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 ICONThink 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 j

True

>>> ‘a’ in p

True

>>> ‘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 Ends

class 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'