Python quickstart for programmers: Python Kung Fu

Python Kung Fu

The Zen of Python

Beautiful is better than ugly.

Explicit is better than implicit.

Simple is better than complex.

Complex is better than complicated.

Flat is better than nested.

Sparse is better than dense.

Readability counts.

Special cases aren't special enough to break the rules.

The Zen of Python cont.

Although practicality beats purity.

Errors should never pass silently.

Unless explicitly silenced.

In the face of ambiguity, refuse the temptation to guess.

There should be one-- and preferably only one --obvious way to do it.

Although that way may not be obvious at first unless you're Dutch.

Now is better than never.

The Zen of Python cont.

Although never is often better than *right* now.

If the implementation is hard to explain, it's a bad idea.

If the implementation is easy to explain, it may be a good idea.

Namespaces are one honking great idea -- let's do more of those!

Uses

Prototyping

Glue language

Web development

System administration

Desktop applications

Games

Everything! Ok not everything but almost

Hello World

print(''Hello world'')

Python 3

print ''Hello World''

Python 2.6

Notice the missing ;

Comments

#This is a comment

types

Numeric

Long or int e.g. 1

Float e.g. 2.5

Complex e.g. 3 + 4j

Boolean

True

False

or

and

Types cont.

Strings

'I am a string'

“Im also a string“

Sequences

Lists (or array) [0,1,3] [2,5,“woo hoo“]

Tuples (1,2,3)

Types cont.

Collections

Dictionary (or hash or map)

{123:“mi casa“, 234:“casa de pepito}

Set

set('hola')

Additional types

unicode, buffer, xrange, frozenset, file

Assingments

id = 10

Watch out for aliasing!

Operators

Cool new stuff

**

//

Sorry no ++ or –-

The rest is about the same

Including this

+= -= *= /= %= **= //=

BIFs

y = abs( x )

z = complex( r, i )

modules

from math import *

from math import sqrt

Objects and Methods

Construction

today = date( 2006, 8, 25 )

Notice the missing 'new'

Methods

which_day = today.weekday()

Cooler printing

print "The sum of %d and %d is %f " % (a, b, sum)

print ''The sum of '' + a + '' and '' + b '' is '' + sum

Selection statements

if value < 0:

print 'Im negative'

elif value == 0:

print 'Im zero'

else

print 'Im positive'

Comparison

Your usual stuff

< > <= >= == !=

Comparison cont.

str1 = "Abc Def"

str2 = "Abc def"

if str1 == str2 :

print "Equal!!"

else :

print "Not Equal!!"

Comparison cont.

name1 = "Smith, John";

name2 = "Smith, Jane";

if name1 < name2 :

print "Name1 comes first."

elif name1 > name2 :

print "Name2 comes first."

else :

print "The names are the same."

Stuff that rules

if "Smith" in name1 :

print name1

Null reference

# The is operator along with None can tests for null references.

result = name1 is None

result2 = name1 == None

Watch out, ther is no Null!

Repetition Statements

theSum = 0

i = 1

while i <= 100:

theSum = theSum + i

i = i + 1

print "The sum = ", theSum

Repetition Statements cont.

for <loop-var> in <object> :

<statement-block>

for i in xrange( 1, 11 ) :

print i

Cool tricks

for i in xrange( 10, 0, -1 ) :

print i

Iterating from zero

for i in xrange( 20 )

print i

Functions

def sumRange( first, last ) :

total = 0

i = first

while i <= last :

total = total + i

i = i + 1

return total

Default values

def sumRange2( first, last, step = 1 ) :

total = 0

i = first

while i <= last :

total = total + i

i = i + step

return total

Clear readable code!

theSum = sumRange2( last = 100, step = 3, first = 1 )

Modules and namespaces

List basics

Creating

gradeList = [ 85, 90, 87, 65, 91 ]

To demonstrate element access in Python, consider the following statements

print "Forward:", gradeList[ 0 ],gradeList[ 3 ]

print "Reverse:", gradeList[ -1 ],gradeList[ -3 ]

which produces the following output

Forward: 85 65

Reverse: 91 87

List length

theLength = len( gradeList )

print "The list contains %d elements." % theLength

Java sucks

// Java array printing.

System.out.print( "[" );

for( int i = 0; i < gradeList.length; i++ ) {

System.out.print( gradeList[ i ] );

if( i < gradeList.length-1 )

System.out.print( ", " );

}

System.out.println( "]" )

Python rules!

print gradeList

Python lists rule

Don't worry about list lengths. The interpreter will take care of that for you.

