python. what is python? a programming language we can use to communicate with the computer and solve...

Python

What is Python?

• A programming language we can use

to communicate with the computer

and solve problems

• We give the computer instructions

that it will execute

What is Python?

• Python is a high level language that we

(humans) can understand

– Other examples include C++ and Java

• A machine can only understand low level

language

• So we process the high level to low level so

the computer will understand!

Translation• How do we translate high level

languages to low level languages?

–We use interpreters and compilers!

– Interpreters process the program a little

bit at a time and runs it

– Compilers translate everything before

running it

Some Python Vocab

• Program – a file of code that may contain functions

• Script – a short code that we can run in a

command line

• Variable – names we assign the values to, allowing

us to reuse them later on

– For example: x = 1 or msg = “Hello world!”

– Variables can be changed throughout a program

– For example: x = 1, x = x + 1

Python Vocab• Comments – notes ignored by the

computer

– For example: x + y # variables

store user input

• Operators – mathematical symbols

–+, -, *, /, ** (exponents), ==

(equality)

Python Vocab

• Keyword – words with meaning/purpose in

Python

– For example: “and”, “print”, “if”

• Expression – statements that produce values

– 3 + 5, “Hello world!”

• Error – program has a problem in the command

area

• Instance – one run-through of a program

Indentation

• A REQUIREMENT IN PYTHON!

• Indenting specifies the “scope” of

different chunks of your code

– Everything indented after a first,

unindented line “belongs” to that line!

Things to Note• Python is case sensitive

– A function called “first” is different than a function called “FIRST”

or “First” or “fiRSt”

• Python doesn’t like spaces or punctuation marks

– You can’t name your function “spam Five” or “spam.Five”

– You could, however, name your function“spamFive” or “spam_Five”

• Some words in Python can’t be used as names

– Keywords can never be used as function/variable names

– Check the colors of the words! Purple and orange are

KEYWORDS!

LET’S PLAY WITH PYTHON!

Our age(number) function

• We had an argument passed into our

function!

• The argument is known as a parameter

• Example

def add(a, b):

print(“This is a + b: “, a+b)

• a and b are the parameters

Data Types

• Numeric

– Integers (5, 2, -1)

– Floating Point Numbers (0.2, 3.14159, 28.92)

• Non-numeric

– String (text), lists, dictionaries, etc

– Basically anything you can’t add up using a

simple plus sign (+)

Not a String? Not a Problem!

• You can format outputting variables

you’ve already defined

x = 42

print “The value of x is”, x,

“.”

• What does this print out?

Not a String? Not a Problem!

• The output is

– The value of x is 42.

• The bottom will cause an error.

x = 42

print “$” + x

• We can’t combine string and numbers. So

what do we do?

Not a String? Not a Problem!

• We can make our numerical variable

a string!

x = 42

print “$” + str(x)

• This will print out $42

More on Variables• Variables can hold all kind of values, including strings,

numbers, and user input

• To assign a string value to a variable, you have to wrap the

string in quotes

firstName = “Jessi”

lastName = “Cheung”

mathProblem = “5 + 5”

print lastName, “,”, firstName, “;”, mathProblem

• What will this print?

More on Variables

• The output is: Cheung, Jessi; 5 + 5

• Variables can also be assigned new values that

are relative to their old values

total = 10

print “Original total:”, total

total = total + 4

print “New total:”, total

• What does this print?

More on Variables• The output is

Original total: 10

New total: 14

• Remember: a variable has to be defined on a previous line

before it can be used on the right-hand side of an equation

ABC = ABC + 4

print “ABC:”, ABC

• ERROR. There was no mention of the value of “ABC”

before the line trying to redefine it.

Python Arithmetic• Try typing the following code in your program area and see what comes out!

def main():

a = 12

b = 2

c = 16

d = 3

e = 2.5

print “The value of a is”, a

print (a / b) * 5

print a + b * d

print (a + b) * d

print b ** d

print c – e

a = a + b

print “The value of a is”, a

Python Arithmetic

• Is this what you got?

the value of a is 12

30

18

42

8

13.5

the value of a is 14

Exercise time!

• Write a program that takes in a

Celsius temperature (celsius) and

returns the temperature in

Fahrenheit

– Hint: To get Fahrenheit, multiply the

Celsius by (9.0/5.0) and add 32

Taking User Input

• Sometimes, instead of passing in an

argument as a parameter, we can

have the computer ask us what we

want!

