1. HANDLE ERROR,
GENERATOR AND
DECORATOR
John
Saturday, December 21, 2013

2. HANDLE ANY
UNEXPECTED
ERROR

3. Brief introduction
• Python provide 2 ways to handle unexpected
error: exception and assert.
• Exception handling: is an event, which occurs
during the execution of a program, that disrupts
the normal flow of the program's instructions.
• The exceptions are defined in the built-in class
exceptions
• For example: If divided by 0, we want to raise an
exception

4. Built-in exceptions

5. Warnings
• It is defined on the warnings module

6. Raising Exceptions
• The raise statement allows the programmer
to force a specified exception to occur
raise NameError('HiThere')
• Raise statement is to raise an exceptions, tryexception-finally clause is to catch an
exceptions and decide how to do.

7. Try …except…finally structure
•
•
•
•
First, the try clause(print 100/0) is executed
If no exception occurs, the except clause is skipped
Otherwise, the rest of the try clause is skipped. Go to the line its type
matches the exception name(ZeroDivisionError).
Clean-up info in the finally sentence. It executed under all conditions

8. Write User-defined Exceptions
>>> class MyError(Exception):
... def __init__(self, value):
...
self.value = value
... def __str__(self):
...
return repr(self.value)
• Define user-defined exception MyError.
• Raise an exception when x == 0. Also write the try-except-finally
clause
• When call f(0,100), the exception is raised and catched.

9. Brief introduction of assert
• The assert clause is used on situation or
condition that should never happen. For
example: assert 1>0
• “assert” statement is removed when the
compilation is optimized (-O and -OO option,
it is because __debug__ change to False
when -O or -OO option are added).
• So It is a convenient way to insert debugging
assertion into a program

10. Quick example
• We can see assert is ignored when add -O
option

11. GENERATOR

12. Brief introduction
• Generator s are a simple and powerful tool for
create iterators.
• Use yield statement instead of return to return
data
• the __iter__() and next() methods are created
automatically. The local and execution state are
saved automatically.
• When generator terminate, it raise StopIteration

13. Quick example
• When you call the generator function, the co
de does not run. It just return the generator
object.

14. The difference between generator
and sequence type
>>> mylist = [x*x for x in range(3)]
>>> mygenerator = (x*x for x in range(3))
•Both mylist and mygenerator are iterable
•But you can only read
generator once.
•Generator do not store all
the values in memory, they
generate the values on the
fly.

15. DECORATOR

16. Brief introduction
• Functions are objects in python.
• We can define other function inside function
definition.
• We can pass a function as argument of other
function.

17. Quick example
•
benchmark function accept
func as input argument.
• We can see @benchmark
equal to
apply benchmark function
on f
f = benchmark(f)
•
This is the typical usage of
decorator: Use func as input
argument. define wrapper
function inside function
definition

18. Official document
• PEP - 318 Decorators for Functions and
Methods

19. DESCRIPTOR

20. Brief introduction
• A descriptor is an object attribute with
“binding behavior”.
• If any of __get__(), __set__() and
__delete__() are defined for an object, it is
said to be a descriptor.

21. Descriptor protocol
• If an object defines both __get__() and __set__(), it is considered
a data descriptor.
• Descriptors that only define __get__() are called non-data
descriptors
• descriptors are invoked by the __getattribute__() method
• overriding __getattribute__() prevents automatic descriptor
calls

22. Descriptor example
• Define __set__ and __get__ method.

23. Implement the property() method
• Calling property() is a succinct way of building
a data descriptor

24. Let us write the similar property()
descriptor

25. Function are non-data descriptor
• All functions include __get__() method for
binding methods.

26. Non-data descriptor staticmethod
• The pure python verson of static method
should be like:
Use static method

27. Non-data descriptor classmethod
• Pure python version of classmethod looks
like: