Python lambda expression

The lambda expression are used to create small anonymous one-line functions. They are basically created at run-time and are not bound to the name of the functions. So they are also called throw-away functions.i.e they are just needed where they have been created.They return definition of the function on the fly. Lambda functions don't have  a return statement , they always return an expression.
They are mostly use along with another function or inside a function definition.
Syntax :

lambda [arg1 [,arg2,.....argn]]:expression 

The lambda operator is mostly used in combination with functions map, filter and reduce. We can put a lambda definition anywhere a function is expected.

For example :
1. use with filter() function

multiple_3 = filter(lambda x: x % 3 == 0, [1, 2, 3, 4, 5, 6, 7, 8, 9])

This function returns array of multiples of 3. Lambda expression has filtered out all the elements that are divisible by 3 from the given array. 

2. use with sorted()  function 

>>> sorted([1, 2, 3, 4, 5, 6, 7, 8, 9], key=lambda x: abs(5-x))
[5, 4, 6, 3, 7, 2, 8, 1, 9]

3. Defining with in another function

def make_incrementor(n):
  return lambda x: x + n
f = make_incrementor(42)
f(0)
>>> 42
f(1)
>>> 43

4. Use with reduce() function 

>>> reduce(lambda a,b: '{}',"{}".format(a,b),[1,2,3,4,5,6,7,8,9])
'1,2,3,4,5,6,7,8,'

5. Making a flatten list out of list of lists 

l = [[1,2,3],[4,5,6],[7,8,9]]
flatten = lambda l: [item for sublist in l for item in sublist]
print flatten(l)

will give output :

[1,2,3,4,5,6,7,8,9]

Exception Handling with Python

When an exception is raised it causes the program to terminate and it is called as an Unhandled Exception. If we know that a line of code might raise an exception when executed we should handle the exception.

What is an Exception
An exception is an event , which occurs during the execution of a program that disrupts the normal flow of the program's instructions. When a python script encounters a situation that it cannot cope with , it raises an exception . An exception basically represents an error.


Handling an Exception 
Exceptions can be handled to avoid program from crashing , this is called Exception handling.
When we can anticipate an error in the program we can use exception handling to stop the program from crashing.

Syntax for exception handling : 

try:

except.............

print 

Example
If we know that in a program we can get such values that a ZeroDivisionError can occur , so we can handle this by except ZeroDivisionError :

>>> while True:
try:
    x = int(raw_input("Please enter a number: "))
    y = int(raw_input("Please enter a number: "))
    z = x/y
    break
        except ZeroDivisionError:
            print "Oops!  That was no valid number.  Try again..."

            
Please enter a number: 3
Please enter a number: 0
Oops!  That was no valid number.  Try again...
Please enter a number: 

First the try clause is executed, if an exception occurs the try block code execution is stopped and handled in except block.. The message given in exception block is printed and execution again passes to try block.