I substitute __str__ method of an instance?

It's not possible. For performance and other reasons most __*__ methods
are looked up on the type only.

Christian

[Diez B. Roggisch]

Re: substitution __str__ method of an instance

Christian Heimes wrote:

netimen wrote:

>How can I substitute __str__ method of an instance?

>
It's not possible. For performance and other reasons most __*__ methods
are looked up on the type only.

Is that documented somewhere? I *know* it is that way, yet I'd like to have
place to read up on it (and point to when this question pops up)

Diez

[Bruno Desthuilliers]

Re: substitution __str__ method of an instance

netimen a écrit :

I couldn't substitute __str__ method of an instance. Though I managed
to substitute ordinary method of an instance:
>
from types import MethodType
>
class Foo(object):
pass
>
class Printer(object) :
>
def __call__(self, obj_self):
return 'printed'
>
f = Foo()
>
f.printer = MethodType(Prin ter(), f, Foo)
print f.printer() # works fine - I get: 'printed'
>
print f # get: <__main__.Foo object at 0x00D69C10>
f.__str__ = MethodType(Prin ter(), f, Foo)
print f # still get: <__main__.Foo object at 0x00D69C10>. Why?
Foo.__str__ = MethodType(Prin ter(), None, Foo)
print f # works fine - I get: 'printed'
>
>
How can I substitute __str__ method of an instance?

Now that others told you you couldn't do so, there's eventually a
workaround - that is, if you have the hand on class Foo:

class Foo(object):
def __str__(self):
printer = getattr(self, 'printer', super(Foo, self).__str__)
return printer()

HTH

[Bruno Desthuilliers]

Re: substitution __str__ method of an instance

Diez B. Roggisch a écrit :

Christian Heimes wrote:
>

>netimen wrote:

>>How can I substitute __str__ method of an instance?

>It's not possible. For performance and other reasons most __*__ methods
>are looked up on the type only.

>
Is that documented somewhere? I *know* it is that way, yet I'd like to have
place to read up on it (and point to when this question pops up)

http://docs.python.org/reference/datamodel.html#special-method-lookup-for-new-style-classes

[Steven D'Aprano]

Re: substitution __str__ method of an instance

On Thu, 23 Oct 2008 10:55:56 +0200, Christian Heimes wrote:

netimen wrote:

>How can I substitute __str__ method of an instance?

>
It's not possible. For performance and other reasons most __*__ methods
are looked up on the type only.
>
Christian

However, you can dispatch back to the instance if you really must:


class MyObj(object):
def __init__(self):
self.__str__ = lambda self: "I'm an object!"
def __str__(self):
return self.__str__(se lf)


But honestly, this sounds like a bad idea. If instances of the one class
have such radically different methods that they need to be treated like
this, I question whether they actually belong in the same class.




--
Steven

[Duncan Booth]

Re: substitution __str__ method of an instance

Steven D'Aprano <steve@REMOVE-THIS-cybersource.com .auwrote:

However, you can dispatch back to the instance if you really must:
>
>
class MyObj(object):
def __init__(self):
self.__str__ = lambda self: "I'm an object!"
def __str__(self):
return self.__str__(se lf)
>
>
But honestly, this sounds like a bad idea. If instances of the one

class

have such radically different methods that they need to be treated

like

this, I question whether they actually belong in the same class.
>

Another option would be to just change the class of the object:

>>class C(object):

pass

>>c = C()
>>print c

<__main__.C object at 0x01180C70>

>>def wrapstr(instanc e, fn=None):

if fn is None:
def fn(self): return "I'm an object"
Wrapper = type(instance._ _class__.__name __, (instance.__cla ss__,),
{'__str__':fn})
instance.__clas s__ = Wrapper

>>wrapstr(c)
>>print c

I'm an object

>>isinstance( c, C)

True

>>type(c)

<class '__main__.C'>

>>wrapstr(c, lambda s: "object %s at %s" % (type(s).__name __, id(s)))
>>print c

object C at 18353264

(I'll leave enhancing wrapstr so that it avoids multiple levels of
wrapping as an exercise for anyone who actually wants to use it.)

--
Duncan Booth http://kupuguy.blogspot.com