presentation.rst

History of Python

Developed by Guido von Rossum in 1989 at CWI

• CWI is a research institution funded by the Dutch government
• Guido needed something that was faster for development than C, less tied to Unix then Perl and bash.

Who uses Python?

• Google (Guido works there)
• DemonWare (Call of Duty servers)
• CCP Games (EVE Online)
• CRC (web development, scientific libraries)
• Dropbox
• Canonical (creators of Ubuntu)

What is Python?

• An object-oriented, dynamically typed, interpreted, general purpose programming language
• Open source, BSD-like license
• Supported on lots of platforms

General Purpose

Used for lots of different programming tasks

• Automatic repetitive administrative tasks
• Scientific calculations (libraries like SciPy and NumPy)
• Web development (Django, Pyramid, Flask, Google App Engine, so on and so forth)

Interpreted

Python code is translated to machine code at runtime, not in a prior compilation step.

Several interpreters

• Standard CPython
• PyPy project
• Stackless

Dynamically Typed

• Languages like Chapel, C, Java require variables to have their type declared on creation
• Python does not; a variable can hold any kind of value.

Examples:

a = "a string"
a = 1

Dynamically Typed

• Variable's properties are important, not it's data type.

• Also known as "duck typing".

  • "If it walks like a duck and quacks like a duck, it's a duck".

Object-Oriented

• Data and the functions related to it is associated through objects
• Objects are instances classes.
• More on this later.

Data Types

• Several generic, basic data types:

  • Integers
  • Floats
  • Strings
  • Booleans
  • Objects

Collections

• Lists
• Dictionaries
• Tuples

Running Python

The interactive interpreter From your shell prompt, type:

$ python

Get something like:

Python 2.6.1 (r261:67515, Jun 24 2010, 21:47:49)
[GCC 4.2.1 (Apple Inc. build 5646)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>>

The Interactive Interpreter

You can type in valid Python expressions in order to test things out:

>>> name = "Guido von Rossum"
>>> year = 1990
>>> print name
Guido von Rossum
>>> print year
1990
>>> 1 + 2
3
>>>

String Interpolation

We assigned some variables and printed some things.

But it can be more interesting.:

>>> print "%s made Python." % name
Guido von Rossum made Python.

We can use string interpolation to substitute a variable's value into a string.

Excercise 1

Make the interactive interpreter return the following string:

"Hello, [your name here]"

Solution 1

One way:

print "Hello, Nolan!"

Another way:

name = "Nolan"
print "Hello %s" % name

Lists

Lists are like arrays in Chapel.

Similar declaration, too.:

numbers = [1, 2, 3]

However, unlike statically typed languages, lists can contain any data type.:

my_list = ["Python", 2, 8.0, []]

Can also add to them and remove elements.

Order matters.

Tuples

Like lists, however, you cannot modify them once created.

Example:

coordinates = (3, 4)

Tuple with a single element:

weird_tuple = ("Guido",)

Parentheses are also used for grouping, hence the comma.

Iteration/Loops

Used to repeat a single instruction multiple times.

Really useful for operating on collections.

For and while loops.

For Loops

Structure::

for variable_name in collection_name:
        # do things

Note: everything inside the loop is indented!

Quick Note on Indentation

• Python uses indentation to control code structure.
• Other languages use braces ( { and } )

• Python only cares that your indentation is consistent

  • Don't mix tabs and spaces
  • Most often, people use 4 spaces as the indent-level

Back to Loops

Example:

>>>> author_names = ["Matz", "Guido", "Larry"]
>>> for name in author_names:
...     print name
...
Matz
Guido
Larry

Exercise 2

Print your name 20 times.

Hint: Instead of a list or tuple in the collection_name spot, use range(0,20).

It's kind of like Chapel's range syntax.

Solution 2

One way:

for i in range(0,20):
        print "Hello, Nolan"

Second way:

name = "Nolan"
for i in range(0,20):
       print "Hello, %s" % name

Dictionaries

• A collection of key, value pairs.
• Associates a key (which can be a number, string, object, whatever), with a value.
• Unordered - when accessing a dictionary, items may not come out in the same order they were added.

