Why is a multiple parent exception not caught?

Question

I toyed around with an exception hierarchy and realized that an exception subclass with several parents is not caught.

For example:

class Error1(Exception): 
    pass

class Error2(Exception): 
    pass

class MixedError(Error1, Error2): 
    pass

try:
    print('before 1')
    raise Error2()
    print('after 1')

except MixedError:
    print('Caught it with a mixin 1!')

except Exception:
    print('Big catcher here 1!')

This prints:

before 1

Big catcher here 1!

Why is a multiple parent exception not caught?

Answer 1

You have a misunderstanding how exceptions work. Exceptions are cached by parent exception, not by child exceptions:

class Error1(Exception):
    pass

class Error2(Exception):
    pass

class MixedError(Error1, Error2):
    pass

try:
    print('before 1')
    raise MixedError()
    print('after 1')

except Error2:
    print('Caught it with a Error2!')

except Exception:
    print('Big catcher here 1!')

prints:

before 1
Caught it with a Error2!

Answer 2

You never raise MixedError . Also, note that parent classes know nothing about child classes, thus, when raising Error1 or Error2 , you are raising an error using the __str__ from that specific class:

class Error1(Exception): 
   pass

class Error2(Exception): 
   pass

class MixedError(Error1, Error2): 
   pass

try:
   raise MixedError('Error here')
except MixedError:
   print("caught 'MixedError'")

Output:

caught 'MixedError'