Introduction to Python Introduction to Python 1 Materials based on - - PowerPoint PPT Presentation

1

Introduction to Python Introduction to Python

Materials based on contents from the course Programming with Python by Chad Haynes

2

Outline Outline

• Overview
• Built-in objects
• Functions and scopes
• Object-oriented programming
• Functional programming
• Exercise

3

Python At First Glance Python At First Glance

import math def showArea(shape): print "Area = %d" % shape.area() def widthOfSquare(area): return math.sqrt(area) class Rectangle(object): def __init__(self, width, height): self.width = width self.height = height def area(self): return self.width * self.height ###### Main Program ###### r = Rectangle(10, 20) showArea(r)

Function definition Class definition Import a library module Calling a function Comment Object instantiation

4

Why use Python? Why use Python?

• Simple, clean syntax
• Portable
• Flexible
• Large standard library
• Short development time
• Lots of 3rd-party tools/add-ons
• Many good implementations

– CPython, PyPy, IronPython, Jython

• Strong support from open-source community

5

Similarities to Java Similarities to Java

• Everything inherits from "object"

– Also numbers, functions, classes, … – Everything is first-class

• Vast, powerful standard library
• Garbage collection
• Introspection, serialization, threads, net,…

6

Similarities to C++ Similarities to C++

• Multi-paradigm

– OOP, procedural, generic, functional (a little)

• Multiple inheritance
• Operator overloading

7

Python vs. Java/C++/C Python vs. Java/C++/C

• Typing: strong, but dynamic

– Names have no type – Objects have types

• No declarations
• Sparse syntax

– No { } for blocks, just indentation – No ( ) for if/while conditions

• Interactive interpreter
• # for comments

if (x < 10) { x = x + tmp; y = y * x; } System.out.println(y);

Java

if x < 10: x = x + tmp y = y * x print y

Python

8

Getting Started Getting Started

• Python already included in most Linux

distributions

• Windows users can download from:

– http://python.org/download – Add python to PATH to run scripts from command line

9

using System; class Hello { static void Main() { Console.WriteLine("Hello, World"); } }

Hello, World! Hello, World!

• C#
• Python

print "Hello, World!"

10

Variables Variables

>>> x = 23 >>> print x 23 >>> x = 'foo' >>> print x foo >>> del x >>> print x Traceback (most recent call last): File "<stdin>", line 1, in <module> NameError: name 'x' is not defined >>> name x means 23 now it means 'foo' x becomes undefined

11

Var1 Var1_copy Var2

Variables Variables

• Reference Model

– Variables refer to an object – More than one variable can refer to the same

• bject

12

Numeric Types Numeric Types

• Integers

– Generally 32 signed bits

• Long Integers

– Unlimited size – Format: <number>L – Example: 4294967296L

• Float

– Platform dependant “double” precision

• Complex

– Format: <real>+<imag>j – Example: 6+3j

13

Strings Strings

• A sequence of characters enclosed with quotes
• 3 quoting styles

– 'Single Quotes' – "Double Quotes" – """Triple Quotes"""

• Examples

>>> print 'This may contain a "' This may contain a " >>> print "A ' is allowed" A ' is allowed >>> print """Either " or ' are OK""" Either " or ' are OK

14

>>> info = raw_input('-> ')

• > Here is info

>>> print info Here is info

Built Built-

• in Function:

in Function: raw_input raw_input

• Syntax: raw_input([prompt])

– Use prompt to ask user to input a string – Example

15

• Arithmetic
• + -

* // / ** % abs

• Example

>>> 5 + 3 # Addition 8 >>> 2 ** 8 # Exponentiation 256 >>> 13 / 4 # Integer (Truncating) Division* 3 >>> float(13) / 4 # Float Division 3.25 >>> 13 % 4 # Remainder 1 >>> abs(-3.5) # Absolute Value 3.5

• * Becomes float division in version 3.x

16

• Comparison
• < <= > >= == != <>
• Results in 1 (true) or 0 (false)
• Example

>>> 4 > 1.5 1 >>> 'this' != 'that' 1 >>> 4+3j == 4-2j >>> '5' == 5 >>> 0 < 10 < 20 1

