A Python Tour: Just a Brief Introduction CS 303e: Elements of Computers and Programming

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A Python Tour: Just a Brief Introduction

CS 303e: Elements of Computers and Programming

"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

Program: n. A magic spell cast over a computer allowing it to turn one's input into error messages.

tr. v. To engage in a pastime similar to banging one's head against a wall, but with fewer

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Useful Definitions

• Programming Language: A language that is designed to express instructions that can be carried out by a computer

• Program: A detailed set of instructions – written in a programming language – for a computer to perform

• Program Execution ("running" a program): The computer performs the program's instructions • Machine language: The low-level language of a computer that consists of each computation's

encoding as a sequence of 0s and 1s. In machine language, a program could look like this: 0110111100001010

1111011110101111 0000111101010111 ...

A sequence of 0s and 1s (called bits) corresponds to operations such as Add, Store, Get Instruction, etc. Each computer (CPU) has its own machine language.

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What is Python?

• A high-level language developed by Guido van Rossum in the

Netherlands in the late 1980s. Released in 1991.

• Named after Monty Python

• Twice received recognition as the language with the largest growth

in popularity for the year (2007, 2010)

• Clean, concise syntax

o Easy to learn

o Can create powerful programs quickly

• Design is compact

o Language constructs are easy to learn

o Easy to remember

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How Does Python Work?

• Python is interpreted:

o Each command is translated to machine language and

executed on the fly

• The other type is compiled:

o All commands are translated into machine language

o The resulting program is stored in a new file

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How Python Works

• 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:

o Type your source code (Python) in a file (that ends in extension .py).

o Invoke the Python interpreter (either from the command line or using IDLE) to translate your instructions into machine language and execute them.

o Or 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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Comments in a Python Program

• Explanations of code purpose in English • Indicated by the # character

• Ignored by the interpreter

• The style guide for this class requires comments: o At the beginning of a module

o At the beginning of a function

o Before a chunk of related statements that carry out some unified action • Example:

# python program example def main():

# Welcome everyone to CS 303e print "Welcome to CS 303e!"

Question: What output do you expect from the above Python program?

A. Welcome to CS 303e! B. "Welcome to CS 303e!"

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Repetition in Python

• In programs, we may want the computer to repeat an instruction or set of instructions more than once

• To do that, we use loops, also known as repetition structures

Examples with a Loop:

for i in [1, 2, 3]:

print i # indentation is important!

Output: 1 2 3 for i in range(3): print i Output: 0 1 2

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Decisions in Python

We may want the computer to perform certain instructions only under certain conditions.

• Test for the condition (to see if it is true)

• If condition is met, perform the action

• If condition not met, skip the action

Example:

i = 5

if i < 10:

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Expected Output

def main():

for i in [2, 4, 6]:

print i

What is the output?

A. 1 B. 2 C. i

2 4 i

3 6 i

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Python Program Structure

• 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 (or a loop body) 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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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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Python Tour: Things to Remember

• Whitespace (blank spaces) matters

o Statements inside functions/loops/conditionals must be indented

§ Tells interpreter which statements belong inside

§ Two spaces too few, six probably too many – be consistent

§ IDLE indents automatically

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A Python Tour: Just a Brief Introduction CS 303e: Elements of Computers and Programming