sys.settrace closure interaction bug


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Oct 1, 2006
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The code below exhibits different behavior depending on whether it invokes sys.settrace ('-t' option) or not. This means that (in a more complicated case) debugging the code (which uses sys.settrace) makes it fail. Any ideas?

Code:
""" Demonstrace that tracing messes up curried class definitions """

# Some simple command line parsing: -t or --trace means trace, nothing means don't trace
import sys

def usage( ):
    print 'Usage:', sys.argv[ 0 ], '[-t | --trace]'
    sys.exit( 1 )

if 1 == len( sys.argv ):
    pass
elif 2 == len( sys.argv ):
    if sys.argv[ 1 ]=='-t' or sys.argv[ 1 ]=='--trace':
        def trace ( frame, event, arg ):
            # print frame, event, arg
            return trace
        sys.settrace( trace )
    else:
        usage( )
else:
    usage( )



# The test: define a class factory with a curried member function

def the_factory( parm1 ):
    class the_class( object ):
        def curried( self ): return parm1
    return the_class

x = the_factory( 42 )

y = x( )

try:
    x.parm1
    print "Failure: x (the manufactured class) has attribute parm1?!"
except AttributeError:
    print "Success with the manufactured class!"

try:
    y.parm1
    print "Failure: y (the instance) has attribute parm1?!"
except AttributeError:
    print "Success with the instance!"

assert y.curried( ) == 42, "Currying didn't work?!"
 
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