Python Introduction

Credit: Slides are modified with permission from Charles Severance at the University of Michigan.

This work is licensed under an Attribution-NonCommercial-ShareAlike 3.0 Unported License

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Act in haste and repent at leisure; code too soon and debug forever.

-- R. Kennington

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xkcd.com/844

Writing good, efficient code requires

thought and planning

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program that uses BLEND interface to automatically manage assignments

program that runs & checks student program submissions

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Users vs Programmers

• Users see computers as a set of tools - word processor, spreadsheet, map, todo list, etc.

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• Programmers know how computers work and how to talk with them in a computer language

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• Programmers often write little “helpers” for themselves to automate a task or sometimes write large complex programs for many many users

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Computers want to be helpful...

• Users have it easy - they just pick the programs they want to use
• Programmers need to speak the computer's language to describe what they want done

• Computers are built for one purpose - to do things for us - really fast

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What is Code? Software? A Program?

A sequence of stored instructions

Program, noun:

A magic spell cast upon a computer to enable it to turn input into error messages.

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Program Steps or

Program Flow

• Like a recipe or a manual, a program is a sequence of steps to be done in order
• Some steps may be conditional/optional and can be skipped
• Sometimes a step or group of steps are to be repeated

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Python was/is developed by Guido van Rossum.

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In the Python community, Van Rossum is known as a "Benevolent Dictator For Life" (BDFL).

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Beginning Programmers

• We need to learn the Python language so we can communicate our instructions to Python interpreter.
• We must communicate very precisely when we write Python code.
• When you make a mistake, the computer does not think you are “cute”. It says “syntax error”.
• Python is much like a dog, which only understands a few key words but constantly looks at you with a sweet look on its face waiting for you to say something it understands.

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Beginning Programmers

• Beginning programmers often take the fact that Python leaves no room for errors as evidence that Python is mean, hateful and cruel.

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• While Python seems to like everyone else, Python knows them personally and holds a grudge against them.

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Common Reaction when program does not work Cub Fan

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Programs do EXACTLY what you tell them to. They don't know any better.

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Programming can be frustrating.

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Early

Greek Programmers

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• Can solve lots and lots of problems
• put a little piece of your intelligence in the computer
• Creative art - particularly when we do a good job on the user experience

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What do we say?

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Statements and Expressions

x = 2

x = x + 2

print(x)

Variable

Operator

Literal

Function Name

Assignment Statement

Assignment with expression

Print statement

"parts of speech"

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to run Python code

Both are accessed using IDLE - Python's default Integrated Development and Learning Environment.

1. Python Shell
• Just type the command after >>> and hit ENTER
• The repl.it shell has a single >
2. Python Edit Window
• Type in program, save, and then run
• NOTE: Program runs in a Python Shell

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Python Shell

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Let's Talk to Python...

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>>> x = 1

>>> print(x)

1

>>> y = x + 1

>>> print(y)

2

>>> y

2

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Sequential Steps

Program

x = 2

print(x)

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2

x = x + 2

print(x)

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4

output to screen.

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x = 2

print(x)

x = x + 2

print(x)

When a program is running, it flows from one step to the next. We as programmers set up “paths” for the program to follow.

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Python Edit Window

Usually we enter our program into the Python Edit Window, save it to a file, and then tell python to run the program.

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Starting Python

2) type Idle

3) right click on Idle Python 3.x and select Pin to taskbar

1) click Start

From then on click on taskbar icon to start

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Edit Window &

Show line numbers

close Idle and open it again

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Useful IDLE Stuff

Quickly open a recent file.

Quickly indent/dedent or comment/uncomment

blocks of code.

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You will submit many of your Python programs using a web application called StudentDrop that was written by a former LASA students.

Programs submitted via StudentDrop end up on my computer so I can run and verify them (using a Python program that I wrote).

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Before you can submit to StudentDrop, we'll first get you registered with StudentDrop.

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Requirements for your program name

• (1) only letters & numbers
• start with a letter
• (2) no spaces

You will submit many of your Python programs using PC². You were emailed a username & password - click on the PC² icon to login.

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PC² submissions can only be made at school.

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PC² submissions are due when I have the PC² judging program running (I usually keep it running at least until the beginning of the class after the class in which we have finished a PC² assignment).

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