Introduction to PYTHON

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Introduction to PYTHON

• Sajid Gul Khawaja

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Introduction to Python

• Python is a high-level programming language
• Open source and community driven
• Standard distribution includes many modules
• Dynamic typed
• Source can be compiled or run just-in-time
• Similar to perl, tcl, ruby

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Introduction to Python contd

• Interpreted scripting language
• Object Oriented
• No pointers garbage collection
• Exception handling stack trace for each error
• Syntax features for manipulating lists (lists,
  dictionaries, sets, list comprehensions, etc.)
• Uses indentation instead of brackets for grouping
  blocks

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More definition

• Python is cross platform (for many platforms).
• The standard library is huge, very useful, and is
  very well documented.
• http//docs.python.org/library/
• Many companies make significant use of it
• Google uses it for a lot of their internal
  software projects
• NASA is using Python to implement a system which
  will be the core infrastructure for its next
  generation collaborative engineering environment

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Features of Python

• A script language
• Interpreted
• No compile/link stage
• Write and run
• Slow compared to C, C
• Elegant design tight
• Designed for
• Quick-and-dirty scripts
• Reusable modules
• Very large systems
• Object-oriented
• Very well-designed
• But you dont have to use it

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Why scripting instead of C/Assembly?

• For performance critical computations (graphics,
  networking), optimized code (C/Assembly)
  outperforms script/interpreted languages
• Video games often require code for Business
  Logic
• Coordinating components
• Glue logic
• Artificial intelligence

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Why scripting instead of C/Assembly?

• The priorities of this business logic are
  mostly
• Development time (Time to write code)
• Coding/output latency (Good for troubleshooting)
• Readability
• Reusability
• Maintenance
• Scripting/interpreted language is often preferred
  for this case

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Python Interfaces

• IDLE a cross-platform Python development
  environment
• PythonWin a Windows only interface to Python
• Python Shell running 'python' from the Command
  Line opens this interactive shell
• For the exercises, we'll use IDLE, but you can
  try them all and pick a favorite

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IDLE Development Environment

• IDLE helps you program in Python by
• color-coding your program code
• debugging
• auto-indent
• interactive shell

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General Information
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Objects All the Way Down

• Everything in Python is an object
• Integers are objects.
• Characters are objects.
• Complex numbers are objects.
• Booleans are objects.
• Functions are objects.
• Methods are objects.
• Modules are objects

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Variables

• No need to declare
• Need to assign (initialize)
• use of uninitialized variable raises exception
• Not typed
• if friendly greeting "hello world"
• else greeting 122
• print greeting
• Everything is a "variable"
• Even functions, classes, modules

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Reference Semantics

• Assignment manipulates references
• x y does not make a copy of y
• x y makes x reference the object y references
• Very useful but beware!
• Example
• gtgtgt a 1, 2, 3
• gtgtgt b a
• gtgtgt a.append(4)
• gtgtgt print b
• 1, 2, 3, 4

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Changing a Shared List
a 1, 2, 3
b a
a
1
2
3
4
a.append(4)
b
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Indentation and Blocks

• Python uses whitespace and indents to denote
  blocks of code
• Lines of code that begin a block end in a colon
• Lines within the code block are indented at the
  same level
• To end a code block, remove the indentation
• You'll want blocks of code that run only when
  certain conditions are met

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Whitespace

• Whitespace is meaningful in Python especially
  indentation and placement of newlines.
• Use a newline to end a line of code.
• Use \ when must go to next line prematurely.
• No braces to mark blocks of code in Python
  Use consistent indentation instead.
• The first line with less indentation is outside
  of the block.
• The first line with more indentation starts a
  nested block
• Often a colon appears at the start of a new
  block. (E.g. for function and class
  definitions.)

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Comments

• Start comments with the rest of line is
  ignored.
• Can include a documentation string as the first
  line of any new function or class that you
  define.
• The development environment, debugger, and other
  tools use it its good style to include one.
• def my_function(x, y)
• This is the docstring. This function does
  blah blah blah. The code would go here...

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None

• None represents the lack of a value.
• Like NULL in some languages or in databases.
• For instance
• gtgtgt if y!0...  fraction  x/y... else...
    fraction  None

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Introduction to Attributes

• Attributes are bits of information attached to
  objects.
•  gtgtgt x  complex(3, 5)gtgtgt x.real3.0gtgtgt x.imag5
  .0

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Introduction to Methods

• Methods are behaviours associated with objects.
• gtgtgt x  complex(3,5) gtgtgt x (35j)
  gtgtgt x.conjugate() (3-5j)

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Basic Data Types
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Data Type

• Boolean
• Integer
• Floating
• Character
• String
• Lists
• Dictionaries
• Tuples

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Booleans

• True and False are the only values of type
  bool.
• True can be treated as 1 and False as 0 in
  mathematical expressions
• gtgtgt print TrueTrueTrue-False3gtgtgt print False
  0True

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Basic Data Types
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Numbers

• The usual suspects
• 12, 3.14, 0xFF, 0377, (-12)3/45, abs(x),
  0ltxlt5
• C-style shifting masking
• 1ltlt16, x0xff, x1, x, xy

