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INTRODUCTION TO PYTHON
Sajid Gul Khawaja
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



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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
Python is cross platform (for many platforms).
The standard library is huge, very useful, and is very
well documented.
FEATURES OF PYTHON




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
o Useful error messages
o Automatic memory handling
o Independent of operating
 No compile/link stage
system
 Write and run
• Works on both Unix and
 Slow compared to C, C++
Windows
Elegant design; “tight”
• Even file system code can
Designed for
be made os-independent
 Quick-and-dirty scripts
o Large library of standard
 Reusable modules
modules
 Very large systems
o Large number of third-party
Object-oriented
modules
 Very well-designed
 But you don’t have to use it o Integration with external C
code
o Well documented
A script language
Interpreted
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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”

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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:
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color-coding your
program code
 debugging
 auto-indent
 interactive shell

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GENERAL INFORMATION
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 = 12**2
print greeting

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
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

>>> a = [1, 2, 3]
>>> b = a
>>> a.append(4)
>>> print b
[1, 2, 3, 4]
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Very useful; but beware!
 Example:

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CHANGING A SHARED LIST
a = [1, 2, 3]
a
2
3
1
2
3
1
2
3
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a
b=a
b
a
a.append(4)
b
4
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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


Use \ when must go to next line prematurely.
No braces { } to mark blocks of code in Python…
Use consistent indentation instead.
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Whitespace is meaningful in Python: especially
indentation and placement of newlines.
 Use a newline to end a line of code.
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


def my_function(x, y):
“““This is the docstring. This
function does blah blah blah.”””
# The code would go here...
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
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: it’s good style to include one.
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NONE
 “None”
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represents the lack of a value.
 Like “NULL” in some languages or in
databases.
 For instance:
>>> if y!=0:
... fraction = x/y
... else:
... fraction = None
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BASIC DATA TYPES
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:

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>>> print True+True+True-False
3
>>> print False==0
True
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BASIC DATA TYPES
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NUMBERS
 The
12, 3.14, 0xFF, 0377, (-1+2)*3/4**5, abs(x), 0<x<=5
 C-style

shifting & masking
1<<16, x&0xff, x|1, ~x, x^y
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
usual suspects
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STRINGS
"hello"+"world" "helloworld" # concatenation
 "hello"*3
"hellohellohello" # repetition
 "hello"[0]
"h"
# indexing
 "hello"[-1]
"o"
# (from end)
 "hello"[1:4]
"ell"
# slicing
 len("hello")
5
# size
 "hello" < "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 var[index], 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

var[n] = object
replaces n with object
var.append(object)

adds object to the end of the list
 Removing

var[n] = []


empties contents of card, but preserves order
var.remove(n)


from the List
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

to the List
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
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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 = key[0]
 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:
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var[‘item’] = “apple”
 var[‘item’] = “banana”
 print var[‘item’] prints just banana
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DICTIONARIES

Hash tables, "associative arrays"

Lookup:
d["duck"] -> "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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d = {"duck": "eend", "water": "water"}
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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
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Keys will be listed in arbitrary order

again, because of hashing
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DICTIONARIES
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BASIC FUNCTIONS
BOOLEAN OPERATORS
A && B
A || B
A == B
A != B
!A
Python
Operator
A and B
A or B
A == B
A != B
A <> B
not A
 Relational
operators are all
the same

5 <= 6
 Boolean


values
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C/Java
Operator
True (case
sensitive)
False
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MATHEMATICAL FUNCTIONS
 abs(x)
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– 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
Concatenation
<seq> * <int-expr>
Repetition
<seq>[]
Indexing
len(<seq>)
Length
<seq>[:]
Slicing
for <var> in <seq>:
Iteration
<expr> in <seq>
Membership (Boolean)
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<seq> + <seq>
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LIST OPERATIONS
Method
Meaning
Add element x to end of list.
<list>.sort()
Sort (order) the list. A comparison function may
be passed as a parameter.
<list>.reverse()
Reverse the list.
<list>.index(x)
Returns index of first occurrence of x.
<list>.insert(i, x)
Insert x into list at index i.
<list>.count(x)
Returns the number of occurrences of x in list.
<list>.remove(x)
Deletes the first occurrence of x in list.
<list>.pop(i)
Deletes the ith element of the list and returns
its value.
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<list>.append(x)
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FUNCTION DEFINITION
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

