january 24, 2006and now for something completely different 1 “and now for something completely...

January 24, 2006 And Now For Something Completely Different

“And Now For Something Completely Different”

A Quick Tutorial of the Python Programming Language

Michael Scherger

Department of Computer Science

Kent State University

Contents

• Python Resources

• Python Basics and Language Elements

• Statements and Control Structures

• Input, Output, and File Access

• Functions

• Classes and Objects

Python Resources

• Python Web Site: http://www.python.org

• Check out the tutorial at: http://www.python.org/doc/tut

• Download current stable version for your PC

Running Python

• PC– c:\>python – Use Python GUI called ‘IDLE’

• Linux• % python

• Mac version available

Running Python

Python Basics and Language Elements

Numbers

• Integers– Decimal

• At least 32 bit

– Octal and hexadecimal

• Long integers– Limited by memory size– Trailing “L”

• Floating point• Complex

– Use “j” instead of “i”

• Boolean

>>> 1+12>>> 1+1.02.0>>> 1+1e02.0>>> >>> 999999999999999999999999999999L>>> 99999999999999999999999999999999999999999999999999999999L>>>9999999999999999999999999999999999999999999999999999999999999999999999999999999999L>>> >>> 1717>>> 02117>>> 0x1117>>>

Variables

• Variables do not have types

• You do not declare variables– They appear when you

assign them– They disappear when

you do not use them anymore

>>> x = 1 + 1>>> x2>>> x = 1 + 1.0>>> x2.0>>> >>> y

Traceback (most recent call last): File "<pyshell#20>", line 1, in -toplevel- yNameError: name 'y' is not defined>>> y = None>>> y>>> y == NoneTrue>>> del y>>> y

Traceback (most recent call last): File "<pyshell#25>", line 1, in -toplevel- yNameError: name 'y' is not defined>>>

• Python’s array like object is a list– Uses [ ]

– Subscripted like arrays in C (zero based)

– Mutable

– Use len(L) to get the length of the list

>>> z = [4, 5, 6]>>> z[1]5>>> z[1] = 7>>> z[4, 7, 6]>>> len( z )3>>>

• Indexing– Uses bracket notation– Indexes start at 0…can have a negative index

name = [“Cleese”, “John”] # example of strings in a list

print name[1], name[0] # prints “John Cleese”

name[0] = “Smith”

x=[[1,2,3],[y,z],[[[]]]] # example of lists in a list

“A” “B” “C” “D” “E”0 1 2 3 4

-5 -4 -3 -2 -1

• Slicing is similar to indexing except you indicate both the start and stop index separated by a colon

x = ["spam1","spam2","spam3","spam4","spam5","eggs","and","spam6"]

print x[5:7] # Prints the list ["eggs","and"]

“A” “B” “C” “D” “E”

0 1 2 3 4 5

-5 -4 -3 -2 -1

List Methods

• append() add an element to the end of the list

• pop() remove an element from the end of a list

• insert() add an element anywhere in the list

• + and * list concatenation and replication

• And a whole bunch more…look them up!

Strings• A string is a sequence of

characters between quotes; either “ “ or ‘ ‘

– There is no character data type in Python…only string

• Strings are not arrays of characters as in C/C++!

– Use subscript notation to read individual characters as a string

– Strings are immutable

– Use len(S) to get the length of the string

>>> x = "abc">>> x[0]'a'>>> len(x)3>>> x[0] = 'd'

Traceback (most recent call last):

File "<pyshell#34>", line 1, in -toplevel-

x[0] = 'd'TypeError: object doesn't

support item assignment>>>

Converting Values

• String values can be converted to integers using the int() function

• String values can be converted to floating points using the float() function

• Ints and floats can be converted to string values str() function

• Example

x = int( “10” )y = float( “10”)yy = float (10)Z = str(3.14159)

String Methodsupper() Returns the uppercase version of the string

lower() Returns the lowercase version of the string

swapcase() Returns a new string where the case of each letter is switched

capitalize() Returns a new string with the first letter capitalized and the remaining letters are in lowercase

title() Returns a new string with the first letter of each word capitalized and all other letters are in lower case

strip() Returns a new string with leading and trailing white space removed.

replace( old, new, [,max]) Returns a new string where occurrences of the string “old” are replaced by “new” up to “max” number of times

Tuples

• Tuples are like lists– Immutable

– Use ( ) notation instead of [ ]’s for creation

– Uses [ ] notation for indexing and slicing

>>> x = (1, "Mike", 2, Laurie", 3, "Sarah")

>>> x(1, 'Mike', 2, 'Laurie', 3, 'Sarah')>>> x[1] = 'Michael'

Traceback (most recent call last): File "<pyshell#46>", line 1, in -

toplevel- x[1] = 'Michael'TypeError: object doesn't support

item assignment>>>

Dictionaries

• Dictionaries are unordered lists– Associative array/list or table or map– List of “key/value” pairs– Use “key” to look up the element or value

person = {'first name': "Robin", 'last name’ : "Hood", 'occupation': "Scoundrel" }

person[‘last name’] = “of Locksley”

Dictionary Methods

• keys() returns a list of keys

• values() returns a list of values

• items() returns a list of key value pairs

• has_key() tests if a key exists

• And a whole bunch more…look them up!