17

• Boolean
• and or not
• Based on Boolean Algebra
• i1

i2 i1 and i2 1 1 1 1 1 i1 or i2 1 1 1 i1 not i1 1 1

18

• Boolean
• Example

>>> 1 == 1 and 2 >= 3 >>> 1 == 1 or 2 >= 3 1 >>> not 5.3 != 2.2 # same as: not (5.3 != 2.2) >>> 2 and '23' > '11' or 0 1

19

• Concatenation (+)
• Syntax: string1 + string2
• Example:

>>> 'Rockefeller' + 'University' 'RockefellerUniversity'

• Repetition (*)
• Syntax: string * number
• Example:

>>> 'dog' * 5 'dogdogdogdogdog'

20

• C-Style formatting (extended printf)
• Examples:

>>> "%i %s in the basket" % (2, "eggs") '2 eggs in the basket' >>> "%f to 2 decimal places: %.2f" %(2.0/9.0, 2.0/9.0) '0.222222 to 2 decimal places: 0.22' >>> length = 5 >>> obj = "fence" >>> "Length of the %(obj)s is %(length)i" % vars() 'Length of the fence is 5'

21

• Syntax: type(object)
• Used to determine the type of an object
• Example

>>> type(2.45) <type 'float'> >>> type('x') <type 'str'> >>> type(2**34) <type 'long'> >>> type(3+2j) <type 'complex'>

22

• Use built-in functions to convert between types
• str() int() float() long() complex() bool()
• Example

>>> str(42.3) '42.3' >>> float('-1.32e-3')

• 0.00132

>>> int('0243') 243 >>> int(2**34) Traceback (most recent call last): File "<pyshell#12>", line 1, in ? int(2**34) OverflowError: long int too large to convert to int >>> long(2**34) 17179869184L

23

• Lists
• Tuples
• Dicts

24

• Construction
• Syntax: [elem1, elem2, …]
• Heterogeneous, ordered sequence
• Mutable
• Example:

>>> list1 = [1, 'hello', 4+2j, 123.12] >>> list1 [1, 'hello', (4+2j), 123.12] >>> list1[0] = 'a' >>> list1 ['a', 'hello', (4+2j), 123.12]

25

• Concatenation (+)
• Syntax: list1 + list2
• Example:

>>> [1, 'a', 'b'] + [3, 4, 5] [1, 'a', 'b', 3, 4, 5]

• Repetition (*)
• Syntax: list * number
• Example:

>>> [23, 'x'] * 4 [23, 'x', 23, 'x', 23, 'x', 23, 'x']

26

• Indexing operator: [ ]
• Positive indices count from the left
• Negative indices count from the right
• 1

2 3 4 5 6

• 7
• 6
• 5
• 4
• 3
• 2
• 1

a b c d e f g sequence[0] == a sequence[-7] == a sequence[6] == g sequence[-1] == g sequence[2] == c sequence[-5] == c

27

• Two indices separated by a colon
• Available for both strings and lists
• Example

>>> sequence = [0, 1, 2, 3, 4, 5, 6, 7] >>> sequence[1:4] [1, 2, 3] >>> sequence[2:-1] [2, 3, 4, 5, 6]

• Missing Index implies end point

>>> sequence[:2] [0, 1] >>> sequence[3:] [3, 4, 5, 6, 7]

28

• Immutable version of list
• Syntax: (elem1, elem2, …)
• Items in tuple can not be altered
• Example:

>>> tuple1 = (1, 5, 10) >>> tuple1[2] = 2 Traceback (most recent call last): File "<pyshell#136>", line 1, in ? tuple1[2] = 2 TypeError: object doesn't support item assignment

29

• Syntax: len(object)
• Return the length of object
• Example

>>> list1 = [1, 2, 3, 4, 5] >>> len(list1) 5 >>> string1 = "length of a string" >>> len(string1) 18

30

• Mapping
• Associate a key with a value
• Each key must be unique
• 'z'

'ab' 2.1 3 keys 10 [2] (3,8) 'hello' values

31

• Construction
• Syntax: {key1: value1, key2: value2 …}
• Unordered map
• Example:

>>> dict1 = {'a': 1, 'b': 2} >>> dict1 {'a': 1, 'b': 2} >>> dict1['a'] 1 >>> dict1['b'] 2

