part0006

Chapter 3 : Python variables, Datatypes & Operators

Important points to note are:

Python variables are used to hold data.

In most programming languages, you need to declare the data type of a variable. For example: int x = 5, String y = “John” etc, where int refers to an integer value and String refers to a sequence of character value.

But in Python, you do not have to declare the data type of a variable. Python understands the data type of any variable simply from its value.

Let us consider the example below:

Output

When we run the above piece of code, Python shows the data types of variable x , y and z .

What is type( ) function?

type( ) function is used to determine the data type of a variable.

Apart from data types like int , float and string , Python also has list , tuple and dictionaries .

3.1: Python String

Important points to note are:

Python string is a sequence of characters.

Python string is immutable meaning that once a string is created it cannot be modified.

3.1.1: What is String Slicing in Python?

Python String slicing is the process of obtaining a sub string of the given string.

Syntax:

string [ index_number_start (including its value) : index_number_stop (excluding its value) ]

Example:

Open Notepad++ and create a new python file and write the following lines of code.

Now run the above piece of code:

Code explanation:

Let’s look into the message “Health is wealth”.

Index number always starts from 0. Letter H is present at index number 0, letter e is present at index number 1, letter a is present at index number 2, letter l is present at index number 3, letter t is present at index number 4, letter h is present at index number 5. Then whitespace is present at index number 6. Letter i is present at index number 7, letter s is present at index number 8. Then again whitespace is present at index number 9. Letter w is present at index number 10, Letter e is present at index number 11, Letter a is present at index number 12, Letter l is present at index number 13, Letter t is present at index number 14, Letter h is present at index number 15.

In the above piece of code, the output of message[2:12] is : print values from 2 (including its value at index position 2) to 12 (excluding its value at index position 12) alth is we

The output of message[:6] is : print values from start (index position 0, including its value) to 6 (excluding its value at index position 6) Health

The output of message[7:] is : print values from 7 (including its value at index position 7) to end is wealth

To print letters or values from the end of a string, minus sign is used. For example: message[-1] will return h , message[-2] will return t and so on.

3.1.2: Important and commonly used String methods

len( ) function returns the length of the string.

Syntax: len(string)

lower( ) function returns the string with lower case characters.

Syntax: string.lower()

upper( ) function returns the string with upper case characters.

Syntax: string.upper()

strip( ) function gets rid of left or right whitespace.

Syntax: string.strip()

lstrip( ) function gets rid of left whitespace.

Syntax: string.lstrip()

rstrip( ) function gets rid of right whitespace.

Syntax: string.rstrip()

index( ) function returns the index number of the given element.

Syntax: string.index(element )

Output:

split( ) function returns the list of substrings separated by whitespace.

Syntax: string.split( )

Output:

replace(old,new) function replaces the old with new.

Syntax: string.replace(old,new)

Output:

join( ) string method returns a string by joining all the elements of an iterable (lists, tuples and string) with the delimiter.

Syntax: delimiter.join(iterable)

Output:

3.1.3: String formatting

format( ) method is used to format a string.

Rules for string formatting:

Create a placeholder with the help of curly brackets {}.
Call the format method and pass variables as parameters. When program executes, the values passed to these parameters will replace the curly brackets placeholders.

Example 1:

In the above example, the values passed to variable animal will fill the first placeholder and the value passed to variable legs will fill the second placeholder.

Output:

Example 2:

If you want to display a float number with two digits after the decimal dot, formatting expression {:.2f} is used.

Output:

3.2: Python List

● A Python List is very similar to an Array . It contains a list of elements separated by commas and its elements are written with square brackets [ …. ] .

● Python list are mutable meaning that we can modify list elements.

● A value from a List can be accessed from its index value.

Syntax for accessing the values from a List is:

List_name [ index_num ]

What is an Array?

An Array is a collection of items or elements all having the same datatype. An element from an array can only be accessed from its index value. Example:

Let’s create an array of cars.

cars = [“Kia”, “Toyota”, “Ford”, “Tesla”]

In order to get the value Ford as output, we need to write print ( cars [ 2 ] )

Example:

Create a new Python file (variables.py ) and write the following lines of code:

After running the above piece of code, we get an output of:

Code explanation:

● In the above piece of code, fruits is a List containing 6 elements. The value at position 0 is “apple ”, at position 1 is “orange ” and so on.

● x is a variable holding a string value. The value at position 0 is “t” , at position 1 is “h ” and so on.