I case you want to simulate a classic array you can do this.

histogram = [ 0 ] * 10

List iteration

total = 0

for value in valueList :

total = total + value

avg = float( total ) / len( valueList )

print "The average value = %6.2f" % avg

Tuples, tuples. Immutable = safety

t = ( 0, 2, 4 ) # 3 element tuple

a = ( 2, ) # 1 element tuple

b = ( 'abc', 1, 4.5, 5 ) # 4 element mixed tuple

c = ( 0, ( 'a', 'b' ) ) # nested tuple

List ninja skills

import random

# Create the empty list.

valueList = []

# Build the list of random values.

for i in range( 1000 ) :

valueList.append( random.random() )

hyadoken!

listA = [ 0, 1, 2, 3 ]

listB = listA.extend( [ 4, 5, 6 ] )

print listB

Atreketroken

values = [ 0, 1, 2, 3, 4 ]

values.insert( 1, 8 )

print values

More classic stuff

list.insert( atIndex, value )

Removing items

x = theList.pop( 1 )

print "list =", theList

print "x =", x

More removal

theList = [ 10, 11, 12, 13 ]

theList.remove( 11 )

print theList # prints [10, 12, 13]

Searching

theList = [ 10, 11, 12, 13 ]

pos = theList.index( 13 )

print pos

# prints 3

This isn't funny anymore

// Java item search.

int i = 0;

while( i < gradeList.length ) {

if( gradeList[ i ] == 100 )

System.out.println( "Someone received at least one "

+ " perfect score." );

i++;

}

Haha

if 100 in gradeList :

print "Student received at least one perfect score."

Lost your index?

print gradeList.index( 90 )

Min max

print "The minimum value = ", min( valueList )

print "The maximim value = ", max( valueList )

List concatenation

listA = [ 1, 2, 3 ]

listB = [ 8, 9 ]

which can be concatenated to produce a third list. If we execute the statements

bigList = listA + listB

print bigList

List duplication

listX = [ 1, 2, 3 ]

listY = listX + []

listY[0] = 0

print "X:", listX

print "y:", listY

Will now produce the following since listY now references a new list which happens to be a duplicate copy of listX.

Reverse

theList = [ 10, 11, 12, 13 ]

theList.reverse()

print theList

# prints [13, 12, 11, 10]

Replication

intList = [ 0, 1, 2, 3, 4 ] * 25

The coolest shit ever

The following are common ways to describe lists (or sets, or tuples, or vectors) in mathematics.

S = {x² : x in {0 ... 9}}

V = (1, 2, 4, 8, ..., 2¹²)

M = {x | x in S and x even}

Wuahahahha!

>>> S = [x**2 for x in range(10)]

>>> V = [2**i for i in range(13)]

>>> M = [x for x in S if x % 2 == 0]

OO programming

# point.py

# Defines a class to represent two-dimensional discrete points.

class Point :

def __init__( self, x = 0, y = 0 ) :

self.xCoord = x

self.yCoord = y

cont.

def __str__( self ) :

return "(" + str( self.yCoord ) + ", " +

str( self.yCoord ) + ")"

def getX( self ) :

return self.XCoord

cont.

def getY( self ) :

return self.yCoord

def shift( self, xInc, yInc ) :

self.xCoord += xInc

self.yCoord += yInc

Object instantiation

from point import *

pointA = Point( 5, 7 )

pointB = Point()

Private members and methods

Private

def __helpermethod

def __privatefield

Inheritance

class DerivedClassName(BaseClassName):

<statement-1>

.

.

.

<statement-N>

Stuff you maybe haven't tried

Multiple Inheritance

Operator overloading

Extra stuff you should know

There are no 'interfaces' but you can emulate them.

There is no 'abstract' or 'virtual' keyword but you can emulate this behaviour.

Abstract base classes serve as THE alternative to interfaces. Python ABC's are somewhat similar to C++ ABC's

Duck typing

If it walks like a duck and quacks like a duck, I would call it a duck.

function calculate(a, b, c) => return (a+b)*c

example1 = calculate (1, 2, 3)

example2 = calculate ([1, 2, 3], [4, 5, 6], 2)

example3 = calculate ('apples ', 'and oranges, ', 3)

print to_string example1

Ta tan!

9

[1, 2, 3, 4, 5, 6, 1, 2, 3, 4, 5, 6]

apples and oranges, apples and oranges, apples and oranges,

Conclusion

Thus, duck typing allows polymorphism without inheritance. The only requirement that function calculate needs in its variables is having the "+" and the "*" methods

Exceptions

try:

myList = [ 12, 50, 5, 17 ]

print myList[ 4 ]

except IndexError:

print "Error: Index out of range."

Raising exceptions

def min( value1, value2 ) :

if value1 == None or value2 == None :

raise TypeError

if value1 < value2 :

return value1

else:

return value2

Black belt python

Class factories

Function factories

Functional programming

Generators

Advanced list idioms

Tons of other tricks

Python Implementations

CPython 2.6 and 3.0

CPython with Psyco

Jython

IronPython

PyPy

Stackless Python

and more...

Python quickstart for programmers: Python Kung Fu

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