32

Control Flow Control Flow

if condition: body elif condition: body else: body

Examples

if x%2 == 0: y = y + x else: y = y - x while i < 0: count = count + 1 for x in [1,2,3]: sum = sum + x while condition: body for name in iterable: body

33

• Syntax: range([start,] stop[, step])
• Generate a list of numbers from start to stop stepping every step
• start defaults to 0, step defaults to 1
• Example

>>> range(5) [0, 1, 2, 3, 4] >>> range(1, 9) [1, 2, 3, 4, 5, 6, 7, 8] >>> range(2, 20, 5) [2, 7, 12, 17]

34

• Using range with for
• Generate list used by for with range
• Example

>>> for i in range(4): print i 1 2 3

35

• The continue statement
• Continue to next iteration of loop, skipping remainder of body
• Example:

>>> for x in range(8): if x%2 == 0: continue print x

• 1

3 5 7

36

• The break statement
• Break out of inner most loop
• Example:

>>> for number in range(10): if number == 4: print 'Breaking' break else: print number 1 2 3 Breaking

37

• Data structures also have methods
• Use built-in function dir to list all available methods
• Example

>>> lst = [1, 3, 2] >>> dir(lst) ['__add__', '__class__', '__contains__', '__delattr__', '__delitem__', '__delslice__', '__doc__', '__eq__', '__ge__', '__getattribute__', '__getitem__', '__getslice__', '__gt__', '__hash__', '__iadd__', '__imul__', '__init__', '__le__', '__len__', '__lt__', '__mul__', '__ne__', '__new__', '__reduce__', '__repr__', '__rmul__', '__setattr__', '__setitem__', '__setslice__', '__str__', 'append', 'count', 'extend', 'index', 'insert', 'pop', 'remove', 'reverse', 'sort']

38

• split
• Syntax: string.split([sep])
• Returns a list of strings
• Example:

>>> text = "1 2 4 5 1" >>> text.split() ['1', '2', '4', '5', '1'] >>> test = "a, b, c, d, e" >>> test.split(',') ['a', ' b', ' c', ' d', ' e']

• !"

!"

39

• strip
• Syntax: string.strip()
• Remove leading and trailing white space (tabs, new lines, etc)
• Example:

>>> padded = " stuff " >>> padded.strip() 'stuff' >>> padded ' stuff ' >>> unpadded = padded.strip() >>> unpadded 'stuff'

• !"

!"

40

• append
• Syntax: list.append(element)
• Add element to end of list
• Example:

>>> list1 = [3, '10', 2] >>> list1.append('new') >>> list1 [3, '10', 2, 'new']

• !"

!"

41

• pop
• Syntax: list.pop([index])
• Remove and return item at position index from list
• Default is to remove last item
• Example:

>>> list1 = [3, '10', 2, 9, 11] >>> list1.pop() 11 >>> list1 [3, '10', 2, 9]

• !"

!"

42

• insert
• Syntax: list.insert(index, element)
• Insert element into list at position index
• Example:

>>> list2 = [0, 1, 2, 3, 4, 5] >>> list2.insert(3, 'new') >>> list2 [0, 1, 2, 'new', 3, 4, 5]

• !"

!"

43

• remove
• Syntax: list.remove(element)
• Removes the first occurrence of element in list
• Example:

>>> list2 = [0, 1, 3, 4, 3, 5] >>> list2.remove(3) >>> list2 [0, 1, 4, 3, 5]

• !"

!"

44

• sort
• Syntax: list.sort([cmpfunc])
• Sort list in place
• Example:

>>> list3 = [4, 12, 3, 9] >>> list3.sort() >>> list3 [3, 4, 9, 12]

• !"

!"

45

• reverse
• Syntax: list.reverse()
• Reverse elements of list in place
• Example:

>>> list3 = [4, 12, 3, 9] >>> list3.reverse() >>> list3 [9, 3, 12, 4]

• !"

!"

46

• keys
• Syntax: dict.keys()
• Return a list of all the keys in dict
• Example:

>>> dict1 = {1: 'a', 9: 'cat', 2: [2, 1]} >>> dict1.keys() [1, 2, 9]

• !"

