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.
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.
10.
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.
12.
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.
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