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>>> def my_function(x):
...
x = 5
...
print "X = ", x
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FUNCTIONS
 Python
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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
>>> my_function
<function my_function at 0x00ABDAF0>
>>> my_function(10)
10
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FUNCTIONS, PROCEDURES
return
# from procedure
return expression
# from function
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def name(arg1, arg2, ...):
"""documentation"""
# optional doc string
statements
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EXAMPLE FUNCTION
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def gcd(a, b):
"greatest common divisor"
while a != 0:
a, b = b%a, a # parallel assignment
return b
>>> gcd.__doc__
'greatest common divisor'
>>> gcd(12, 20)
4
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DEFAULT VALUES

box(10, 10, 20)
box(10, 10, 0)
box(10, 10) # same as above
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Parameters can have default values:
def box(width, height,depth=0):
...do something ...
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FUNCTIONS – INNER BLOCK INDENTATION



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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Is a good habit to be consistent with the
indentation per level to avoid Python compiler
errors:
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FUNCTIONS – SCOPE
 You
>>> y = 3
>>> def foo():
...
y = 2
...
print "Y = ", y
>>> foo()
>>> print "Y = ", y
# Prints 2
# Prints 3
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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)
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FUNCTIONS – RETURN VALUE
 Functions

If no return statement is given, Python will return
None when after executing the last statement of a
function
>>> def foo(x): x = x + 1
>>> print foo(5)
None
>>> if foo(5):
...
print "Impossible..."
... else:
...
print "None is like null in Java“
None is like null in Java
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can have a return statement to
terminate the execution of a function and
return a value
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BASIC FLOW CONTROL
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
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if j=="Hello":
doSomething()
elif j=="World":
doSomethingElse()
else:
doTheRightThing()
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WHILE STATEMENT
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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”.
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while True:
string = input()
if string==“exit”:
break
print (string)
print ("Done”)
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FOR STATEMENT
for i in range( 10 ):
print i
for i in range( len( myList ) ):
if myList[i]=="c":
myList[i]=None
 Can “break” out of for-loops.
 Can “continue” to next iteration.
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myList = ["a", "b", "c", "d", "e"]
for i in myList:
print i
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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 < a; x += c)
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RANGE FUNCTION

>>> range(10)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> range(5, 10)
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The len and range functions can be combined to
get the index value of a list while iterating
through it
[5, 6, 7, 8, 9]
>>> range(1, 10, 2)
[1, 3, 5, 7, 9]
>>> for i in range(len(a)):
...
print i, a[i]
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LOOP CONTROL STATEMENTS
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
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BREAK AND CONTINUE STATEMENT
 For/While loops can have else statement
 Runs at the end of the for/while loop
 Break skips else statement, continue does not
>>>
...
...
...
>>>
...
...
...
for i in range(10):
if i == 9: continue
else:
print "bye“
# Bye is printed
for i in range(10):
if i == 9: break
else:
print "bye“
# Bye is not printed
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 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
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PASS STATEMENT

Is used to make the Python compiler happy when
you want an empty function or class
...
pass
...
# Do nothing
...
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>>> for i in range(10):
>>> class DummyClass:
...
pass
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TASK
TASKS

Factorial

Fibonacci series

Write a function fib that calculates the Fibonacci
series till N.
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
Write a function fact that calculates factorial of N.
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IMPORTING AND MODULES
IMPORTING AND MODULES

A Python module is a file with the same name (plus the .py
extension)

Like Java import, C++ include.

Three formats of the command:
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
Use classes & functions defined in another file.
import somefile
from somefile import *
from somefile import className
What’s 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

Module: sys



- Lots of handy stuff.
Maxint
Module: os
Module: os.path
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
Some useful modules to import, included with
Python:
- OS specific code.
- 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

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