!"

47

• has_key
• Syntax: dict.has_key(key)
• Determines if key is in dict
• Example:

>>> dict1 = {'x': 1, 'y': 2} >>> dict1.has_key('x') 1 >>> dict1.has_key('w')

• !"

!"

48

• values
• Syntax: dict.values()
• Returns a list of all values in dict
• Example:

>>> dict1 = {(1,2): 'a', (3,4): 'b', (9,8,7): 'c'} >>> dict1.values() ['a', 'c', 'b']

• !"

!"

49

Getting Help Getting Help

• For interactive use, calling help function will invoke the

built-in help system

• Call help() without argument for interactive mode

>>> help(str) Help on class str in module __builtin__: class str(basestring) | str(object) -> string | | Return a nice string representation of the object. | If the argument is a string, the return value is the same object. | | Method resolution order: | str | basestring | object | | Methods defined here: | | __add__(...) | x.__add__(y) <==> x+y | | __contains__(...) | x.__contains__(y) <==> y in x :

50

Functions and Scopes Functions and Scopes

51

• Syntax: def func(arg1, …):

body

• Body of function must be indented
• If no value is returned explicitly, function will return None
• Example:

>>> def average(num1, num2, num3): sum = num1 + num2 + num3 avg = sum / 3.0 return avg

# #

52

• Parameters
• Parameters can be any type
• A function can take any number of parameters (or none at all)
• Example:

>>> def usage(programName, version): print ‘%s Version %i' % (programName, version) print 'Usage: %s arg1 arg2‘ % (programName) >>> usage('Test', 1.0) Test Version 1.0 Usage: Test arg1 arg2

53

• Default Parameters
• One or more parameters can be given a default value
• The function can be called with fewer arguments than there are

parameters

• All non-default (required) parameters must precede default parameters
• Example:

>>> def printName(last, first, mi=""): print "%s, %s %s" % (last, first, mi) >>> printName("Smith", "John") Smith, John >>> printName("Smith", "John", "Q") Smith, John Q

54

• Calling functions
• Syntax: func(arg1, arg2, … argn)
• Order of arguments must match order of declared parameters
• No type checking is done
• Example

>>> def display(arg1, arg2, arg3): print arg1 print arg2 print arg3 >>> display(1, 'x', 4.3) 1 x 4.3

55

• Keyword arguments
• Functions can be called using the keyword of the argument
• Syntax: func(keyword=value, …)
• The order of the values passed by keyword does not matter
• Example

def keywords(key1="X", key2="X", key3="X",key4="X"): print key1, key2, key3, key4 >>> keywords(key3="O", key2="O") X O O X >>> keywords() X X X X

56

• Functions as variables
• Functions can be assigned
• Example

def sub(a, b): return a-b >>> op = sub >>> print op(3, 5)

• 2

>>> type(op) <type 'function'>

57

• Functions as parameters
• Functions can be passed to other functions
• Example

def convert(data, convertFunc): for i in range(len(data)): data[i] = convertFunc(data[i]) return data >>> convert(['1', '5', '10', '53'], int) [1, 5, 10, 53] >>> convert(['1', '5', '10', '53'], float) [1.0, 5.0, 10.0, 53.0] >>> convert(['1', '5', '10', '53'], complex) [(1+0j), (5+0j), (10+0j), (53+0j)]

58

• Returning multiple values
• Return a tuple containing the values to return
• Example

def separate(text, size=3): start = text[:size] end = text[-size:] return (start, end) >>> separate('sample text') ('sam', 'ext') >>> start, end = separate('sample text') >>> print start sam >>> print end ext

59

Generators Generators

• Generators are functions that generate

sequence of items

– Generated sequence can be infinite

def fibonacci(): i = j = 1 while True: r, i, j = i, j, i+j yield r for rabbits in fibbonacci(): print rabbits if rabbits > 100: break 1 1 2 3 5 8 13 21 34 55 89 144

60

Namespaces and Scopes Namespaces and Scopes

• Namespace

– A mapping from names to objects – (Currently) implemented as Python dictionaries

• Scope

– A region of program where a namespace is directly accessible – Name references search at most 3 scopes: local, global, built-in – Assignments create or change local names by default – Can force arguments to be global with global command

