Metaclass Recipe: Automatic Debugger Methods

Step 1

For now, let’s not worry about invoking the debugger. We’ll start by printing a traceback whenever an error occurs. For that, we’ll use a decorator function called get_debugger_func, which is already defined for you.

Your task is to define the __debugger__ metaclass.

import inspect, traceback, pdb

def get_debugger_func(func):
    def new_func(*args):
        try:
            return func(*args)
        except:
            traceback.print_exc()

    return new_func

class __debugger__(type):
    """Define this class"""

class Example:
    __metaclass__ = __debugger__

    def divide(self, v):
        # exception if v == 0
        result = 100 / v
        return result

    def interpolate(self, *args):
        # exception if len(args) != 2
        result = "%s is %s!" % args
        return result

if __name__ == '__main__':
    e = Example()

    print e.divide(0)

    print '-'*80

    print e.interpolate('Feihong', 'very', 'cool')

Expected output:

Traceback (most recent call last):
  File "D:\Projects\PyCon 2008 Talks\framework-tutorial\src\metaclass\debugger\solutions\debugger1.py", line 9, in new_func
    return func(*args)
  File "D:\Projects\PyCon 2008 Talks\framework-tutorial\src\metaclass\debugger\solutions\debugger1.py", line 29, in divide
    result = 100 / v
ZeroDivisionError: integer division or modulo by zero
None
--------------------------------------------------------------------------------
Traceback (most recent call last):
  File "D:\Projects\PyCon 2008 Talks\framework-tutorial\src\metaclass\debugger\solutions\debugger1.py", line 9, in new_func
    return func(*args)
  File "D:\Projects\PyCon 2008 Talks\framework-tutorial\src\metaclass\debugger\solutions\debugger1.py", line 34, in interpolate
    result = "%s is %s!" % args
TypeError: not all arguments converted during string formatting
None
>>>

Hints

1. You will need to iterate through all the keys in classDict.
2. You have to test each value in classDict to see if it’s a method.
3. Remember that metaclasses cannot see methods as method objects, they only see function objects.
4. Use the inspect.isfunction function.
5. You need to replace each method in the target class with a method transformed by get_debugger_func.

Solution: solutions/debugger1.py

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