Question

I am learning from the book Python for Absolute Beginners and am up to the chapter on exceptions. The authors explanations have been increasingly short, and with this piece of code, i am completely confused and there is no explanation! Can someone explain please, line by line?

#!/usr/bin/env python
store = []
try: {}["foo"]
except KeyError as e: store.append(e)
try: 1/0
except ZeroDivisionError as e: store.append(e)
try: "".bar()
except AttributeError as e: store.append(e)
for exceptionobject in store:
    ec = exceptionobject.__class__
    print(ec.__name__)
    indent = " +-"
    while ec.__bases__:
        ec = ec.__bases__[0]
        print(indent + ec.__name__)
        indent = " " + indent

Answer 1

# create a list
store = [] 

try:
    # if something in this block would throw an exception,
    # code could continue in a controlled way in an except block

    # force a KeyError by looking up a non-existent key in an empty dictionary
    {}["foo"] 

except KeyError as e: 
    # store the exception object in the list
    store.append(e)

# same scheme here; construct something that fails (1/0), 
# then instead of quitting the interpreter, continue operations
try: 
    1/0
except ZeroDivisionError as e: 
    store.append(e)

# pythons exceptions hierarchy is diverse and exceptions carry meaningful
# names expressing their context
try:
    # here we attept to lookup an attribute (that's happening technically
    # before the call), which does not exists (because strings don't know
    # how to bar... 
    "".bar()

except AttributeError as e:
    # ... and the the appropriate exception here is AttributeError
    store.append(e)

At this point the list has three elements, which are exception objects.

# loop over list
for exceptionobject in store:

    # get the class of the object via special method __class__
    # __class__ returns an object whose class is type actually;
    # but don't be too confused by this 
    ec = exceptionobject.__class__

    # print the name of the exception class, now this is just a plain string
    print(ec.__name__)

    indent = " +-"

    # use another special method __bases__ to get all superclasses
    # of the exception class; all exceptions inherit from BaseException
    # loop over the base classes
    while ec.__bases__:

        # get the first base class
        ec = ec.__bases__[0]

        # print its name an indent more
        print(indent + ec.__name__)
        indent = " " + indent

The result should look something like this:

KeyError
 +-LookupError
  +-StandardError
   +-Exception
    +-BaseException
     +-object
ZeroDivisionError
 +-ArithmeticError
  +-StandardError
   +-Exception
    +-BaseException
     +-object
AttributeError
 +-StandardError
  +-Exception
   +-BaseException
    +-object

showing a part of the exception hierarchy.