61

Scope Example Scope Example

x = 99 def func(Y): Z = X+Y # X is not assigned, so it's global return Z func(1)

62

• A file containing Python definitions and statements
• Modules can be “imported”
• Module file name must end in .py
• Used to divide code between files

! !

import math import string … math.py string.py

63

• Syntax 1: import <module name>
• Module name is the file name without the .py extension
• You must use the module name to call the functions
• Example

>>> import math >>> dir(math) ['__doc__', '__name__', 'acos', 'asin', 'atan', 'atan2', 'ceil', 'cos', 'cosh', 'e', 'exp', 'fabs', 'floor', 'fmod', 'frexp', 'hypot', 'ldexp', 'log', 'log10', 'modf', 'pi', 'pow', 'sin', 'sinh', 'sqrt', 'tan', 'tanh'] >>> print math.e 2.71828182846 >>> print math.sqrt(2.3) 1.51657508881

import import

64

• Syntax 2: from <module> import <name>
• Import only a specific name from a module into global namespace
• Module name is not required to access imported name
• Example

>>> from math import sqrt >>> sqrt(16) 4 >>> dir(math) Traceback (most recent call last): File "<stdin>", line 1, in <module> NameError: name 'math' is not defined

import import

65

• Syntax 2a: from <module> import *
• Import everything from a module into global namespace
• Example

>>> dir() ['__builtins__', '__doc__', '__name__'] >>> from time import * >>> dir() ['__builtins__', '__doc__', '__name__', 'accept2dyear', 'altzone', 'asctime', 'clock', 'ctime', 'daylight', 'gmtime', 'localtime', 'mktime', 'sleep', 'strftime', 'struct_time', 'time', 'timezone', 'tzname'] >>> time() 1054004638.75

import import

66

Python Standard Libraries Python Standard Libraries

• Some examples

sys - System-specific parameters and functions time - Time access and conversions thread - Multiple threads of control re - Regular expression operations email - Email and MIME handling package httplib - HTTP protocol client Tkinter - GUI package based on Tcl/Tk

• See http://docs.python.org/library/index.html

67

• Opening a file
• Syntax: open(fileName, mode)
• Mode is one of:
• 'r' : Read
• 'w' : Write
• 'a' : Append
• If a file opened for writing does not exist it will be created
• Example

>>> inFile = open('input.txt', 'r') >>> type(infile) <type 'file'>

$% $%

68

• read([size])
• Read at most size bytes and return text as a string
• readlines([size])
• Read the lines of the file into a list of strings.
• Use size as an approximate bound on the number of bytes returned

$% $% !" !"

69

• write(text)
• Write text to the file
• writelines(string_sequence)
• Write each string in the sequence to the file
• New lines are not added to the end of the strings

$% $% !" !"

70

• Definition
• Errors detected during execution
• Basic Example
• Divide by zero

>>> 1 / 0 Traceback (most recent call last): File "<pyshell#0>", line 1, in ? 1 / 0 ZeroDivisionError: integer division or modulo by zero

& &

71

• Motivation
• Move error handling code away from main code block
• Deal with “exceptional” cases separately
• How it works
• Exceptions are thrown (or raised) and caught
• Control exits the current code block when the exception is thrown
• An exception can then be caught by a catching code block

& &

72

• Throwing
• Many common operations may throw an exception
• List index out of bounds
• Invalid type conversions
• Exceptions can be thrown manually using the raise keyword
• >>> raise ValueError, "Bad Value“
• Catching
• Thrown exceptions must be caught
• If the exception is never caught the progam will terminate

& &

73

• Handling Syntax

try: <try code block> except <Exception List>: <exception code block> except <Exception List>: <exception code block> except: <exception code block> else: <else code>

& &

74

• Example

try: x = 1 / 0 except ZeroDivisionError: print 'Divide by zero error' Divide by zero error

& &

75

• Example

try: x = 1 / 0 except IOError: print 'Input/Output error' except: print 'Unknown error' Unknown error

& &

76

• Types of Exceptions
• There is a hierarchy of exceptions
• All built-in exceptions are derived from Exception
• An exception will be caught by any type higher up in the hierarchy

