This document provides an introduction and overview of key concepts in Python, including variables, types, functions, and conditional statements. It discusses how variables store and reference values, Python's dynamic typing, and how functions are defined with the def keyword. Conditionals like if/else statements are explained, with the general form being an if conditional block followed by optional elif and else blocks. Homework involves implementing functions for logical operations, distance calculation, and percent calculation.

1. Introduction to Python
Lecture 2
Kasyanov Anton
IASA 2011

2. Plan
 Variables
 Types
 Functions
 If operator

3. Variables
 A variable is a name that refers to a value.
 Variables let us store and reuse values in
several places.
 Python is dynamically typed. That means that
we don't need to specify variable's type.

4. Variables, assignment
 Form: variable = expression
 An expression is a legal sentence in python that
can be evaluated.
 So we will put math expressions into the shell and
seen them be evaluated to single numbers.
 What it does:
 1. Evaluate the expression on the RHS.(This value
is a memory address)
 2. Store the memory address in the variable.

5. Variables, assignment
 What this means is that a variable is a name
and a memory address. The name points to a
memory address where the value is stored.

6. Practice
 Let's play with python shell

7. Functions
 Useful but simple to create.
 First let's think about what it means to define a
function in math.
 Consider f(x)=x^2.
 In python we can do the same with:
 def f(x):
return x**2


8. Functions
 A function definition has the form:
def function_name(parameters):
block
 def is a python keyword; it cannot be used for
naming functions or variables.
 A parameter of a function is a variable. A function
can have any number of parameters, including 0.
 A block is a sequence of legal python statements.
 A block must be indented.
 If the block contains the keyword return, it returns a
value; otherwise it returns the special value None.

9. Functions documentation
 We can use the built-in function help() to get
information on functions or modules.
 We can do this on functions that we've defined
as well, but it doesn't give much information.
 We can add useful documentation with
docstrings.
 A docstring is surrounded by ''' and must be the first
line of a module or function.

11. Docstrings
 If the first line of a function or module is a
string, we call it a docstring.
 Short for documentation string.
 Python saves the string to return if the help
function is called.
 Convention: Leave a blank line after but not
before a docstring.
 All functions should have docstrings.

13. Naming conventions
 Naming rules and conventions apply to
functions, variables and any other kind of name
that you will see.
 Must start with a letter or underscore.
 Can include letters, numbers, and underscores
and nothing else.
 Case matters, so age is not same name as
Age.

14. Name conventions
 For variables and functions pothole_case is
used
 variable_name, useful_function
 CamelCase is sometimes used, but not for
variables and functions
 MyClass

15. Types
 Every variable has a type
 Use built-in function type()
 Type converting is available

16. Booleans
 Can have two values True, False.
 Have three operations: not, and, or.
 not changes a True to a False and vice versa.
 and returns False unless all the arguments are
True.
 or returns True unless all the arguments are
False.

17. Booleans
 We can use relational operators.
 <,>,<=,>=,!=, ==
 These are all comparison operators that return
True or False.
 == is the equality operator.
 != is not equals.

18. Practice
 Let's use Python shell and play with types.

19. If statement
 The general form of an if statement is:
if condition:
block
 Example:
if grade >=50:
print “pass”

20. If statement
 The general form of an if statement is:
if condition:
block
 The condition is a boolean expression.
 Recall that a block is a series of python
statements.
 If the condition evaluates to true the block is
executed.

21. If statement
 If we want to execute different lines of code
based on the outcome of the boolean
expression we can use:
if condition:
block
else:
block
 The block under the else is executed if the
condition evaluates to false.

22. Elif
if condition1:
block
elif condition2:
block
elif condition3:
block
else:
block

23. Practice
 Changing photo to make it look like it's sunset
using Python.
 Sunset is when red color is main on the photo.
 Let's decrease blue and green components of
each pixel.
 We will use Python Imaging Library (PIL).

24. Workflow
 Load an image
 Get it's size
 Step through all pixels
 Get color of this pixel
 Change it
 Put it back
 Save the image

25. Home assignment
 Create all following functions:
 xor(bool, bool) – returns result of xor operation
 distance(int, int, int, int) – returns (float)
distance between (x1,y1) and (x2, y2)
 percent(int, int) - if a<b then returns (int)
percent of a according to b, else returns -1

26. Home assignment
 Send to mind_master@ukr.net due to 17.10
(Monday)
 Just functions.py file
 First line is import math
 Math.sqrt() is useful

27. Any questions?