TypeError: unbound method DefaultTracer() must be called with MyClass instance as first argument (got str instance instead)

Kannan Varadhan kvaradhan3 at gmail.com
Thu Jul 7 10:08:30 CEST 2011


Hi Folks,

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)

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.

Thanks,

Kannan



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