& &

Exception StandardError StopIteration SystemExit ArithmeticError LookupError ValueError IndexError KeyError OverflowError ZeroDivisionError

77

• Example

try: x = 1 / 0 except Exception: print 'Exception caught' Exception caught

& &

78

• Propagation
• If no proper except block can be found in the current block, the

exception propagates back to the calling function

def func1(): try: a = 1 / 0 … except ValueError: print 'first' def func2(): try: func1() except: print 'second' >>> func2() second

& &

79

• Example

try: x = 1 / 0 except Exception: print 'Exception caught' Exception caught

& &

80

OO Programming OO Programming

81

Defining a Class Defining a Class

• Syntax:
• Creating a class with no superclass
• Basically, classes are simply namespaces

class MyClass(object): myvar = 30 >>> MyClass.myvar 30

class name[(base)]: body class name: body

Old-style class New-style class

class name(object): body

82

Class Example Class Example

• All instance methods must explicitly take an

instance as the first parameter

– self is a commonly used name

class Rectangle(object): def __init__(self, width, height): self.width = width self.height = height def area(self): return self.width * self.height >>> r = Rectangle(10, 20) >>> Rectangle.area(r) 200 >>> r.area() 200

Constructor

83

Inheritance Inheritance

• Subclass must invoke parent's constructor

explicitly

class Square(Rectangle): def __init__(self, width): Rectangle.__init__(self, width, width) >>> s = Square(100) >>> s.area() 10000

84

Polymorphism Polymorphism

• All methods are virtual

import math class Circle(object): def __init__(self, radius): self.radius = radius def area(self): return math.pi*self.radius*self.radius >>> shapes = [Square(5), Rectangle(2,8), Circle(3)] >>> for x in shapes: ... print x.area() ... 25 16 28.2743338823

85

Python Object Hooks Python Object Hooks

• Objects can support built-in operators by

implementing certain special methods

– The usual operators: +, -, *, /, **, &, ^, ~, != – Indexing (like sequences): obj[idx] – Calling (like functions): obj(args,...) – Iteration and containment tests

• for item in obj:...
• if item in obj:...

86

Functional Programming Functional Programming

87

• Taken from functional languages
• Lisp/Scheme
• Haskell
• Added to Python as built-in functions
• map()
• filter()
• reduce()
• zip()

'" '"

88

• Perform an operation on each element of a list
• A function is applied to each element
• The results of each function call are used to generate a new list
• The resulting list is always the same length as the original list
• The original list is not altered
• #

#map map

89

• #

#map map

func y1 y2 y3 y4 y5 y6 y7 y8 ŷ1

90

• #

#map map

func y1 y2 y3 y4 y5 y6 y7 y8 ŷ2 ŷ1

91

• #

#map map

func y1 y2 y3 y4 y5 y6 y7 y8 ŷ3 ŷ1 ŷ2

92

• #

#map map

func y1 y2 y3 y4 y5 y6 y7 y8 ŷ4 ŷ1 ŷ2 ŷ3

93

• #

#map map

func y1 y2 y3 y4 y5 y6 y7 y8 ŷ5 ŷ1 ŷ2 ŷ3 ŷ4

94

• #

#map map

func y1 y2 y3 y4 y5 y6 y7 y8 ŷ6 ŷ1 ŷ2 ŷ3 ŷ4 ŷ5

95

• #

#map map

func y1 y2 y3 y4 y5 y6 y7 y8 ŷ7 ŷ1 ŷ2 ŷ3 ŷ4 ŷ5 ŷ6

96

• #

#map map

func y1 y2 y3 y4 y5 y6 y7 y8 ŷ8 ŷ1 ŷ2 ŷ3 ŷ4 ŷ5 ŷ6 ŷ7

97

• Syntax 1: map(func, list)
• Example: Convert a list of integers to strings

>>> lst1 = [1, 2, 3, 4] >>> lst2 = map(str, lst1) >>> print lst2 ['1', '2', '3', '4']

• The function (str) takes one argument
• The result (lst2) is the same length as the original (lst1)
• #

#map map

98

• What if the function requires more than one

argument?