Assignments

• Assignment uses =

• Equality uses ==

• Not Equal uses != or <>

x, y, z = 1, 2, 3

first, second = second, first

a = b = 123

Other Assignment Operators

• Augmented assignment operators– A combination of assignment and a mathematical

operation

• *= x *= 5 x = x * 5• /= x /= 5 x = x / 5• %= x %=5 x = x % 5• += x += 5 x = x + 5• -= x -= 5 x = x - 5

Logical Values

• Python 2.3 has a Boolean type– Prior versions do not

– False is zero or empty

– True is not False (non-zero or not empty)

– And, Or, Not are short-circuited

– Can do lexicographic comparison of sequences (lists, strings, tuples)

• 1 < 2 # True• 1 > 2 # False• [1,2] < [3,4] # True• [1,2] == [1,2] # True• “ab” == “a” + “b” # True• (1,2) == [1,2] # False• not [] # True• not [1,2,3] #

False• not ‘’ # True• not ‘ab’ # False

Statements and Control Structures

Blocks

• Blocks are indicated using indentation only– No BEGIN/END or { }

if x < 5 or (x > 10 and x < 20):print “The value is OK.”

if x < 5 or 10 < x < 20:print “The value is OK.”

If-Elif-Else

if color == Green:do_green_function()

elif color == Blue:do_blue_function()

elif color == Red:do_red_function()

else:report_error()

While Loops

• Example of a while loops

x = 10

while x >= 0:

print “x is still negative.”

x = x-1

For Loops

• To make an “ordinary” for loop use the built-in function range()

# Print out the values from 0

# to 99 inclusive

for value in range(100):

print value

For Loops• More examples of for loops

# print values in a listfor value in [1, 2, 3, 4, 5, 4, 3, 2, 1]:

print value

# print 10 thru 19for value in range(10,20):

print value

# print 10 12 14 16 18for value in range(10, 20, 2):

print value

Input, Output, and File Access

Getting User Input

• The raw_input function returns a string.– Be careful!

• 10 is not the same as “10”• Other functions convert strings to integers and vice

versa.

• Example: Personal Greeter

A Tiny Algorithm

• Prompt user for a number• Print out the square of the number

x = input( “Please enter a number: “)

print “The square of that number is”, x * x

• Could also use… x = raw_input( “Please enter a number: “)

x = int(x)

print “The square of that number is”, x * x

File Input / Output

• Example

filespec = open( “somefile”, “r” )

s = filespec.readline() # read one line

s = filespec.readlines() # read all lines

filespec.close()

File Input / Output

• Example

filespec = open( “somefile”, “w” )

filespec.writeline(s) # write one string

filespec.writelines(l) # write string list

filespec.close()

Functions

• Use the keyword def– Passing parameters– Named arguments– Default values– Variable scoping

def square(x):return x*x

print square(2) # prints out 4

Functions

• When you pass a parameter to a function, you bind the parameter to the value

• Creates a new reference

def change(some_list): some_list[1] = 4

x = [1,2,3] change(x) print x # Prints out [1,4,3]

Functions

• Another example

def nochange(x): x = 0

y = 1 nochange(y) print y # Prints out 1

• Example: Balanced Parentheses and Fibonocci Numbers

Functions

• Example

>>>def sumdif(a, b):return a+b, a-b

>>>sumdif( 5, 7 )(12,2)>>>

Putting It All Together

• Write a Python version of wc (word count)– Writes the counts of the number of characters, words,

and lines in a file

• Algorithm– open the file– count the characters, words, and lines– write out the counts– close the file

• Example: wc– % python wc.py somefile.ext

Classes and Objects

class Basket:def __init__(self, contents=None):self.contents = contents or []

def add(self, element):self.contents.append(element)

def print_me(self):result = “”for element in self.contents:

result = result + “ “ + `element`print “Contains: “ + result

Classes and Objects• All methods (functions in an object) received an additional argument at the

start of the argument list containing the object itself. (Called self which is customary)

• Methods are called like this: object.method( arg1, arg2)

• Some method names, like __init__ (two underscores on each side), are predefined and mean special things (constructor)

• Some arguments are optional and given and default value

• Short circuit evaluation and assignment

• Backquotes convert an object to its string representation

• Addition sign (+) is used for string concatenation

Classes and Objects

• Instead of short circuit to assign contents…could have written the statement like this

if contents:self.contents = contents

else:self.contents = []

def __init__(self,contents=[])self.contents = contents[:]

Classes and Objects

• To create an instance of a class (object)

a = Basket()

b = Basket( [“apple”, “lemon”])

b.add( “orange” )

b.print_me()

Classes and Objects

• Convert object to string– Use the __str__ method

def __str__(self)

result = “”

for element in self.contents:result = result + “ “ + `element`

return “Contains: “ + “ “ + result

Classes and Objects• Example: Basket Class

• >>> b = Basket()• >>> b• <__main__.Basket instance at 0x00A09B98>• >>> print b• Contains:• >>> b.add( "Foo" )• >>> print b• Contains: 'Foo'• >>> b.add (["Foo", "Bar"])• >>> b.print_me()• Contains: 'Foo' ['Foo', 'Bar']• >>> b.add ((1,2,3))• >>> print b• Contains: 'Foo' ['Foo', 'Bar'] (1, 2, 3)• >>>

january 24, 2006and now for something completely different 1 “and now for something completely...

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