● len( ) function is used to get the length of the list .

● In Line 4 and 5 we are performing Slicing operation .

print ( fruits [ 2 : 4] ) means print elements from position 2 (including its value ) to position 4 (excluding its value ).

Output: [‘banana’, ‘mango’]

print ( x [ 2 : 8 ] ) means print the values of x (“this apple is tasty ”) from position 2 (including its value ) to position 8 (excluding its value ).

Output: is app

3.2.1: Important and commonly used List methods

append( ) method inserts the element at the end of the list.

Syntax: list.append(element )

Output:

insert( ) method inserts an element at the specified index number.

Syntax: list.index(index_num , element )

Output:

remove( ) method removes the first occurrence of element specified.

Syntax: list.remove(element )

Output:

3.3: Python Tuple

● A Tuple is very much like a List . It contains elements separated by commas within open and close parenthesis ( …. ) .

● Python tuple are immutable meaning once a tuple is declared, it cannot be modified.

Difference between Tuple and List

Tuple	List
Tuple contains elements within (… ) parenthesis	List contains elements within [ …] square brackets
Once a tuple is created, no item can be added, updated or deleted	List gives us the ability to add, update and delete items.

Example:

After running the above piece of code, we get an output of:

Code explanation:

del ( ) function is used to delete element from a certain position. When applying delete function to fruits list , the value at position 1 (orange ) was deleted successfully. But in case of tuple , when we tried to delete an element at position 2, it threw an error.

3.3.1: What is unpacking a tuple mean?

When we create a tuple and assign new values into it. This is called packing a tuple.

When we extract those values and store them into a variable. This is called unpacking a tuple.

Output:

3.4: Python Dictionary

Python dictionaries are used to store key - value pairs where the key should be a unique value.

A dictionary is written within { … } curly braces.

Python dictionary are mutable meaning that its content can be modified.

Example:

After running the above piece of code, we get an output of:

3.4.1: Important Dictionary operations and methods

Now let’s insert some new records.

To insert new records into the dictionary, the syntax is:

dictionary_name [ key ] = value

Output:

Update a record

To update a record, the syntax is same as insert a new record.

Output:

In the above piece of code, we see the new value (70 ) assigned to key John replaces the old value (95 ).

Delete a record

To delete a record from a dictionary, del keyword is used. Syntax:

del dictionary_name [ key ]

Important Dictionary Methods:

1. To iterate through dictionary, items( ) method is used. Syntax: dictionary.items( ) (example present in chapter 5 )

2. To display only dictionary keys, key( ) method is used.

Syntax: dictionary.keys( )

3. To display only dictionary values, values( ) method is used.

Syntax: dictionary.values( ).

3.5: Python Operators

3.5.1: Commonly used Arithmetic operators

Operator	Description
+	Addition
-	Subtraction
*	Multiplication
/	Division
%	Modulus. This sign returns the remainder.
**	Exponentiation. For example: 2 ** 3 means 2 to the power of 3 and that is equal to 8
//	Floor division. This operator returns only the whole integer number. For example: 23 // 2 will give 11

3.5.2: Commonly used Comparison Operators

Operator	Description
= =	Equals: x = = y
!=	Not Equals: x != y
<	Less than: x < y
<=	Less than or equal to: x <= y
>	Greater than: x > y
>=	Greater than or equal to: x >= y

3.5.3: Commonly used Logical Operators

Operator	Description
and	Example: x = 123 and y = “John” executes certain block of code if both statements are TRUE
or	Example: x = 123 or y = “John” executes certain block of code if one of the statements is TRUE
in	Example: x = [ 123 , 345 ] if 123 in x returns TRUE, then executes certain block of code
not in	Example: x = [ 123 , 345 ] if 12 not in x returns TRUE, then executes certain block of code

3.5.4: Commonly used Assignment Operators

Operator	Description
=	Example: x = 2, this means value 2 is assigned to x
+= (x += 1)	This is same as x = x + 1. If value of x = 3, then the new value of x is 3 + 1 = 4
-= (x -= 2)	This is same as x = x – 2. If value of x = 5, then the new value of x is 5 – 2 = 3
= (x = 5)	This is same as x = x * 5. If value of x = 2, then the new value of x is 2 * 5 = 10
/= (x /= 2)	This is same as x = x / 2. If value of x is 10, then the new value of x is 10 / 2 = 5
%= (x %= 2)	This is same as x = x % 2. If value of x is 10, then the new value of x is 10 % 2 = 0