• Multiple lists can be passed to the map function
• Syntax 2: map(func, list1, …, listn)
• All lists must be of same length
• The function must take n parameters
• #

#map map

99

• Example: adding numbers

def add2(a, b): return a+b >>> lst1 = [0, 1, 2, 3] >>> lst2 = [4, 5, 6, 7] >>> print map(add2, lst1, lst2) [4, 6, 8, 10]

• #

#map map

100

• lst1 = [1, 2, 3, 4]

lst2 = [] for element in lst1: lst2.append(str(element)) lst1 = [1, 2, 3, 4] lst2 = map(str, lst1)

101

• lst1 = [0, 1, 2, 3]

lst2 = [4, 5, 6, 7] lst3 = [] for index in range(len(lst1)): lst3.append(add2(lst1[index], lst2[index])) lst1 = [0, 1, 2, 3] lst2 = [4, 5, 6, 7] lst2 = map(add2, lst1, lst2)

102

• The map function can be used like an expression
• Can be used as a parameter to a function
• Example:

>>> lst1 = [1, 2, 3, 4] >>> string.join(lst1) # Error because lst1 contains ints … TypeError: sequence item 0: expected string, int found >>> string.join( map(str, lst1) ) # Correct '1 2 3 4'

# #

103

• Remove elements of a list based on a condition
• Each element of a list is tested, those that fail are removed
• The resulting list is the same length or shorter than the original
• The original list is not altered
• #

#filter filter

104

• Syntax: filter(func, list)
• Example: Remove all negative numbers

def removeNegative(number): return number >= 0 >>> lst1 = [1, 2, -3, 4] >>> lst2 = filter(removeNegative, lst1) >>> print lst2 [1, 2, 4]

• The function (str) takes one argument and returns 0 or 1
• The result (lst2) is shorter than the original (lst1)
• #

#filter filter

105

• lst1 = [1, 2, -3, 4]

lst2 = [] for element in lst1: if removeNegative(element): lst2.append(element) lst1 = [1, 2, -3, 4] lst2 = filter(removeNegative, lst1)

106

• Apply a function cumulatively to a sequence
• The function must take 2 parameters
• Function is applied to parameters from left to right
• The list is reduced to a single value
• The original list is not altered
• #

#reduce reduce

107

• #

#reduce reduce

func y1 y2 y3 y4 t1 func t2 func t3

108

&($' &($'

1+10 1 10 20 30 11 11+20 31 31+30 61

109

• Syntax 1: reduce(func, list)
• Example: Find the sum of a list of integers

>>> lst1 = [1, 2, 3, 4] >>> sum = reduce(operator.add, lst1) >>> print sum 10

• The function (operator.add) takes two arguments
• The result is a single value
• #

#reduce reduce

110

• Syntax 2: reduce(func, list, initialValue)
• The initialValue is applied to func with the first value in the list
• If the list is empty, the initialValue is returned
• Example: Concatenating lists

>>> lst1 = [ [2, 4], [5, 9], [1, 7] ] >>> result = reduce(operator.add, lst1, [100]) >>> print result [100, 2, 4, 5, 9, 1, 7]

• #

#reduce reduce

111

• lst1 = [1, 2, 3, 4]

sum = operator.add(lst1[0], lst1[1]) for element in lst1[2:]: sum = operator.add(sum, element) lst1 = [1, 2, 3, 4] sum = reduce(operator.add, lst1)

112

• lst1 = [ [2, 4], [5, 9], [1, 7] ]

result = operator.add([100], lst1[0]) for element in lst1[1:]: result = operator.add(sum, element) lst1 = [ [2, 4], [5, 9], [1, 7] ] result = reduce(operator.add, lst1, [100])

113

• Combine multiple lists into one
• Elements are paired by index
• The resulting list is the same length as the shortest list supplied
• Each element of resulting list contains a tuple
• The original lists are not altered
• #

#zip zip

114

• #

#zip zip

y1 y2 y3 y4 x1 x2 x3 x4 (x1, y1) (x2, y2) (x4, y4) (x3, y3)

115

• Syntax: zip(list1, …, listn)
• Example: Combine two lists

>>> lst1 = [1, 2, 3, 4] >>> lst2 = ['a', 'b', 'c', 'd', 'e'] >>> result = zip(lst1, lst2) >>> print result [(1, 'a'), (2, 'b'), (3, 'c'), (4, 'd')]

