Variables 


A Byte of Python 

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Using just literal constants can soon become boring - we need some way of storing any information and manipulate them as well. This is where variables come into the picture. Variables are exactly what they mean - their value can vary i.e. you can store anything using a variable. Variables are just parts of your computer's memory where you store some information. Unlike literal constants, you need some method of accessing these variables and hence you give them names.

Identifier Naming

Variables are examples of identifiers. Identifiers are names given to identify something. There are some rules you have to follow for naming identifiers:

  • The first character of the identifier must be a letter of the alphabet (upper or lowercase) or an underscore ('_').

  • The rest of the identifier name can consist of letters (upper or lowercase), underscores ('_') or digits (0-9).

  • Identifier names are case-sensitive. For example, myname and myName are not the same. Note the lowercase n in the former and the uppercase N in te latter.

  • Examples of valid identifier names are i, __my_name, name_23 and a1b2_c3.

  • Examples of invalid identifier names are 2things, this is spaced out and my-name.

Data Types

Variables can hold values of different types called data types. The basic types are numbers and strings, which we have already discussed. In later chapters, we will see how to create our own types using classes.

Objects

Remember, Python refers to anything used in a program as an object. This is meant in the generic sense. Instead of saying 'the something', we say 'the object'.

Note for Object Oriented Programming users

Python is strongly object-oriented in the sense that everything is an object including numbers, strings and even functions.

We will now see how to use variables along with literal constants. Save the following example and run the program.

How to write Python programs

Henceforth, the standard procedure to save and run a Python program is as follows:

  1. Open your favorite editor.

  2. Enter the program code given in the example.

  3. Save it as a file with the filename mentioned in the comment. I follow the convention of having all Python programs saved with the extension .py.

  4. Run the interpreter with the command python program.py or use IDLE to run the programs. You can also use the executable method as explained earlier.

Example 4.1. Using Variables and Literal constants

			
# Filename : var.py

i = 5
print i
i = i + 1
print i

s = '''This is a multi-line string.
This is the second line.'''
print s
			
			

Output

			
$ python var.py
5
6
This is a multi-line string.
This is the second line.
			
			

How It Works

Here's how this program works. First, we assign the literal constant value 5 to the variable i using the assignment operator (=). This line is called a statement because it states that something should be done and in this case, we connect the variable name i to the value 5. Next, we print the value of i using the print statement which, unsurprisingly, just prints the value of the variable to the screen.

The we add 1 to the value stored in i and store it back. We then print it and expectedly, we get the value 6.

Similarly, we assign the literal string to the variable s and then print it.

Note for C/C++ Programmers

Variables are used by just assigning them a value. No declaration or data type definition is needed/used.