Python III # Real Learning

What can you learn here?

** How present data into easily understandable format? ** Data Formatting

** How to raise custom exception? ** Exception Handling

** How can we write linked lists in Python? ** Linked Lists

** How to implementing stacks with Python lists? ** Stacks

How to format string output data?

Data representation in easily understand manner as important as data. It created a readability for user to work better for example,. output of ls -l, ps -ef in unix os.

In Python, we have “format” mechanisms which we use for formatting data in easily readable format.

>>> def test():
	out_format = '%-15s%-15s%s' 
    sample_data = [ ('1', '22', '333'), ('111', '22', '3'), ('Tom', 'Dick', 'Harry')]
    # first 15 characters for first column %-15s
    #  next 15 characters for second column
    #  last - no need to format
    # if '-' is present it means, align data left in column space
	for items in sample_data:
		print out_format % items

>>> test()
1              22             333
111            22             3
Tom            Dick           Hary

** How to handle floats - formats? **

>>> "%-20s %12.2f" % ('Server Bios Version', 2.0212)
'Server Bios Version          2.02'
>>> "%-20s %12.4f" % ('Server Bios Version', 2.0212)
'Server Bios Version        2.0212'
>>> 





def report (wages) :
    students = wages.keys()
    students.sort()
    for student in students :
    print "%-20s %12.2f" % (student, wages[student])

To test this function, we’ll create a small dictionary and print the contents:

>>> wages = {’mary’: 6.23, ’joe’: 5.45, ’joshua’: 4.25}
>>> report(wages)
joe             5.45
joshua          4.25
mary            6.23

Gist

Meaning of ‘%-15s%-15s%s’:

first 15 characters for first column %-15s

next 15 characters for second column

last - no need to format

if ‘-‘ is present it means, align data left in column space

How to raise custom exception?

>>> def test():
	x = input()
	if x == 42:
		raise ValueError,'input other than 42'

	
>>> test()
1212
>>> 1212
1212
>>> test()
42

Traceback (most recent call last):
  File "<pyshell#96>", line 1, in 
    test()
  File "<pyshell#93>", line 4, in test
    raise ValueError,'input other than 42'
ValueError: input other than 42

Linked lists

class Node:
    def __init__(self, cargo=None, next=None):
        self.cargo = cargo
        self.next = next
    def __str__(self):
        return str(self.cargo)

Implementing stacks with Python lists

class Stack :
    def __init__(self) :
        self.items = []
    def push(self, item) :
        self.items.append(item)
    def pop(self) :
        return self.items.pop()
    def isEmpty(self) :
        return (self.items == [])