And Now For Something Completely Different A Quick Tutorial of the Python Programming Language

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And Now For Something Completely DifferentA
Quick Tutorial of the Python Programming Language

• Michael Scherger
• Department of Computer Science
• Kent State University

2
Contents

• Python Resources
• Python Basics and Language Elements
• Statements and Control Structures
• Input, Output, and File Access
• Functions
• Classes and Objects

3
Python Resources
4
Python Resources

• Python Web Site http//www.python.org
• Check out the tutorial at http//www.python.org/d
  oc/tut
• Download current stable version for your PC
  (version 2.3.4)

5
Running Python

• PC
• c\gtpython
• Use Python GUI called IDLE
• Linux
• python
• Mac version available

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Running Python
7
Running Python
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Python Basics and Language Elements
9
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

• gtgtgt 11
• 2
• gtgtgt 11.0
• 2.0
• gtgtgt 11e0
• 2.0
• gtgtgt
• gtgtgt 999999999999999
• 999999999999999L
• gtgtgt 9999999999999999999999999999
• 9999999999999999999999999999L
• gtgtgt99999999999999999999999999999999999999999
• 99999999999999999999999999999999999999999L
• gtgtgt
• gtgtgt 17
• 17
• gtgtgt 021
• 17
• gtgtgt 0x11

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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

• gtgtgt x 1 1
• gtgtgt x
• 2
• gtgtgt x 1 1.0
• gtgtgt x
• 2.0
• gtgtgt
• gtgtgt y
• Traceback (most recent call last)
• File "ltpyshell20gt", line 1, in -toplevel-
• y
• NameError name 'y' is not defined
• gtgtgt y None
• gtgtgt y
• gtgtgt y None
• True
• gtgtgt del y
• gtgtgt y

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Lists

• Pythons 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

• gtgtgt z 4, 5, 6
• gtgtgt z1
• 5
• gtgtgt z1 7
• gtgtgt z
• 4, 7, 6
• gtgtgt len( z )
• 3
• gtgtgt

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Lists

• Indexing
• Uses bracket notation
• Indexes start at 0can have a negative index
• name Cleese, John example of strings in
  a list
• print name1, name0 prints John Cleese
• name0 Smith
• x1,2,3,y,z, example of lists in a
  list

0
1
2
3
4
A
B
C
D
E
-1
-5
-4
-3
-2
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Lists

• 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 x57 Prints the list "eggs","and"

0
1
2
3
4
5
A
B
C
D
E
-6
-1
-5
-4
-3
-2
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Lists

• More on slicing
• x "spam1","spam2","spam3","spam4","spam5","eggs
  ","and","spam6"
• x3 prints spam1, spam2, spam3
• x3 prints spam4, spam5,
  eggs,and,spam6
• x prints all of it!

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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 morelook them up!

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Strings

• A string is a sequence of characters between
  quotes either or
• There is no character data type in Pythononly
  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

• gtgtgt x "abc"
• gtgtgt x0
• 'a'
• gtgtgt len(x)
• 3
• gtgtgt x0 'd'
• Traceback (most recent call last)
• File "ltpyshell34gt", line 1, in -toplevel-
• x0 'd'
• TypeError object doesn't support item assignment
• gtgtgt

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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)

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String Methods

• upper() 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

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Tuples

• Tuples are like lists
• Immutable
• Use ( ) notation instead of s for creation
• Uses notation for indexing and slicing

• gtgtgt x (1, "Mike", 2, Laurie", 3, "Sarah")
• gtgtgt x
• (1, 'Mike', 2, 'Laurie', 3, 'Sarah')
• gtgtgt x1 'Michael'
• Traceback (most recent call last)
• File "ltpyshell46gt", line 1, in -toplevel-
• x1 'Michael'
• TypeError object doesn't support item assignment
• gtgtgt

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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"
• personlast name of Locksley

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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 morelook them up!

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Assignments

• Assignment uses
• Equality uses
• Not Equal uses ! or ltgt
• x, y, z 1, 2, 3
• first, second second, first
• a b 123

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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

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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 lt 2 True
• 1 gt 2 False
• 1,2 lt 3,4 False
• 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

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Statements and Control Structures
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Blocks

• Blocks are indicated using indentation only
• No BEGIN/END or
• if x lt 5 or (x gt 10 and x lt 20)
• print The value is OK.
• if x lt 5 or 10 lt x lt 20
• print The value is OK.

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If-Elif-Else

• if color Green
• do_green_function()
• elif color Blue
• do_blue_function()
• elif color Red
• do_red_function()
• else
• report_error()

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While Loops

• Example of a while loops
• x 10
• while x gt 0
• print x is still negative.
• x x-1

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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

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For Loops

• More examples of for loops
• print values in a list
• for value in 1, 2, 3, 4, 5, 4, 3, 2, 1
• print value
• print 10 thru 19
• for value in range(10,20)
• print value
• print 10 12 14 16 18
• for value in range(10, 20, 2)
• print value

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Input, Output, and File Access
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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

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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

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File Input / Output

• Example
• filespec open( somefile, r )
• s filespec.readline() read one line
• s filespec.readlines() read all lines
• filespec.close()

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File Input / Output

• Example
• filespec open( somefile, w )
• filespec.writeline(s) write one string
• filespec.readlines(l) write string list
• filespec.close()

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Functions
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Functions

• Use the keyword def
• Passing parameters
• Named arguments
• Default values
• Variable scoping
• def square(x)
• return xx
• print square(2) prints out 4

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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_list1 4
• x 1,2,3
• change(x)
• print x Prints out 1,4,3

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Functions

• Another example
• def nochange(x)
• x 0
• y 1
• nochange(y)
• print y Prints out 1
• Example Balanced Parentheses and Fibonocci
  Numbers

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Functions

• Example
• gtgtgtdef sumdif(a, b)
• return ab, a-b
• gtgtgtsumdif( 5, 7 )
• (12,2)
• gtgtgt

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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

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Classes and Objects
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Classes and Objects

• class Basket
• def __init__(self, contentsNone)
• 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

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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

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Classes and Objects

• Instead of short circuit to assign contentscould
  have written the statement like this
• if contents
• self.contents contents
• else
• self.contents
• def __init__(self,contents)
• self.contents contents

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Classes and Objects

• To create an instance of a class (object)
• a Basket()
• b Basket( apple, lemon)
• b.add( orange )
• b.print_me()

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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

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Classes and Objects

• Example Basket Class
• gtgtgt b Basket()
• gtgtgt b
• lt__main__.Basket instance at 0x00A09B98gt
• gtgtgt print b
• Contains
• gtgtgt b.add( "Foo" )
• gtgtgt print b
• Contains 'Foo'
• gtgtgt b.add ("Foo", "Bar")
• gtgtgt b.print_me()
• Contains 'Foo' 'Foo', 'Bar'
• gtgtgt b.add ((1,2,3))
• gtgtgt print b
• Contains 'Foo' 'Foo', 'Bar' (1, 2, 3)
• gtgtgt