• The ‘e’ element was truncated since lst1 only has 4 elements
• The result is a list of tuples
• #

#zip zip

116

• lst1 = [1, 2, 3, 4]

lst2 = ['a', 'b', 'c', 'd', 'e'] lst3 = [] for index in range(min(len(lst1), len(lst2)): lst3.append( (lst1[index], lst2[index]) ) lst1 = [1, 2, 3, 4] lst2 = ['a', 'b', 'c', 'd', 'e'] lst3 = zip(lst1, lst2)

117

• Iterate over two lists simultaneously

>>> produce = ['apples', 'oranges', 'pears'] >>> prices = [0.50, 0.45, 0.55] >>> for fruit, cost in zip(produce, prices): print '%s cost $%.2f'%(fruit, cost) apples cost $0.50

• ranges cost $0.45

pears cost $0.55

# #zip zip

118

• Create a dictionary using dict()

>>> produce = ['apples', 'oranges', 'pears'] >>> prices = [0.50, 0.45, 0.55] >>> priceDict = dict(zip(produce, prices)) >>> print priceDict {'pears': 0.55, 'apples': 0.5, 'oranges': 0.45}

# #zip zip

119

• Generate new lists from old ones
• Can simultaneously map and filter
• More flexible than the built-in functions

" "

120

• Basic Syntax: [<exp> for <var> in <list>]
• The resulting list is the result of exp evaluated for each var in list
• Example: Increase each element by 1

>>> [x+1 for x in range(5)] [1, 2, 3, 4, 5]

• Example: Convert each element to a string

>>> [str(x) for x in range(5)] ['0', '1', '2', '3', '4']

" "

121

• Syntax: [<ex1> for <var> in <list> if <ex2>]
• ex1 is only evaluated if ex2 is true
• Example: Remove smallest element

>>> lst1 = [5, 10, 3, 9] >>> [x for x in lst1 if x != min(lst1)] [5, 10, 9]

• Example: Sum all lists of size greater than 2

>>> lst1 = [[1, 2, 4], [3, 1], [5, 9, 10, 11]] >>> [reduce(operator.add, x) for x in lst1 if len(x) > 2] [7, 35]

!" !"

122

• Multiple for loops can be included
• The loops will be nested
• Example: Generate all possible combinations of letters a, c, g, t

>>> nucleo = ['a', 'g', 'c', 't'] >>> [a+b for a in nucleo for b in nucleo] ['aa', 'ag', 'ac', 'at', 'ga', 'gg', 'gc', 'gt', 'ca', 'cg', 'cc', 'ct', 'ta', 'tg', 'tc', 'tt']

!" !"

123

• nucleo = ['a', 'g', 'c', 't']

results = [] for a in nucleo: for b in nucleo: results.append(a+b) nucleo = ['a', 'g', 'c', 't'] results = [a+b for a in nucleo for b in nucleo]

124

• Anonymous functions
• Body can consist of only a single expression
• Execute slightly slower than normal functions
• Can take any number of parameters
• In general, lambda functions should be avoided

$ $

125

• Syntax : lambda p1[,…,pn]: <exp>
• Expression should not use return
• Example: adding two numbers

>>> add2 = lambda a,b: a+b >>> print add2(1, 5) 6

• Example: simple factorial

>>> fac = lambda x: reduce(lambda a,b: a*b, range(1, x+1)) >>> print fac(5) 120

$ $

126

References References

• Python Documentation

http://docs.python.org

• Python for Programmers by Alex Martelli
• Advanced Python (Understanding Python)

by Thomas Wouters

127

Exercise Exercise

• Write a Python program freq.py to:

– Fetch a text file from the web – Then report the most 10 frequently used words

$ python freq.py http://www.cpe.ku.ac.th/~cpj/gpl.txt 345: the 221: of 192: to 184: a 151: or 128: you 102: license 98: and 97: work 91: that $

128

Exercise Exercise – – Hints Hints

• Accessing command-line arguments
• Reading a webpage
• Extracting all English words from text

import urllib contents = urllib.urlopen(url).read() import sys url = sys.argv[1] import re words = re.findall('[A-Za-z]+', text)