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Strings

• "hello""world" "helloworld" concatenation
• "hello"3 "hellohellohello" repetition
• "hello"0 "h" indexing
• "hello"-1 "o" (from end)
• "hello"14 "ell" slicing
• len("hello") 5 size
• "hello" lt "jello" 1 comparison
• "e" in "hello" 1 search
• "escapes \n etc, \033 etc, \if etc"
• 'single quotes' """triple quotes""" r"raw
  strings"

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Lists

• Think of a list as a stack of cards, on which
  your information is written
• The information stays in the order you place it
  in until you modify that order
• Methods return a string or subset of the list or
  modify the list to add or remove components
• Written as varindex, index refers to order
  within set (think card number, starting at 0)
• You can step through lists as part of a loop

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List Methods

• Adding to the List
• varn object
• replaces n with object
• var.append(object)
• adds object to the end of the list
• Removing from the List
• varn
• empties contents of card, but preserves order
• var.remove(n)
• removes card at n
• var.pop(n)
• removes n and returns its value

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Lists
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Tuples

• Like a list, tuples are iterable arrays of
  objects
• Tuples are immutable once created, unchangeable
• To add or remove items, you must redeclare
• Example uses of tuples
• County Names
• Land Use Codes
• Ordered set of functions

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Tuples

• key (lastname, firstname)
• point x, y, z parentheses optional
• x, y, z point unpack
• lastname key0
• singleton (1,) trailing comma!!!
• empty () parentheses!
• tuples vs. lists tuples immutable

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Tuples
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Dictionaries

• Dictionaries are sets of key value pairs
• Allows you to identify values by a descriptive
  name instead of order in a list
• Keys are unordered unless explicitly sorted
• Keys are unique
• varitem apple
• varitem banana
• print varitem prints just banana

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Dictionaries

• Hash tables, "associative arrays"
• d "duck" "eend", "water" "water"
• Lookup
• d"duck" -gt "eend"
• d"back" raises KeyError exception
• Delete, insert, overwrite
• del d"water" "duck" "eend", "back" "rug"
• d"back" "rug" "duck" "eend", "back"
  "rug"
• d"duck" "duik" "duck" "duik", "back"
  "rug"

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Dictionary Details

• Keys must be immutable
• numbers, strings, tuples of immutables
• these cannot be changed after creation
• reason is hashing (fast lookup technique)
• not lists or other dictionaries
• these types of objects can be changed "in place"
• no restrictions on values
• Keys will be listed in arbitrary order
• again, because of hashing

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Dictionaries
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Basic Functions
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Boolean Operators
C/Java Operator Python Operator
A B A and B
A B A or B
A B A B
A ! B A ! B A ltgt B
!A not A

• Relational operators are all the same
• 5 lt 6
• Boolean values
• True (case sensitive)
• False

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Mathematical Functions

• abs(x) absolute value
• divmod(a, b) - divide a by b and return both the
  quotient and the remainder
• hex( x) - Convert an integer number (of any size)
  to a hexadecimal string.
• oct( x) - Convert an integer number (of any size)
  to an octal string.
• round( x, n) - Return the floating point value
  x rounded to n digits after the decimal point.

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List Operations
Operator Meaning
ltseqgt ltseqgt Concatenation
ltseqgt ltint-exprgt Repetition
ltseqgt Indexing
len(ltseqgt) Length
ltseqgt Slicing
for ltvargt in ltseqgt Iteration
ltexprgt in ltseqgt Membership (Boolean)
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List Operations
Method Meaning
ltlistgt.append(x) Add element x to end of list.
ltlistgt.sort() Sort (order) the list. A comparison function may be passed as a parameter.
ltlistgt.reverse() Reverse the list.
ltlistgt.index(x) Returns index of first occurrence of x.
ltlistgt.insert(i, x) Insert x into list at index i.
ltlistgt.count(x) Returns the number of occurrences of x in list.
ltlistgt.remove(x) Deletes the first occurrence of x in list.
ltlistgt.pop(i) Deletes the ith element of the list and returns its value.
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Function Definition
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Functions

• Function syntax consist of
• The def keyword
• Function name
• Parenthesis with arguments separated by commas
• Sequence of statements with an indentation higher
  than the function definition statement
• gtgtgt def my_function(x)
• ... x 5
• ... print "X ", x

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Functions

• Python separates memory for this sequence of
  statements and gives the pointer to the variable
  given in the function definition
• Printing the variable containing the pointer
  (my_function) will print the pointer value
• Parenthesis (with all needed arguments) are
  needed to deference the pointer
• gtgtgt my_function
• ltfunction my_function at 0x00ABDAF0gt
• gtgtgt my_function(10)
• 10

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Functions, Procedures

• def name(arg1, arg2, ...)
• """documentation""" optional doc string
• statements
• return from procedure
• return expression from function

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Example Function

• def gcd(a, b)
• "greatest common divisor"
• while a ! 0
• a, b ba, a parallel assignment
• return b
• gtgtgt gcd.__doc__
• 'greatest common divisor'
• gtgtgt gcd(12, 20)
• 4