Taking User Inputname = requestString("Enter your name:") 

print name

first pops up a dialog box (where you can enter a name, say

‘John Doe’): 

then outputs

John Doe

Taking User Input

• Let’s try it with numbers!

def requestNumber():

num = input(“Enter a number:”)

print “Your number is:”, num

print “Your number squared:”,

num*num

Taking User Input

• What if you tried inserting a string into…

def requestNumber():

num = input(“Enter a number:”)

print “Your number is:”, num

• If you type hello, there will be an error. If you

type “hello”, it will work

• This is where raw_input comes into play!

Taking User Input

• raw_input will take exactly what you

type and make it into a string

def requestName():

name = raw_input(“Enter your

name:”)

print name, “is awesome!”

• Try typing in a number!

Let’s Write a Program!

• Let’s write a program that will calculate the area and

the circumference of a circle!

• Open a new window (File/New Window)!

• At the top of your (blank) file, write the following:

# file name: circle.py

# author: Jessi Cheung

# description: a program to calculate the area and

the circumference of a circle

• Save the program as circle.py

Let’s Write a Program

• Let’s define this program as main.

def main():

• Your turn!

– Use input to ask the user for the

radius!

Let’s Write a Program

• We now have (besides our heading)

def main():

radius = input(“What is the radius? “)

• Now let’s start the calculations!

– Circumference of a circle: Pi (3.14) times (radius

times two)

– Area of a circle: Pi (3.14) times (radius squared)

Let’s Write a Program!

• We now have (besides our heading)

def main():

radius = input(“What is the radius?

“)

circumference = 3.14 * (2 * radius)

area = 3.14 * (radius * radius)

Let’s Write a Program!

• Now for the finishing touches!

• Let’s print out the output so we can

see it!

Let’s Write a Program

def main():

radius = input(“What is the radius? “)

circumference = 3.14 * (2 * radius)

area = 3.14 * (radius * radius)

print “The radius of our circle is”, radius

print “The circumference of our circle is”,

circumference

print “The area of our circle is”, area

Let’s RUN the Program!

• Once you save your program, press

F5 on your keyboard

• Nothing happens?!

Let’s RUN the Program!

• You must call your program!

• Call using main()

Let’s RUN the Program

• Another way you could run the

program…

For Loops

• Also known as the “definite loop” – we know

exactly how many times the loop will happen!

• Allows you to specify a list of items (numbers,

words, letters, etc.) and specify actions to be

performed on each one

• The official syntax for the for loop is:

for <var> in <sequence>:

<body>

Help the Kittens!

• You are working at an animal shelter, and you’re asked

to take a group of kittens and bathe, dry, and feed each

one individually

Use a Loop!

• Using a for-loop type notation, your

instructions would look like this:

Kittens = [kitty #1, kitty #2, kitty

#3, ...]

for kitty in Kittens:

bathe kitty

dry kitty

feed kitty

Basic Loop

• See what happens when you put in

this:

phrase = “Hello world!”

for letter in phrase:

print “the next letter is:”, letter

Basic Loop• The output!

the next letter is: H

the next letter is: e

the next letter is: l

the next letter is: l

the next letter is: o

the next letter is:

the next letter is: w

the next letter is: o

the next letter is: r

the next letter is: l

the next letter is: d

the next letter is: !

What Just Happened?

• Python went through the string one

character at a time, treating the

string like a sequence

• That means that the string can be

split into its components (the

characters)

Accumulator Variables

• When you’re using a for loop,

sometimes you might want to keep a

running total of numbers you’re

calculating, or recombine bits of a

string

Accumulator Variables

• Steps:

1. Define a variable for the first time before the loop starts

2. Redefine it as itself plus some operation in the body of

the for loop

total = 0

for num in [1,2,4,10,20]:

total = total + num

print “Total:”, total

• This will give the output

Total: 37

Accumulator Variables

• What is the point of accumulator

variables?

– Counting

– Keeping score

– Debugging

Conditional Statements

• Equals: ==

• Does not equal: !=

• Try this:

x = 1

if (x != 2):

print “Artemis rocks”

Want to learn more?

• Go to:

wiki.python.org/moin/BeginnersGuide