"The only way to learn a new programming language is by writing programs in it."
          - B. Kernighan and D. Ritchie

Computers are good at following instructions, but not at reading your mind."
          - D. Knuth

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• Python uses an interpreter, which is a program that translates each command in your program into machine language and executes it. 

• Here's what you do:

• Type your source code (Python) in a file (that ends in .py).
• Invoke the Python interpreter (either from the command line or using IDLE) to translate your instructions into machine language and execute them. 

• You may also use Python interactively, and enter one command at a time:

piglet.cs.utexas.edu:eberlein> python
Python 2.5.4 (r254:67916, Jan 16 2009, 16:27:43)
[GCC 4.2.2] on linux2
Type "help", "copyright", "credits" or "license" for more information
>>>> print "welcome to cs 303e"
welcome to cs 303e
>>>> for i in range(3):
...              print i
...
0
1
2
>>>>

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An Example: Some Source Code in Python


def main(): # definition of main function
    print "hello"
    print "world"
    print "hello", "world"

myNum = 13
print myNum*myNum

# call the main function
main()

Output:
169
hello
world
hello world

   

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• In our programs, we often want the computer to carry out some set of instructions over and over again.
  
• To do this, we use what is called a loop. There are different types of loops:

                for i in [1, 2, 3]:
                    print i

                for i in range(2):
                    print i

                i = 3
                while i < 10:
                    print i
                    i = i+1

• We also want to perform certain actions only under certain conditions:

                           i = 5
                    if i < 10:
                        print i, "is less than 10"

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• A source file my contain function definitions, which are a collection of Python statements that will be executed together when the function is invoked. Function definitions start like this:
• def myFunctionName():
• Statements that are part of the function must be indented (say 4 spaces) underneath the above line - this indentation tells the Python interpreter which statements are part of your function.
• Functions are not executed until you call them. 
• It is common to include a function called main which is executed first. 
• A Python program can contain statements which are not part of any function.
• A comment is included after a # sign and is ignored by the python interpreter.
  

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