The Python Programming Language

Matt Campbell | Steve Losh

From the Creators…

“The language is named after the BBC show ``Monty Python's Flying Circus''

and has nothing to do with nasty reptiles. Making references to Monty Python skits in documentation is not

only allowed, it is encouraged! “

Origins• Created in the early 1990s by Guido van

Rossum at Stichting Mathematisch Centrum (CWI ) in the Netherlands

• Successor language to ABC• Rossum remains the principle author of

the language today

Overview of the Language• Python is an interpreted language• Like Scheme, it is an interactive

language• Very high-level data types• Code is very human readable

Extensibility• Python is a very extensible language• You can write modules in C to link

python to other binary libraries• You can even link the interpreter itself

into an application written in C and use python as an extension or command language for that application

Lexical Aspects• Input Format:

– Line oriented– White space is not ignored

• Comments:– Denoted by hash mark (#) to end of line

• Delimiters:– End of line

• Keywords:– Reserved

• Names:– Case sensitive– Variable names can consist of letters, numbers, and/or

underscores– Underscores sometimes have special meaning, so their use

is not highly recommended

Data Types• Scalars:– Integer, Float, Boolean

• Aggregate Types– Complex Number, String, List, Dictionary,

Tuple, File, Set• Python is not strongly typed• Python does not require declaration of

variables before their use

Literals• Integers: 2, 4, -3• Floats: 2.0e10, 3.5, .03• Boolean: True, False• Strings: ‘cat’, “cat”• Lists: [12, 3.4, ‘cat’, lambda x: x+3]• Sets: set([12, 3.4, ‘cat’, lambda x: x+3])• Dictionaries: dict = {‘cat': 2, 6: ‘dog’} • Functions: Can be mapped to names via ‘def’ and

‘lambda’ just as in Scheme. They can be returned by functions, placed in lists, etc.

• Files: open('/path/file', ‘r+') • Null: None• ‘_’: holds the most recently returned value

Variable Typing• Variables in Python do not need to be

declared as a specific type– Example:

• A, B = 3, ‘cat’

• A variable’s type is dynamic, and will changed whenever it is reassigned– Example:

• a, b = 1, ‘cat’• a, b = .3, lambda x: x*x

• No such thing as “const” in Python

Quick & Dirty Input

>>> x = int(raw_input("Please enter an integer: "))

Slicing• Aggregate slicing syntax is similar to ICON

Think of indices as pointing between elements in a list.[ ‘cat’, ‘dog’, 3, 4.5 ] 0 1 2 3 4

>>> animals = [‘cat’, ‘dog’, ‘mouse’, ‘bird’]>>> print animals[0:1][‘cat’, ‘dog’]>> print animals[1:][‘dog’, ‘mouse’, ‘bird’]

>>> tmp = list(“Shrubbery”)>>> tmp[:1] = tmp[-7:]>>> tmp[‘r’, ’u’, ’b’, ’b’, ’e’, ’r’, ’y’, ’S’, ’h’, ’r’, ’u’, ’b’, ’b’, ’e’, ’r’, ’y’]

Ranges• Python has a range function to easily

form lists of integers.

>>> range(5) [0, 1, 2, 3, 4]>>> range(2,5) [2, 3, 4]>>> range(0, 10, 2) [0, 2, 4, 6, 8]>>> range(5, 0, -1) [5, 4, 3, 2, 1]

in• The in keyword checks if the given object is

contained within the aggregate.

>>> p = “cat”>>> j = [‘cat’, ‘dog’]>>> p in jTrue>>> ‘a’ in pTrue>>> ‘t’ in p[:2]False

Subroutines• Python supports both procedures and

functions– Procedure:• def proc1():

print ‘Hi!’– Function:• def func1():

return ‘Hi!’

Subroutines (continued)• Python does not support name

mangling as in C++• Anything can be returned from a

function, including None and other functions

• Recursion is allowed• Python has support for calling

subroutines in modules written in C• Parameters are passed by value

Scope• Lexical• Global/local scope• Similar to Scheme• No names need to be declared before

use

Lifetime / Actions• Variables are alive as long as they can

be referenced, similar to Scheme• Python supports standard arithmetic

precedence and association with ()’s• Result type is defined the more

descriptive of the operands

Control Structures• if statements work as expected

>>> if x < 0:… print ‘Negative’… elif x == 0:… print ‘Zero’… else:… print “Positive”…

Control Structures continued• for loops differ from c++ and/or java.

They iterate over an aggregate.

>>> animals = [‘cat’, ‘dog’, ‘horse’]>>> for x in animals:… print x…

Control Structures Continued• for loops can iterate over multiple lists at the

same time

>>> questions = ['name', 'quest', 'favorite color']>>> answers = ['lancelot', 'the holy grail', 'blue']>>> for q, a in zip (questions, answers):... print 'What is your %s? It is %s.' % (q, a)...What is your name? It is lancelot.What is your quest? It is the holy grail.What is your favorite color? It is blue.

Pass• The pass command does nothing.

Functions>>> def fib(n): ... a, b = 0, 1... while b < n:... print b,... a, b = b, a+b...

Functions continued>>> def makeIncFunc ( n = 1 )… return lambda x: x + n…>>> tmp = makeIncFunc()>>> print tmp(3)4>>> tmp = makeIncFunc(2)>>> print tmp(3)5

Default Value Side Effects

>>> def f(a, L=[]):… L.append(a)… return L…>>> print f(1)[1]>>> print f(2)[1, 2]>>> print f(3)[1, 2, 3]

Classes• Python implements classes in a similar

way to Java and C++

>>> class Complex:... def __init__(self, realpart, imagpart):... self.r = realpart... self.i = imagpart...>>> x = Complex(3.0, -4.5)>>> x.r, x.i(3.0, -4.5)

Inheritance• “Of course, a language feature would

not be worthy of the name ``class'' without supporting inheritance. “

class DerivedClassName(BaseClassName):<statement-1> . . .<statement-N>

Multiple Inheritance!

class DerivedClassName(Base1, Base2, Base3):<statement-1>. . .<statement-N>

Odds and Endsclass Employee:

pass

john = Employee() john.name = 'John Doe‘john.dept = 'computer lab‘john.salary = 1000

Pickling• Python’s equivalent to Serialization

>>> pickle.dump( anyobject, fileopenedforwriting )

>>> objecttoloadto = pickle.load( fileopenedforreading )

What this has to do with Legos

• A python library calls Pylnp which allows remote control of your robot through the IR tower

import lnp lnp.iwrite('hello')lnp.iread() 'world'