Python Language Part-2

 Flow Control

Boolean Values:- 

While the integer, floating-point, and string data types have an unlimited number of possible values, the Boolean data type has only two values: True and False. (Boolean is capitalized because the data type is named after mathematician George Boole.) When typed as Python code, the Boolean values True and False lack the quotes you place around strings, and they always start with a capital T or F, with the rest of the word in lowercase. Enter the following into the interactive shell. (Some of these instructions are intentionally incorrect, and they’ll cause error messages to appear).

>>> spam = True

 >>> spam 

True 

>>> true 

Traceback (most recent call last):

 File "", line 1, in

true 

NameError: name 'true' is not defined w

 >>> True = 2 + 2 

SyntaxError: assignment to keyword

Comparison Operators 

Comparison operators compare two values and evaluate down to a single Boolean value. 

==  Equal to 

!= Not equal to 

< Less than 

> Greater than 

<= Less than or equal to 

>= Greater than or equal to

These operators evaluate to True or False depending on the values you give them. As you might expect, == (equal to) evaluates to True when the values on both sides are the same, and != (not equal to) evaluates to True when the two values are different. The == and != operators can actually work with values of any data type.  

The not Operator 

Unlike and and or, the not operator operates on only one Boolean value (or expression). The not operator simply evaluates to the opposite Boolean value. 

>>> not True 

False 

>>> not not not not True 

True

Elements of Flow Control 

Flow control statements often start with a part called the condition, and all are followed by a block of code called the clause. Before you learn about Python’s specific flow control statements, I’ll cover what a condition and a block are. 

Conditions 

The Boolean expressions you’ve seen so far could all be considered conditions, which are the same thing as expressions; condition is just a more specific name in the context of flow control statements. Conditions always evaluate down to a Boolean value, True or False. A flow control statement decides what to do based on whether its condition is True or False, and almost every flow control statement uses a condition. 

Blocks of Code 

Lines of Python code can be grouped together in blocks. You can tell when a block begins and ends from the indentation of the lines of code. There are three rules for blocks. 1. Blocks begin when the indentation increases. 

2. Blocks can contain other blocks. 

3. Blocks end when the indentation decreases to zero or to a containing block’s indentation.

Program Execution 

In the previous chapter’s hello.py program, Python started executing instructions at the top of the program going down, one after another. The program execution (or simply, execution) is a term for the current instruction being executed. If you print the source code on paper and put your finger on each line as it is executed, you can think of your finger as the program execution. Not all programs execute by simply going straight down, however. If you use your finger to trace through a program with flow control statements, you’ll likely find yourself jumping around the source code based on conditions, and you’ll probably skip entire clauses.