Python language essentials

In this section we will go through the idea of variables, strings, data types, networking, and exception handling. For an experienced programmer, this section will be just a summary of what you already know about Python.

Variables and types

Python is brilliant in case of variables. Variables point to data stored in a memory location. This memory location may contain different values, such as integers, real numbers, Booleans, strings, lists, and dictionaries.

Python interprets and declares variables when you set some value to this variable. For example, if we set a = 1 and b = 2.

Then we print the sum of these two variables with:

print (a+b) 

The result will be 3 as Python will figure out that both a and b are numbers.

However, if we had assigned a = "1" and b = "2". Then the output will be 12, since both a and b will be considered as strings. Here, we do not have to declare variables or their type before using them as each variable is an object. The type() method can be used to get the variable type.

Strings

As with any other programming language, strings are one of the important things in Python. They are immutable. So, they cannot be changed once defined. There are many Python methods which can modify strings. They do nothing to the original one, but create a copy and return after modifications. Strings can be delimited with single quotes, double quotes, or in case of multiple lines, we can use triple quotes syntax. We can use the \ character to escape additional quotes which come inside a string.

Commonly used string methods are as follows:

• string.count('x'): This returns the number of occurrences of 'x' in the string
• string.find('x'): This returns the position of character 'x' in the string
• string.lower(): This converts the string into lowercase
• string.upper(): This converts the string into uppercase
• string.replace('a', 'b'): This replaces all a with b in the string

Also, we can get the number of characters, including white spaces, in a string with the len() method:

#!/usr/bin/python 
a = "Python" 
b = "Python\n" 
c = "Python  " 
 
print len(a) 
print len(b) 
print len(c) 

You can read more about the string function here: https://docs.python.org/2/library/string.html.

Lists

Lists allow us to store more than one variable inside it and provide a better method for sorting arrays of objects in Python. They also have methods which help to manipulate the values inside them:

list = [1,2,3,4,5,6,7,8] 
print (list[1])  

This will print 2, as Python index starts from 0. To print out the whole list, use the following code:

list = [1,2,3,4,5,6,7,8]
for x in list:
 print (x)

This will loop through all elements and print them.

Useful list methods are as follows:

• .append(value): This appends an element at the end of the list
• .count('x'): This gets the number of 'x' in the list
• .index('x'): This returns the index of 'x' in the list
• .insert('y','x'): This inserts 'x' at location 'y'
• .pop(): This returns the last element and also removes it from the list
• .remove('x'): This removes first 'x' from the list
• .reverse(): This reverses the elements in the list
• .sort(): This sorts the list alphabetically in ascending order, or numerical in ascending order

Dictionaries

A Python dictionary is a storage method for key:value pairs. Python dictionaries are enclosed in curly braces, {}. For example:

dictionary = {'item1': 10, 'item2': 20} 
print(dictionary['item2']) 

This will output 20. We cannot create multiple values with the same key. This will overwrite the previous value of the duplicate keys. Operations on dictionaries are unique. Slicing is not supported in dictionaries.

We can combine two distinct dictionaries to one by using the update method. Also, the update method will merge existing elements if they conflict:

a = {'apples': 1, 'mango': 2, 'orange': 3} 
b = {'orange': 4, 'lemons': 2, 'grapes ': 4} 
a.update(b) 
 
Print a 

This will return the following:

{'mango': 2, 'apples': 1, 'lemons': 2, 'grapes ': 4, 'orange': 4} 

To delete elements from a dictionary we can use the del method:

del a['mango'] 
print a 

This will return the following:

{'apples': 1, 'lemons': 2, 'grapes ': 4, 'orange': 4}

Networking

Sockets are the basic blocks behind all network communications by a computer. All network communications go through a socket. So, sockets are the virtual endpoints of any communication channel that takes place between two applications which may reside on the same or different computers.

The socket module in Python provides us a better way to create network connections with Python. So to make use of this module, we have to import this in our script:

import socket 
socket.setdefaulttimeout(3) 
newSocket = socket.socket() 
newSocket.connect(("localhost",22)) 
response = newSocket.recv(1024) 
print response 

This script will get the response header from the server. We will discuss more about networking in our later chapters.

Handling exceptions

Even though we wrote syntactically correct scripts, there will be some errors while executing them. So, we have to handle the errors properly. The simplest way to handle exceptions in Python is by using try-except:

Try to divide a number by zero in your Python interpreter:

>>> 10/0
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
ZeroDivisionError: integer division or modulo by zero

So, we can rewrite this script with try-except blocks:

try: 
   answer = 10/0 
except ZeroDivisionError, e: 
   answer = e 
print answer 

This will return the error integer division or modulo by zero.

Tip

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