Unit 1 Basic Python programs functions Special thanks to Roy McElmurry John Kurkowski Scott Shawcroft Ryan Tucker Paul Beck for their work Except where otherwise noted this work is licensed under http creativecommons org licenses by nc sa 3 0 Python Created in 1991 by Guido van Rossum now at Google Named for Monty Python Useful as a scripting language script A small program meant for one time use Targeted towards small to medium sized projects Used by Google Yahoo Youtube Many Linux distributions Games and apps e g Eve Online 2 Installing Python Windows Download Python from http www python org Install Python Run Idle from the Start Menu Mac OS X Python is already installed Open a terminal and run python or run Idle from Finder Linux Chances are you already have Python installed To check run python from the terminal If not install from your distribution s package system Note For step by step installation instructions see the course web site 3 Interpreted Languages interpreted Not compiled like Java Code is written and then directly executed by an interpreter Type commands into interpreter and see immediate results Java Python Code Compiler Code Runtime Environment Computer Interpreter Computer 4 The Python Interpreter Allows you to type commands one at a time and see results A great way to explore Python s syntax Repeat previous command Alt P 5 Our First Python Program Python does not have a main method like Java The program s main code is just written directly in the file Python statements do not end with semicolons hello py 1 print Hello world 6 The print Statement print text print a blank line Escape sequences such as are the same as in Java Strings can also start end with swallows py 1 2 3 4 print Hello world print print Suppose two swallows carry it together print African or European swallows 7 Comments Syntax comment text one line swallows2 py 1 2 3 4 5 6 Suzy Student CSE 142 Fall 2097 This program prints important messages print Hello world print blank line print Suppose two swallows carry it together print African or European swallows 8 Functions Function Equivalent to a static method in Java Syntax def name statement statement statement hello2 py 1 2 3 4 5 6 7 Prints a helpful message def hello print Hello world main calls hello twice hello hello Must be declared above the main code Statements inside the function must be indented 9 Whitespace Significance Python uses indentation to indicate blocks instead of Makes the code simpler and more readable In Java indenting is optional In Python you must indent hello3 py 1 2 3 4 5 6 7 8 Prints a helpful message def hello print Hello world print How are you main calls hello twice hello hello 10 Exercise Rewrite the Figures lecture program in Python Its output STOP 11 Exercise Solution def egg top bottom print def top print print print def cup bottom line print def bottom print print def stop top print bottom print def hat top line print def line print STOP main egg cup stop hat 12
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