dynamically defined class not visible in importing module python

Question

A temporary exception class is defined dynamically using 'type' in a python script meant to be used as module. When an instance of this class is caught in importing script it doesn't recognize the class. Below is code snippet

# the throwing module, defines dynamically
def bad_function():
    ExceptionClass = type( "FooBar", (Exception,),
        { "__init__": lambda self, msg: Exception.__init__(self, msg) })
    raise ExceptionClass("ExceptionClass")

the using code

import libt0

try:
    libt0.bad_function()
#except libt0.FooBar as e:
#print e
except Exception as e:
    print e
    print e.__class__

can it be explained why libt0.FooBase is not visible to this script? observer output of last line.

John La Rooy · Accepted Answer · 2013-06-18 07:13:05Z

2

It's not clear how you expect FooBar to exist without doing something like this

def bad_function():
    ExceptionClass = type( "FooBar", (Exception,),
        { "__init__": lambda self, msg: Exception.__init__(self, msg) })
    globals()['FooBar'] = ExceptionClass
    raise ExceptionClass("ExceptionClass")

answered Jun 18, 2013 at 7:13

John La Rooy

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1 Comment

user2496012 Over a year ago

Thanks, this works. I must admit, my python knowledge is quite shallow. All I have learned in python is syntax and some libraries for survival. I am mostly C/C++ guy. Help really appreciated @gnibbler.

BrenBarn · Accepted Answer · 2013-06-18 07:07:35Z

2

You created the class inside a function, so it doesn't exist as a name in the module's global namespace. In fact, it doesn't exist at all except while bad_function is executing. It's the same reason why this fails:

# file1.py
def good_function():
   x = 2

# file2.py
import file1
print file1.x

Your exception class is just a local variable inside bad_function.

answered Jun 18, 2013 at 7:07

BrenBarn

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