An Introduction to Python

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An Introduction to Python
Michael DiRamio District School Board of Niagara

• Imperial Oil Summer Institute
• 2006

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Summary

• IDLE a Python IDE
• Output
• Arithmetic
• Variables
• Input
• Looping
• Selection

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Most Important Feature in a Language
Documentation

• The symbol to start documentation in Python is

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Interpreted Programming Languages

• Some programs are compiled, meaning that the code
  you write is translated to something the computer
  understands.
• Others are interpreted, meaning that your code is
  read, as is, by another program that then does
  what the program specifies.
• Python is an interpreted language.

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IDLE A Python IDE

• Free IDE download at www.python.org
• Has both interactions window and program files

Interactions Window
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IDLE Interaction

• Allows you to try out an expression with
  immediate results.
• Output is the result of the expression

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Activity Exploration

• Play with the interactions window, see what the
  results are.
• Exploration is a good way to learn, and this
  window provides an easy opportunity to explore.

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IDLE Writing Programs

• Programs can be written in files, saved, and run.
• The results are produced in the interactions
  window.

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Output

• The output keyword in Python is print. print
  will output anything, Strings, numbers, etc.
• Note the difference between simply putting
  Hello in the interactions window, and putting
  print Hello

gtgtgt 10 Hello
print 4 6 print "Hello"
program
output
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Activity First Program

• Write a program (not in the interactions window,
  in a separate file) that prints the phrase Hello
  World on the screen
• Run this program (Run -gt Run Module) and see the
  output produced in the interactions window

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Concatenation

• String concatenation is the symbol

gtgtgt 'Hello ' 'World' 'Hello World'
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or ?

• Python allows you to use either single or double
  quotes for Strings.
• This prevents needing special characters if you
  want to use a quote in your string.

gtgtgt print 'This contains "double quotes"' This
contains "double quotes" gtgtgt print "And here are
'single quotes' inside" And here are 'single
quotes' inside
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Math

• The standard arithmetic operations , -, /, ,
  (remainder)
• Note if you divide two integers, the result will
  be an integer
• More math functions like ceiling and floor are in
  the math module (see intermediate Python)

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Activity Division

• Explore the difference between
• 5 / 2
• and
• 5.0 / 2

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Variables

• Python variables dont carry type. You can assign
  a variable to be a string, then assign it to an
  integer later

gtgtgt x 6 gtgtgt print x 3 9 gtgtgt x "Hello" gtgtgt
print x Hello gtgtgt print x " world" Hello world
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Input

• The method raw_input(string) returns a string the
  user has typed in
• The parameter to raw_input is the prompt given to
  the user.

x raw_input("ltTYPE SOMETHINGgt") print "The user
typed " x
Program
gtgtgt ltTYPE SOMETHINGgtThis is my input The user
typed This is my input
Output
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Input

• The string input can be converted to other types
  using conversion methods like int(string) and
  float(string)

x raw_input("ltTYPE A NUMBERgt ") y
int(x) print y 2
Program
gtgtgt ltTYPE A NUMBERgt 11 22
Output
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Activity Input Conversion

• Write a program that reads in a temperature in
  Fahrenheit and converts it to Celsius
• C (F - 32) 5 / 9

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Indentation

• Python doesnt use brackets to define blocks of
  code
• Indentation tells the program where a section
  ends
• If a program isnt properly indented

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Loops
gtgtgt 0 1 2 3 4 5 6 7 8 9 done!
i 0 while i lt 10 print i i i
1 print "done!"

• The loop continues iterating as long as the
  condition following while remains true

Program
Output
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Activity Importance of Indentation

• Create a program that uses a loop, like the
  previous example.
• Now remove the indentation and try running the
  program

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Comparisions

• is used for equality testing
• Inequality is !
• Inequalities are straight forward lt, gt, lt, gt

gtgtgt print 8 lt 15 True gtgtgt print 9 ! 7 True
Interactions Window
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Selection Statements

• if/elif/else
• Must have one if clause
• Followed by 0 or more elif (else if) clauses
• Followed by 0 or 1 else clause

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Selection Statements Example

• Notice what happens when you remove the
  indentation.

x raw_input("input please") x int(x) if x lt
10 print "Small" elif x lt 20 print
"Medium" elif x lt 30 print "Big" else
print "HUGE!" print "Done"
gtgtgt input please21 Big Done
Program
Output
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Activity Selection

• Write a program that reads in a number and
  determines if that number is even or odd.

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Use in Computer Programming

• Free IDE
• The interactions window allows students to
  quickly test out ideas.
• Lack of brackets keeps code uncluttered.
• Students must indent their code properly or it
  wont function properly.
• Lightweight syntax prevents students from
  becoming lost in the picky details of a language.