Python 101:Python for Absolute Beginners

PyTexas 2014

What is programming?

A computer is a machine that stores pieces of information.

A computer also moves, arranges, and controls that information (or data).

A program is a detailed set of instructions that tells a computer what to do with that data.

CodeSkulptor!

CodeSkulptor Developed by Scott Rixner of

Rice University to use for COMP 200.

Based on CodeMirror and Skulpt. www.codeskulptor.org If you want to learn more about

using Python with CodeSkulptor after this class, check out the Coursera course “An Introduction to Interactive Programming in Python”! (9/15 – 11/16)

https://www.coursera.org/course/interactivepython

Interacting with CodeSkulptor Run Save Fresh URL Open Local Reset

Additional Resources Docs (documentation) Demos Viz Mode

Let’s see what this thing can do…

Recap:Explore! Makes changes, see how

they impact the program as a whole.

When in doubt, check the documentation.

When super in doubt, Google it.

Math

Math

Try typing this into CodeSkuptor:

>>> print 3 + 12

>>> print 12 - 3

>>> print 9 + 5 – 15 + 12

Operators:add: +

subtract: -

Note: don’t type the arrows >>> !

MathRule: If you want Python to answer in floats, you have to talk to it in floats.

More operators:divide: /multiply: *

>>> print 3 * 12>>> print 12 / 3>>> print 11 // 3>>> print 12.0 / 3.0>>> print 11.0 / 3.0>>> print 11.0 // 3.0

Math

Comparison operators:

== Equal to!= Not equal to< Less than> Greater than<= Less than or equal to>= Greater than or equal to

Math

Practice:

>>> print 2 < 3True>>> print 2 <= 2 False>>> print 3 > 2True>>> print 2 != 3True>>> print False < True True

Strings

Strings

Examples:

Try typing one without quotes: What’s the result?

>>> “It’s a beautiful day!”>>> “Goodbye, cruel world.”

>>> Aggies

>>> “Aggies”>>> “Rice fight, never die!”>>> “3 + 2”

Strings

String operators: concatenation: + multiplication: *

Try concatenating:Try multiplying:

>>> print “Hello” + “ “ + “world!”

>>> print “HAHA” * 250

Variables

Variables

Calculate a value:

How can you save that value?

Give the value a name:

>>> print 21 + 21

42

>>> ultimate_answer = 42

>>> ultimate_answer

42

VariablesCreate a variable and give it a value:

Now assign a new value:

>>> headmaster = “Dumbledore”

>>> print headmaster

‘Dumbledore’

>>> headmaster = “Hardcastle”

>>> print headmaster

‘Hardcastle’

>>> color = “Brad Neely”

>>> color = 12

Variables You can calculate a variable once, but keep the result to use later. You can keep the same name for a variable, but change the value.

Some other things that we can do with variables:

Get an index from a string:

Do some math:

>>> headmaster = “Dumbledore”

>>> print headmaster[2]

>>> number = 3

>>> print headmaster[number - 2]

Types of data

Data TypesWe already know about three types of data: “Whoop!” string 42

integer 3.14159 float

Python can tell us about types using the type() function:>>> print type(“Whoop!”)

<type ‘str’>

How would we get Python to output int and float types?

Data Type: Lists

ListsList: a sequence of objects>>> Beatles = [“John”, “Paul”, “George”, “Ringo”]

>>> grades = [82, 93, 67, 99, 100]

Guess what this will output:

>>> type(Beatles)

>>> type(grades)

ListsList: a sequence of objects>>> Beatles = [“John”, “Paul”, “George”, “Ringo”]

>>> grades = [82, 93, 67, 99, 100]

Guess what this will output:

>>> type(Beatles)

<type ‘list’>

>>> type(grades)

<type ‘list’>

ListsIndex: Where an item is in the list>>> Beatles = [“John”, “Paul”, “George”, “Ringo”]

>>> Beatles[0]

‘John‘

[“John”, “Paul”, “George”, “Ringo”]

0 1 2 3

Python always starts at zero!

Data Type: Booleans

BooleansA boolean value can be:

Is 1 equal to 1?

Is 15 less than 5?

True or False.