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Default Values

• Parameters can have default values
• def box(width, height,depth0) ...do something .
  ..box(10, 10, 20)box(10, 10, 0)box(10, 10)  s
  ame as above

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Functions inner block indentation

• Is a good habit to be consistent with the
  indentation per level to avoid Python compiler
  errors
• Level 1 3 spaces, level 2 6 spaces, level 3
  9 spaces, etc.
• Tabs are not recommended, because they are
  interpreted differently between text editors and
  operating systems

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Functions Scope

• You can not modify a variable of an outer scope,
  because the Python interpreter will think that it
  is a local variable and will create a new
  variable instead
• This can be avoided by using dot notation on an
  outside module or object (more on this later)
• gtgtgt y 3
• gtgtgt def foo()
• ... y 2
• ... print "Y ", y
• gtgtgt foo() Prints 2
• gtgtgt print "Y ", y Prints 3

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Functions Return value

• Functions can have a return statement to
  terminate the execution of a function and return
  a value
• If no return statement is given, Python will
  return None when after executing the last
  statement of a function
• gtgtgt def foo(x) x x 1
• gtgtgt print foo(5)
• None
• gtgtgt if foo(5)
• ... print "Impossible..."
• ... else
• ... print "None is like null in Java
• None is like null in Java

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Basic Flow Control
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Basic Flow Control

• if/elif/else (test condition)
• while (loop until condition changes)
• for (iterate over iteraterable object)
• Range
• Loop control statements

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IF Statement

• if j"Hello" doSomething()elif j"World"
      doSomethingElse()else    doTheRightThing()
  

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WHILE Statement

• str""while str!"quit"        str
  input()        print (str)print ("Done)

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Looping Forever

• Use a value that will always be true to loop
  until a break.
• while True
• string input()
• if stringexit
• break
• print (string)
• print ("Done)

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FOR Statement

• myList  "a", "b", "c", "d", "e"for i in myLis
  t        print ifor i in range( 10 )        
  print ifor i in range( len( myList ) )        
  if myListi"c"                myListiNone
  
• Can break out of for-loops.
• Can continue to next iteration.

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Range function

• Creates a list of integer values usually used to
  iterate in a for loop
• range(n) will give 0, 1, 2, ., (n 2), (n
  1)
• Starting value and increment/decrement can be
  provided
• These features allows to do any type of iteration
  as a C for statement would allow

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Range function

• range(a, b, c)
• a Starting value, default value is 0
• b Terminating condition, must always be given
• c Increment/decrement, default value is 1
• Equivalent in C to
• for(int x b x lt a x c)

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Range function

• The len and range functions can be combined to
  get the index value of a list while iterating
  through it
• gtgtgt range(10)
• 0, 1, 2, 3, 4, 5, 6, 7, 8, 9
• gtgtgt range(5, 10)
• 5, 6, 7, 8, 9
• gtgtgt range(1, 10, 2)
• 1, 3, 5, 7, 9
• gtgtgt for i in range(len(a))
• ... print i, ai

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Loop Control Statements
break Jumps out of the closest enclosing loop
continue Jumps to the top of the closest enclosing loop
pass Does nothing, empty statement placeholder
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Break and Continue Statement

• Break and continue work like in C/Java
• break statement allows you to jump out of the
  middle of a loop
• The continue statement is a short-cut way of
  jumping to the top of the loop
• For/While loops can have else statement
• Runs at the end of the for/while loop
• Break skips else statement, continue does not
• gtgtgt for i in range(10)
• ... if i 9 continue
• ... else
• ... print "bye Bye is printed
• gtgtgt for i in range(10)
• ... if i 9 break
• ... else
• ... print "bye Bye is not printed

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Pass Statement

• Is used to make the Python compiler happy when
  you want an empty function or class
• gtgtgt for i in range(10)
• ... pass
• ... Do nothing
• ...
• gtgtgt class DummyClass
• ... pass

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Task
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Tasks

• Factorial
• Write a function fact that calculates factorial
  of N.
• Fibonacci series
• Write a function fib that calculates the
  Fibonacci series till N.

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Importing and Modules
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Importing and Modules

• Use classes functions defined in another file.
• A Python module is a file with the same name
  (plus the .py extension)
• Like Java import, C include.
• Three formats of the command
• import somefile
• from somefile import
• from somefile import className
• Whats the difference? What gets imported from
  the file and what name refers to it after it has
  been imported.

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import

• import somefile
• Everything in somefile.py gets imported.
• To refer to something in the file, append the
  text somefile. to the front of its name
• somefile.className.method(abc)
• somefile.myFunction(34)

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Commonly Used Modules

• Some useful modules to import, included with
  Python
• Module sys - Lots of handy stuff.
• Maxint
• Module os - OS specific code.
• Module os.path - Directory processing.

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More Commonly Used Modules

• Module math - Mathematical code.
• Exponents
• sqrt
• Module Random - Random number code.
• Randrange
• Uniform
• Choice
• Shuffle