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To: python-list@python.org
From: Peter Otten <__peter__@web.de>
Subject: Re: TypeError: unbound method DefaultTracer() must be called with MyClass instance as first argument (got str instance instead)
Date: Thu, 07 Jul 2011 11:04:15 +0200
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Kannan Varadhan wrote:

> Here is something I don't fully understand.
> 
>   1 def DefaultTracer(fmt, *args):
>   2   fmt = 'in DefaultTracer ' + fmt
>   3   print(fmt % args)
>   4   pass
>   5 def myTracer(fmt, *args):
>   6   fmt = 'in myTracer ' + fmt
>   7   print(fmt % args)
>   8
>   9 class MyClass:
>  10   ClassDefaultTracer = DefaultTracer
>  11   def __init__(self):
>  12     self.InstanceTracer = MyClass.ClassDefaultTracer
>  13   def trace(self, fmt, *args):
>  14     self.InstanceTracer(fmt, *args)
>  15
>  16
>  17 DefaultTracer("Testing %s", 'at first')
>  18 myTracer("Testing %s", 'at first')
>  19
>  20 MyClass().trace('Using ClassDefaultTracer')
>  21
>  22 # override ClassDefaultTracer for all new instances
>  23 MyClass.ClassDefaultTracer = myTracer
>  24 MyClass().trace('Using Overridden ClassDefaultTracer')
> 
> I want ClassDefaultTracer to store a reference to DefaultTracer and be
> used by instances of MyClass.  Why does line20 give me the following
> error:
> 
> $ python foo.py
> in DefaultTracer Testing at first
> in myTracer Testing at first
> Traceback (most recent call last):
>   File "foo.py", line 20, in <module>
>     MyClass().trace('Using ClassDefaultTracer')
>   File "foo.py", line 14, in trace
>     self.InstanceTracer(fmt, *args)
> TypeError: unbound method DefaultTracer() must be called with MyClass
> instance as first argument (got str instance instead)

Functions written in Python are also "descriptors", they have a __get__() 
method. The seemingly harmless

whatever = MyClass.ClassDefaultTracer

therefore results in something like

whatever = MyClass.__dict__["ClassDefaultTracer"].__get__(None, MyClass)

internally, and whatever becomes an "unbound method", essentially the 
DefaultTracer function wrapped in a check that its first argument is a 
MyClass instance.

The simplest workaround is probably to spell out the dictionary access

whatever = MyClass.__dict__["ClassDefaultTracer"]

If you want inheritance to work properly you need something more invoved, 
perhaps (untested)

f = self.ClassDefaultTracer
try: 
    f = f.im_func
except AttributeError: 
    pass
self.InstanceTracer = f

or you wrap the callable in a descriptor:

>>> def DefaultTracer(*args): print args
...
>>> class D(object):
...     def __init__(self, f):
...             self.f = f
...     def __get__(self, *args):
...             return self.f
...
>>> class MyClass(object):
...     old = DefaultTracer
...     new = D(DefaultTracer)
...
>>> MyClass.old
<unbound method MyClass.DefaultTracer>
>>> MyClass.new
<function DefaultTracer at 0x7f0f1c0c45f0>
>>> m = MyClass()
>>> m.old
<bound method MyClass.DefaultTracer of <__main__.MyClass object at 
0x7f0f1cda6fd0>>
>>> m.new
<function DefaultTracer at 0x7f0f1c0c45f0>

Not pretty, but in Python 3 the problem is gone...

> Alternately, how can I achieve what I want, i.e. a class-wide default
> used by all instances created off it, but
> itself be changeable, so that once changed, the changed default would
> be used by subsequent newer instances of that class.