Python Programming Language | Part 1

In last article we saw all the technical details, it’s time to start having fun with Python. In this article we will learn about python programming language basics. The ultimate goal is to make the computer do our bidding. To this end, we will write programs that control the computational processes inside the machine. You have already seen that there is no magic in this process, but in some ways programming feels like magic. The computational processes inside the computer are like magical spirits that we can harness for our work.

Unfortunately, those spirits only understand a very arcane language that we do not know. What we need is a friendly Genie that can direct the spirits to fulfill our wishes. Our Genie is a Python interpreter. We can give instructions to the Python interpreter, and it directs the underlying spirits to carry out our demands. We communicate with the Genie through a special language of spells and incantations (i.e., Python).

The best way to start learning about Python is to let our Genie out of the bottle and try some spells. You can start the Python interpreter in an interactive mode and type in some commands to see what happens. When you first start the interpreter program, you may see something like the following:

Python 2.1 (#1, Jun 21 2001, 11:39:00)

[GCC pgcc-2.91.66 19990314 (egcs-1.1.2 release)] on linux2

Type "copyright", "credits" or "license" for more information. >>>

The >>> is a Python prompt indicating that the Genie is waiting for us to give it a command. In programming languages, a complete command is called a statement. Here is a sample interaction with the Python interpreter.

>>> print "Hello, World"

 Hello, World

 >>> print 2 + 3

5

 >>> print "2 + 3 =", 2 + 3

 2 + 3 = 5

Here I have tried out three examples using the Python print statement. The first statement asks Python to display the literal phrase Hello, World. Python responds on the next line by printing the phrase.

 The second print statement asks Python to print the sum of 2 and 3. The third print combines these two ideas. Python prints the part in quotes “2 + 3 =” followed by the result of adding 2 + 3, which is 5. This kind of interaction is a great way to try out new things in Python. Snippets of interactive sessions are sprinkled throughout this article. When you see the Python prompt >>> in an example, that should tip you off that an interactive session is being illustrated. It’s a good idea to fire up Python and try the examples for yourself.

 Usually we want to move beyond snippets and execute an entire sequence of statements. Python lets us put a sequence of statements together to create a brand-new command called a function. Here is an example of creating a new function called hello.

>>> def hello():

print "Hello"

print "Computers are Fun"

>>> 

The first line tells Python that we are defining a new function called hello. The following lines are indented to show that they are part of the hello function. The blank line (obtained by hitting the <Enter> key twice) lets Python know that the definition is finished, and the interpreter responds with another prompt. Notice that the definition did not cause anything to happen. We have told Python what should happen when the hello function is used as a command; we haven’t actually asked Python to perform it yet. A function is invoked by typing its name. Here’s what happens when we use our hello command.

>>> hello()

Hello

Computers are Fun

 >>>

Do you see what this does? The two print statements from the hello function are executed in sequence. You may be wondering about the parentheses in the definition and use of hello. Commands can have changeable parts called parameters that are placed within the parentheses. Let’s look at an example of a customized greeting using a parameter. First the definition:

>>> def greet(person):

 print "Hello", person

 print "How are you?"

Now we can use our customized greeting.

>>> greet("John")

 Hello John How are you?

 >>> greet("Emily")

 Hello Emily How are you?

 >>>

Can you see what is happening here? When we use greet we can send different names to customize the result. We will discuss parameters in detail later on. For the time being, our functions will not use parameters, so the parentheses will be empty, but you still need to include them when defining and using functions.

One problem with entering functions interactively at the Python prompt like this is that the definitions go away when we quit Python. If we want to use them again the next time, we have to type them all over again. Programs are usually created by typing definitions into a separate file called a module or script. This file is saved on a disk so that it can be used over and over again.

 A module file is just a text file, and you can create one using any program for editing text, like a notepad or word processor program (provided you save your program as a “plain text” file). A special type of program known as a programming environment simplifies the process. A programming environment is specifically designed to help programmers write programs and includes features such as automatic indenting, colour highlighting, and interactive development. The standard Python distribution includes a programming environment called Idle that you may use for working on the programs.

 Let’s illustrate the use of a module file by writing and running a complete program. Our program will illustrate a mathematical concept known as chaos. Here is the program as we would type it into Idle or some other editor and save in a module file:

# File: chaos.py

# A simple program illustrating chaotic behaviour.

def main():

 print "This program illustrates a chaotic function"

 x = input("Enter a number between 0 and 1: ")

for i in range(10):

 x = 3.9 * x * (1 - x)

print x

main()

This file should be saved with the name chaos.py. The .py extension indicates that this is a Python module. You can see that this particular example contains lines to define a new function called main. (Programs are often placed in a function called main.) The last line of the file is the command to invoke this function. Don’t worry if you don’t understand what main actually does; we will discuss it in the next section. The point here is that once we have a program in a module file, we can run it any time we want.

This program can be run in a number of different ways that depend on the actual operating system and programming environment that you are using. If you are using a windowing system, you can run a Python program by double-clicking on the module file’s icon. In a command-line situation, you might type a command like python chaos.py. If you are using Idle (or another programming environment) you can run a program by opening it in the editor and then selecting a command like import, run, or execute. One method that should always work is to start the Python interpreter and then import the file. Here is how that looks.

>>> import chaos

 This program illustrates a chaotic function

 Enter a number between 0 and 1: .25

0.73125

0.76644140625

0.698135010439

0.82189581879

0.570894019197

0.955398748364

0.166186721954

0.540417912062

0.9686289303

0.118509010176

>>> 

Typing the first line import chaos tells the Python interpreter to load the chaos module from the file chaos.py into main memory. Notice that I did not include the .py extension on the import line; Python assumes the module will have a .py extension.

 As Python imports the module file, each line executes. It’s just as if we had typed them one-by-one at the interactive Python prompt. The def in the module causes Python to create the main function. When Python encounters the last line of the module, the main function is invoked, thus running our program. The running program asks the user to enter a number between 0 and 1 (in this case, I typed “.25”) and then prints out a series of 10 numbers.

When you first import a module file in this way, Python creates a companion file with a .pyc extension. In this example, Python creates another file on the disk called chaos.pyc. This is an intermediate file used by the Python interpreter. Technically, Python uses a hybrid compiling/interpreting process. The Python source in the module file is compiled into more primitive instructions called byte code. This byte code (the .pyc) file is then interpreted. Having a .pyc file available makes importing a module faster the second time around. However, you may delete the byte code files if you wish to save disk space; Python will automatically re-create them as needed.

 A module only needs to be imported into a session once. After the module has been loaded, we can run the program again by asking Python to execute the main command. We do this by using a special dot notation. Typing chaos.main() tells Python to invoke the main function in the chaos module. Continuing with our example, here is how it looks when we rerun the program with 26 as the input.

>>> chaos.main()

Enter a number between 0 and 1:.26

0.75036

0.73054749456

0.767706625733

0.6954993339

0.825942040734

0.560670965721

0.960644232282

0.147446875935

0.490254549376

0.974629602149

>>>