Python Notes – 2 : Variables, Statements, Expressions, Operators, and Functions

Welcome to our second note in our Python learning process. In this note we will talk about variables, statements, expressions, operators, comments, and functions. These are the very basic building blocks of you programs whatever its final size

Variables

The assignment statement creates new variables and gives them values

>>> message = "What's up, Doc?"

>>> n = 17

>>> pi = 3.14159

Python is a dynamically typed language. Which means that variables' types don't have to be defined before the variables use. The python interpreter figure out what type a variable is when you first assign it a value.

Variables naming

• Variable name can contain letters, numbers, and underscore.
• Variable name must begin with letter.
• Variable names are case-sensitive ( x different than X ).
• Variable names can't be one of the Python language reserved words, which are :

and	continue	else	for	import	not	raise
assert	def	except	from	in	or	return
break	def	exec	global	is	pass	try
class	elif	finally	if	lambda	print	while

Statements

A statement is an instruction that the Python interpreter can execute. A Python script is a sequence of statements. The results appear one at a time as the statements execute.

For example, the script

>>> print 1

>>> 1

>>> x = 2

>>> print x

>>> 2

Expressions

An expression is a combination of values, variables, and operators. If you type an expression on the command line, the interpreter evaluates it and displays the result:

>>> 1 + 1

2

Boolean Expressions

A boolean expression is an expression that is either true or false. Boolean expressions are expressions that uses logical operators. There are three logical operators: and, or, not. The operands of logical operands should be boolean expressions. In Python there is no boolean data type instead 0, '', [], (), {}, and None are false in a boolean context; everything else is true. The absence of boolean data types results in the following behavior of logical operators:

• All the behaviors depends on the order of expressions evaluation, which is from left to right.
• and : returns the first false value, if all values are true, returns the last value.

>>> 'a' and 'b'

'b'

>>> ' ' and 'b'

' '

>>> 'a' and 'b' and 'c'

'c'

• or : returns the first true value, if all values are false, returns the last value

>>> 'a' or 'b'

'a'

>>> ' ' or 'b'

'b'

>>> ' ' or [] or {}

{}

• not : evaluates the boolean expression that follows it and inverts it whole value.

>>> not (1 and 0)

True

>>> not (1 or 0)

False

Operators and operands

Operators are special symbols that represent computations like addition and multiplication. The values the operator uses are called operands.

The symbols +, -, and /, and the use of parenthesis for grouping, mean in

Python what they mean in mathematics. The asterisk (*) is the symbol for multiplication, and ** is the symbol for exponentiation.

Order of operations

When more than one operator appears in an expression, the order of evaluation depends on the rules of precedence. The following is operators from highest precedence to lower precedence:

• Parentheses
• Exponentiation
• Multiplication / Division
• Addition / Subtraction

Operators with the same precedence are evaluated from left to right.

Comments

Comments start with #.

Built-in Functions

Here are some of the built-in functions available in Python:

• Type conversion functions which converts values from one type to another. Like:
  • int() :

>>> int("32")

32

>>> int("Hello")

ValueError: invalid literal for int(): Hello

• float():

>>> float(32)

32.0

>>> float("3.14159")

3.14159

• str():

>>> str(32)

'32'

>>> str(3.14149)

'3.14149'

• Math functions which provides most of the familiar mathematical functions. These functions are implemented in a module called math, we will talk about modules soon. But for now, if you want to use these functions, you have to import the math module. This is very straight foreword, using following statement
  • >>> import math
  • After the import statement, you can simply call the math functions.

>>> decibel = math.log10 (17.0)

>>> angle = 1.5

>>> height = math.sin(angle)

>>> degrees = 45

>>> angle = degrees * 2 * math.pi / 360.0

>>> math.sin(angle)

0.707106781187

>>> math.sqrt(2) / 2.0

0.707106781187

There are many other built-in functions, we mentioned these two types just as examples.

Creating your own functions

A function is a named sequence of statements that performs a desired operation. This operation is specified in a function definition.

The syntax for a function definition is:

def FUNCTION_NAME( LIST OF PARAMETERS ):

STATEMENTS

• def is a keyword.
• FUNCTION_NAME could be anything except Python keywords.
• LIST OF PARAMETERS are the ordered list parameters the function needs to operate. The empty parentheses indicate no parameters needed.
• The function body doesn't have starting or ending characters (like "{" and "}" in C like languages ). Indentation is the only scope identifier for statements in Python. The first statement that doesn't follow the indentation, will be considered out of the function and will mark the function termination.

