Python Programming Essentials - M5 - Variables

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Variables
Pavan Verma
@YinYangPavan
Python Programming Essentials

© SkillBrew http://skillbrew.com
What is a variable
2
>>> print 100
100
>>> counter = 100
>>> print counter
100

What is a variable (2)
3
• A variable is a memory location that stores a value
• The value of a variable can change. That’s why it’s
called a variable!
• A variable is the basic unit of storing data in a
computer program
100counter
Somewhere in memory

What is a variable (3)
4
>>> name = 'Henry Ford'
>>> print name
Henry Ford
• Variables can store values of different types
• In this slide, name is a variable of type ‘string’
• In previous slide, counter is a variable of type
‘int’

Multiple variable assignments in one statement
5
>>> length = height = breadth = 2
>>> length
2
>>> height
2
>>> breadth
2
2length
2breadth
2height

Multiple variable assignments in one statement
6
>>> x, y = 2, 3
>>> x
2
>>> y
3
2
3y
x

Standard data types
Standard Data Types
Number String List Tuple Dictionary
7

Standard data types (2)
>>> length = 10 #integer
>>> length
10
>>> motive = 'to learn Python' #string
>>> motive
'to learn Python'
8

>>> colors = ['brown', 'black', 'orange']
#list
>>> colors
['brown', 'black', 'orange']
>>> logs = ('skillbrew.com', 500) #tuple
>>> logs
('skillbrew.com', 500)
9

>>> contacts = {'Monty': 123445, 'Guido':
67788} #dictionary
>>> contacts
{'Monty': 123445, 'Guido': 67788}
10

type function
>>> var = "foo"
>>> type(var)
<type 'str'>
>>> type(8)
<type 'int'>
>>> type(9.0)
<type 'float'>
11

type function
>>> type([1, 2, 3])
<type 'list'>
>>> type((1, 2, 3))
<type 'tuple'>
>>> type({'1': 'one'})
<type 'dict'>
12

Python is a dynamically typed language
 Variable type is determined at runtime
 A variable is bound only to an object
and the object can be of any type
 If a name is assigned to an object of
one type it may later be used to an
object of different type
13
Variable
Object
Type
is of

Python is a dynamically typed language (2)
14
>>> x = 10
>>> x
10
>>> type(x)
<type 'int'>
>>> x = 'foo'
>>> x
'foo'
>>> type(x)
<type 'str'>
>>> x = ['one', 2, 'three']
>>> type(x)
<type 'list'>

Dynamically typed vs Statically typed
 The way we used variable in previous slide
won’t work in statically typed languages
like C, C++, Java
 In a statically typed language, a variable is
bound to:
• an object – at runtime
• a type – at compile time
15
Variable
Object
Type
is of
Type
must match

Statically Typed (C/C++/Java)
int x;
char *y;
x = 10;
printf(“%d”, x)
y = “Hello World”
printf(“%s”, y)
Dynamically Typed – Python
x = 10
print x
x = “Hello World”
print x
16
Dynamically typed vs Statically typed (2)

Statically Typed (C/C++/Java)
 Need to declare variable
type before using it
 Cannot change variable
type at runtime
 Variable can hold only
one type of value
throughout its lifetime
Dynamically Typed – Python
 Do not need to declare
variable type
 Can change variable type
at runtime
 Variable can hold
different types of value
through its lifetime
17
Dynamically typed vs Statically typed (3)

Python is a strongly typed language
>>> 10 + "10"
TypeError: unsupported operand type(s) for
+: 'int' and 'str'
>>> 10 + int("10")
20
18
In a weakly typed language (eg. perl), variables can be
implicitly coerced to unrelated types
Not so in Python! Type conversion must be done explicitly.

Variable naming rules
 Variable names must begin with a letter [a-zA-
Z] or an underscore (_)
 Other characters can be letters[a-zA-Z],
numbers [0-9] or _
 Variable names are case sensitive
 There are some reserved keywords that cannot
be used as variable names
19

Variable naming rules (2)
20
length
Length
myLength
_x
get_index
num12
1var

>>> 1var = 10
SyntaxError: invalid syntax
>>> var1 = 10
>>> v1VAR = 10
>>> myVar = 10
>>> _var = 10
>>> Var = 10
21

>>> myvar = 20
>>> myVar
10
>>> myvar
20
>>> myVar == myvar
False
22

Accessing non-existent name
 Cannot access names before declaring it
23
>>> y
Traceback (most recent call last):
File "<pyshell#46>", line 1, in <module>
y
NameError: name 'y' is not defined

Reserved keywords
and
del
from
not
while
as
elif
global
or
with
assert
else
if
pass
yield
break
except
import
print
class
exec
in
raise
continue
finally
is
return
def
for
lambda
try
24

Cannot use reserved keywords as variable names
>>> and = 1
>>> if = 1
>>>
25

Summary
 What is a variable
 Different types of variable assignments
 Brief introduction to standard data types
 type function
 Python is a dynamically typed language
 Dynamically types versus statically types languages
 Python is a strongly typed language
 Variable naming rules
 Reserved keywords
26

Resources
 Python official tutorial
http://docs.python.org/2/tutorial/introduction.html
 Blog post on python and Java comparison
http://pythonconquerstheuniverse.wordpress.com/2009/10/03/pyt
hon-java-a-side-by-side-comparison/
 Python guide at tutorialspoint.com
http://www.tutorialspoint.com/Python/Python_quick_guide.html
 Python type function
http://docs.Python.org/2/library/functions.html#type
 Blog post on static vs dynamic languages
http://Pythonconquerstheuniverse.wordpress.com/2009/10/03/stat
ic-vs-dynamic-typing-of-programming-languages/
27

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