Dictionary Examples

A basic dictionary:

>>> {"shoe_size": 12}

Multiple items:

>>> {"name": "Frank", "height": 6.0}

Accessing a Dictionary

First, let's create a dictionary, with a variable:

>>> my_dictionary = {"name": "Fred"}

Next, we can retrieve the value associated with the name key:

>>> my_dictionary["name"]
'Fred'

Looping with Dictionaries

Using just the dictionary in the for statement we saw before only works on the dictionary keys.

To access both, we use the items method on the dictionary. (Those will be explained soon):

>>>> language_authors = {"Matz": "Ruby", "Guido": "Python",
... "Larry": "Perl"}
>>> for key, value in language_authors.items():
...     print "%s wrote %s" % (key, value)
...
Larry wrote Perl
Matz wrote Ruby
Guido wrote Python

Things to note

Instead of one loop variable, we had two.

When we used multiple values in string interpolation, we used a tuple.

The loop didn't print the values in the same order we put them in.

Functions

Functions break up programs into logical pieces

Very much like procedures in Chapel

A stepping stone to objects.

An example:

def adder(val1, val2):
        return val1 + val2

Functions have names (adder), take arguments (val1, val2) and can return a result.

Arguments

We use the previous function like this:

>>> adder(2, 3)
5

The value 2 is put into val1, and 3 into val2. val1 and val2 are confined to the adder function:

>>>> val1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'val1' is not defined

This is called scope.

Null arguments

You can also have functions without any arguments:

def print_hello():
        print "Hello"

2 things here:

• The parentheses are simply empty.
• Notice we didn't return anything; in Python we don't have to.

Interlude - Using Python Files

You can use gedit to edit Python files, saving them with the extension .py.

Then, you can run the files with this command:

$ python my_file.py

Control Flow

Python uses if statements that look similar to Chapel's, but without the braces.

Basic structure:

if something:
        transform(1,2)
elif something_else:
        transform(2,3)
else:
        transform(3,4)

Check if a value is in a collection

Using a conditional with the in keyword to see if a particular value is contained in a collection:

>>> if "Yes" in ["Yes", "No"]:
...     print "Yep, it's there."
...
Yep, it's there.
>>> if "Joe" not in ["Sam", "Frank"]:
...     print "Joe's not there."
...
Joe's not there.

Exercise 3

Create a function that returns "weekday" if a day's name is a weekday, "weekend" if it's not.

Hints:

• Just worry about lower case values

• Getting user input:

  >>>> day = raw_input("Input a day's name >> ")
  Input a day's name >> Monday
  >>> day
  'Monday'
  

Bonus: If the word given isn't a valid calendar day, return "neither"

Solution 3

Object Oriented Programming

• Object-oriented programming tries to model program strutures after things in the real world.

• Objects have:

  • properties that describe them (the bike is red)
  • methods that make them do things (the man runs).

• Together, these are called members
• Benefit: information is contained only in the area it's necessary ("information hiding")

• Objects are:

  • defined by writing a class
  • created by instantiating a class

Classes

Think of classes like a template for objects; it describes how they will work

But, the objects contain specific information

Example

• The Car Class describes cars:

  • They have four wheels, color(s), number of seats, make, model, VIN, can accelerate, deccelerate, turn, etc.

• The actual color, number of seats, VIN, make, model corresponds to the specific car.
• The verbs (the things they can do) are shared amongst all of them.

Example

• The Dog Class describes dogs: They have a color, four legs, eye color, breed(s), can bark, run, etc.
• Fido is a black labrador
• Butch is a white bulldog

Making the connection

• Properties are variables - these can be any valid data type, including other objects!
• Methods are functions - these do things, and are common to all instances of the object
• Objects hold state in their properties, which is then changed by methods.