# just to show the function scope and structure

def sample_func():

    Print "we are in the function body scope"

    Print "we still in the function scope"

Print "we are out of the function scope"

# another function with parameters

def sum(x, y):

    print x + y

• To call a function just use the function name followed by the list of parameters between parentheses

>>> sample_func()

>>> sum(1,2)

3

• Functions can call other functions in their body.
• Functions can return values to its caller

def sum(x,y):

return x +y

def print_sum(x,y):

print sum(x,y)

print_sum(2,3)

5

• When you create a local variable inside a function, it only exists inside the function, and you cannot use it outside. The following will generate a run-time error when trying to access Z variable.

def sum(x,y):

Z = x +y

print Z # will generate a runtime error

Returning multiple values

def divide(a,b):

q = a/b

r = a - q*b

return q,r

>>> x,y = divide(42,5) # x = 8, y = 2

>>> x

8

>>> y

2

In this note we talked about variables, statements, expressions, operators, comments, and functions. These are the very basic building blocks of you programs whatever its final size. We will continue our learning in the upcoming notes.

Python Notes – 1 : Setup

I write this notes about Python during my learning process of it. I'm not an expert in Python. I'm just writing this notes while I learn Python aiming to help others who are learning it too. Never hesitate to comment on anything you want.

Origins of Python

• Developed by Guido van Rossum.
• Derived from ABC, a teaching language that was developed in 1980s.

Why Pyhton ?

• Python code is pretty simple, compact and easy to learn.
• Code simplicity let you focus on the core program functionality.
• Suitable for programming languages introductory courses.
• Provides a balance between the practical and the conceptual. You start writing after the first tutorial; and if you want to go further in advanced topics you will find a large library of modules that can be used to do all sorts of tasks ranging from web-programming to graphics.
• Python borrows features from both functional programming languages and object-oriented programming languages, which enables it to serve as an excellent foundation for introducing important computer science concepts.
• Python allow you to see higher level of success and a lower level of frustration :)

How to setup Python ?

Python is free downloadable from many source, and it have also many IDEs and editors. Aiming to start coding quickly without wasting time in IDE exploration, setup, configuration problems; we will use just the ActivePython. Let's see how to do that:

1. Download ActivePython, it is free.
2. After you install ActivePython you will got that the following change in your start menu. We will not use Python Intercative Shell nor PythonWin Editor in our first steps, we will use them in upcoming notes.

     

1. In our first note we will use Python from the Windows Command window. To do that we need to add the Python installation path to the Environment Variables of the operating system.
2. If you are using Windows XP or Windows 2000 do the following (if you are using Windows Vista go for step 5):
   1. Right-click My Computer and select Properties from the displayed context menu. You will got the System Properties dialog

     

1. Then click the Advanced Tab

     

1. Then click the Environment Variables button near the left bottom. You will got the Environment variables dialog

     

1. Select the variable path from the second list called System Variables. Then click Edit button, you will got the Edit System Variable dialog box

     

1. Append the Python installation path to the string in Variable value textbox. The default installation path for ActivePython in our case is C:\Python25\; the semicolon ';' is not part of the path but is mandatory to be added at the end. This semicolon acts as the delimiter that enables the operating system to differentiate between this list of paths.

1. If you using Windows Vista, do the following (very similar steps):
   1. Right-click My Computer and select Properties from the displayed context menu. You will got the System Properties dialog

     

1. Then the link Advanced system settings on the left menu, you will got the System Properties dialog box

     

1. Then click the Advanced tab

     

1. Click the Environment Variables button near the right bottom, you will got Environment Variables dialog box

     

1. Select the variable path from the second list called System Variables. Then click Edit button, you will got the Edit System Variable dialog box

     

1. Append the Python installation path to the string in Variable value textbox. The default installation path for ActivePython in our case is C:\Python25\; the semicolon ';' is not part of the path but is mandatory to be added at the end. This semicolon acts as the delimiter that enables the operating system to differentiate between this list of paths.

Now, we have Python installed and configured on our machines.

Let's begin in Python

Python is a high-level programming language. It is interpreted language, which means that its programs are executed by an interpreter rather than a compiler. There are two ways to use the interpreter:

• Command-line mode where you type your python programs at the command-line and the interpreter prints the result.
  • Open the windows command window and type python, to invoke the python interpreter.
  • Type print "Hello world !" and press enter. Guess what, you have wrote your first program in Python :)

      

• Script mode where you write your program in a file and use the interpreter to execute the contents of the file. Such a file called script. By convention python files have extension .py.
  • To try that mode, create a file script1.py in your Python installation folder ( default is : C:\Python25 ).
  • In this file write print "Hello world !" , save and close.
  • Open the windows command window and type python, to invoke the python interpreter.
  • Type import script1 and press enter. You just ran your first Python program.

      

We will use the script mode in most of our notes.

This is just the beginning of our notes about Python. Now we have Python installed and configured on our machines. We will continue our learning in the upcoming notes.