>>> print 1 == 1

True

>>> print 15 < 5

False

BooleansWhat happens when we type Boolean values in the interpreter?

When the words ‘True’ and ‘False’ begin with upper caseletters, Python knows to treat them like Booleans

instead of strings or integers.

>>> True

>>> False

>>> true

>>> false

>>> type(True)

>>> type(“True”)

BooleansandIf both comparisons are True:

If only one of the comparisons is True:

If both of the comparisons are False:

>>> 1==1 and 2==2

True

>>> 1==1 and 2==3

False

>>> 1==2 and 2==3

False

BooleansorIf both comparisons are True:

If only one of the comparisons is True:

If both of the comparisons are False:

>>> 1==1 or 2==2

True

>>> 1==1 or 2!=2

True

>>> 1==2 or 2==3

False

BooleansnotYou can use the word not to reverse the answer that Python gives:

Any expression that is True can become False:

>>> 1==1

True

>>> not 1==1

False

>>> not True

False

BooleansYou can also use Booleans in their own expressions:

>>> True and True>>> True and False>>> False and False

>>> True or True>>> False or True>>> False or False

>>> not True and True>>> not True or True

Logic

if Statements

if Statements

Making decisions:

If a condition is met, perform an action:

“If you’re hungry, let’s eat lunch.”“If you like Frisbee, let’s play!”

>>> state = “Texas”

>>> if state == “Texas”:

print “TX”

TX

if Statements

Adding a choice:

Adding a choice in our code with the else clause:

“If you’re hungry, let’s eat lunch. Or else we can eat in an hour.”“If you like Frisbee, let’s play! Or else we can play rugby.”

>>> if state == “Texas”

print “TX”

else:

print “[inferior state]”

if Statements

Adding many choices:

Adding more choices in our code with the elif clause:

“If you like Frisbee, let’s play! Or

else we can play rugby. Or else we can play Bioshock, or Half-Life, or Portal…”

>>> if name == “Paige”

print “Hi Paige!”

elif name == “Walker”:

print “Hi Walker!”

else:

print “Imposter!”

Loops

LoopsLoops are chunks of code that repeat a task over and over again.

Counting loops repeat a certain number of times. Conditional loops keep going until a certain thing happens

(or as long as some condition is True).

LoopsCounting loops repeat a certain number of times – they keep going until they get to the end of a count.

>>> for mynum in [1, 2, 3, 4, 5]:

print "Hello", mynum

Hello 1

Hello 2

Hello 3

Hello 4

Hello 5

The for keyword is used to create this kind of loop, so it is usually just called a for loop.

LoopsConditional loops repeat until something happens (or as long as some condition is True).

>>> count = 0

>>> while (count < 4):

print 'The count is:', count

count = count + 1

The count is: 0

The count is: 1

The count is: 2

The count is: 3

The while keyword is used to create this kind of loop, so it is usually just called a while loop.

Algorithms

AlgorithmsReally just means “set of

instructions”

Secret: computers aren’t very smart.

How would I make a pot of coffee?1. Get a flavor of ground coffee.2. Get a coffee maker.3. Get filter paper.4. Get a pot of water.5. Make sure the coffee maker is plugged in…

…and on, and on, and on.

But to us, it’s just “make a pot of coffee”.

Functions

Remember how Algorithms are just instructions? Functions are just a concise way to group instructions into a bundle.

What it's like in our minds: “Make a pot of coffee.” bundle

In Python, you could say it like this: make_coffee(coffee_grounds, coffee_pot, water, filter_paper)

^ ^-----------------^---------------^-----------------^ function name function parameters

FunctionsLet’s define a function in CodeSkulptor:

>>> def beverage():print ‘Have you had a cup of coffee today?’

Now we’ll call the function:

>>> beverage()Have you had a cup of coffee today?

FunctionsBut what if not everyone wants a cup of coffee?

Let’s define a function with parameters:

>>> def beverage(drink):print “Have you had a cup of ” + drink + “ today?’

Now we’ll call the function:

>>> beverage(“Monster Zero”)Have you had a cup of Monster Zero today?

Functions Functions are defined using def. Functions are called using parentheses (). Functions take parameters and return outputs. print displays information, but does not give a value. return gives a value to the caller.

Thanks so much!Any questions?