Defining a class

Example:

>>>> class Dog(object):
...     def __init__(self, name=None, breed=None):
...             self.name = name
...             self.breed = breed
...     def bark(self):
...             print '%s says, "Woof!"' % self.name
...     def bark(self, target):
...             print "%s barks at %s!" % (self.name, target.name)
...
>>> fido = Dog(name="Fido", breed="Black Lab")
>>> fido.bark()
Fido says, "Woof!"

Things of note

• All classes:

  • should be defined as class ClassName(object)

    • ClassName inherits from object.

  • define an initializer function, __init__

    • This is a "magic method" that Python uses.
    • Similar to C++/Java constructors - it sets up initial state

Things of note

• All methods take self as the first argument
• Different methods can have the same name, if they take different number of arguments.
• self is not passed in when you call the function; Python's interpreter uses it, not the programmer.
• Variables attached to self can be accessed in any other method in that class.
• I snuck in named arguments there; done for clarity.

More Theory: Inheritance

• Classes can inherit from others.

  • This means that the subclass shares properties and methods with it's superclass
  • Also referred to as child and parent classes

Examples:

• A parent class could be Vehicle, and subclasses would be Car, Motorcycle, Truck, Tractor, etc
• Another parent class could be Animal, subclasses being Cat, Dog, Giraffe, Elephant, etc

Hierarchies

You can also have hierarchies:

• Animal

  • Mammal

    • Dog
    • Cat

  • Bird

    • Eagle

Inheritance in Python

class ChildClass(ParentClass)

except for the parent class, which looks like

class ParentClass(object)

Why?

• object is a type, just like int or str. Helps Python allocate memory correctly.

Exercise 4

Create a Python class that represents a mathematical vector (http://en.wikipedia.org/wiki/Euclidean_vector)

Vectors are pairs of x, y coordinates. We want the following members:

• x coordinate
• y coordinate
• adding 2 vectors (x1 added to x2, y1 added to y2)
• subtracting 2 vectors (x1 minus x2, y1 minus x2)
• a __repr__(self) method that returns the coordinates as the string "(x, y)"

Exercise 4 Hints

Should look something like this:

v1 = Vector(1,3)
v2 = Vector(8,9)
v3 = v1.add(v2)
print v3
(9, 12)

Bonus: Write a method that multiplies both x and y by a single number (scalar multiplication)

Solution 4

Modules and Packages

• Python code is organized into modules and packages.
• Modules are individual Python files.
• Packages are directories that contain Python modules
• Modules and packages are used to distribute re-usable code.

Standard Library

• Python has a philosophy of "batteries included".

• LOTS of modules/packages bundled with Python.

• Examples:

  os
  zlib
  datetime
  hashlib
  threading
  socket
  unittest
  

• External packages, too, like SciPy, NumPy, PyCUDA, Django, SQLAlchemy, Pygments, etc

Using packages and modules

2 forms:

• import os
• from os import path

Getting help with packages/modules

• Use the dir function to inspect an object's members, packages and modules included
• Use the help function to get help for an object.

Example (output shortened):

>>>> import os
>>> dir(os)
[.... 'walk', 'write']
>>> help(os.walk)
Help on function walk in module os:

walk(top, topdown=True, onerror=None, followlinks=False)
    Directory tree generator.

Exercise 5

Using the vector class we created last time, add a length method to your vector class that does the following:

• Squares x and y (** is the exponential operator)
• Add the squares together
• Return the square root of that sum

Use the sqrt function from the math package.

Solution 5

Python 2 or 3?

For most things, Python 2 is probably the best choice.

• More mature libraries available
• Better tested

However, Python 3 is the future.

• Python 2.7 is the last release of the 2 line.
• Overall, a much better language.

Further Resources

• Python Homepage: http://python.org

• Learn Python the Hard Way: http://learnpythonthehardway.org/ (2nd edition released today!)

• Learning the Zen of Python - from the Python prompt, type::

  import this
  

• Also: The Pragmatic Programmer, by Andrew Hunt and David Thomas