Python conditions

Conditional execution is often required in an application as some actions need to be performed only when a certain condition is met.
Example, when dividing two numbers, you need to check the value of numbers and perform division only when the divisor is greater than zero.
Or when creating a menu based application, different actions need to be performed as per the menu option selected by the user.
Python provides if, if-else and if-elif statments to control execution flow based on conditions.

Python if statement
Python if statement is followed by a conditional expression and a set of statements that form part of it.
The conditional statement ends with a colon(:) and should return either True or False.
If it returns True, then the statements that are part of it(also called if-block) are executed else they are by-passed as if they do not exist.
Syntax of python if statement is

if condition:
  # statements
# other code

Python if statement example
An example of if statement follows

x = 5 
if x == 5: 
  print("Condition is true")
print("Outside if block")

Above code contains an if statement  with a conditional statement that compares the value of a variable with 5.
Notice the comparison operator(==) and a colon(:) at the end.
Also, note that the print statement is one tab or a group of spaces away. This is python way of grouping statements together.
It is not necessary to give multiple spaces, a single space will also be sufficient.

Condition of if statement can also be enclosed between parenthesis as if (x == 5):
As explained above, python follows indentations to identify statements that belong to a block. This means all statements at same indentation level are treated as belonging to same block.

Thus, to make multiple statements as part of if block, place all of them at the same indentation level as show below.

x = 5 
if x == 5: 
  # if-block begins
  print("Condition is true") 
  x = x + 10 
  print("x is increased by 10") 
  # if-block ends
print("Not part of if block")

Above code has if-block which contains 3 statements and will be executed only when the condition is True.
Last statement is not part of if block and is executed irrespective of the result of condition.

Python if statement : Multiple conditions
Till now, we saw only one condition after if statement.
But it is also possible to use multiple conditions in if statement. Conditions are either separated by and operator or an or operator.
When conditions are separated by and, this means that when all conditions will be True, then if-block will be executed.
When conditions are separated by or, then any one condition should be True for if-block to be executed. Example,

x = 5 
y = 10 
# two conditions separated by and 
if x == 5 or y == 20: 
  print("x is 5") # will execute
# two conditions separated by or 
if x == 5 and y == 20: 
print("x = 5, y = 20 ") # will not execute

Output of this code will be

x is 5

Python if-elif statment
There are times when you need to test multiple related conditions but execute only one of them.

Example, calculate ticket charges based on the age of a person.
Ticket prices are different for children between 1-5 years, 6-17 years, young persons between age groups 18-30 etc.
One way is to put multiple if-blocks checking each age group. But, the problem with this approach is that all if conditions will be evaluated, which is not correct.
Another way is to use Python if-elif statement which is specially designed for this purpose.

Syntax of if-elif statement is

if condition: 
  statement 1 
  statement 2 
  # more statements 
elif condition: 
  statement 1 
  statement 2

Syntax is same as if statement in that, every if is followed by a condition and a set of statements.
Statements will be executed only when the condition of if statement evaluates to True. Also, all statements at same indentation level after if are treated to be of same block.
Next condition is given using elif statement whose syntax is same as that of if statement.

There may be any number of elif statements after an if statement. Also, if one elif statement is executed, other elif  statements are not tested.

Python if-elif example

age = 5 
if 1 <= age <= 5: 
  print('Age between 1 and 5') 
elif 6 <= age <= 17: 
  print('Age between 6 and 17') 
elif 18 <= age <= 30: 
  print('Age between 18 and 30')

Above code checks different age groups and executes print statement of the block whose condition returns True.
Remember that only one block will be executed. That is the main benefit of if-elif statement.

Python else statement
There may be some set of statements that should be executed,  when none of the if or elif blocks are executed. These statements are written inside an else block.
else block can be used with a single if block or if-elif blocks.

When used with any of these, else block should always be placed at last since it will be executed only when all other conditions evaluate to False. Example,

age = 0 
if 1 <= age <= 5: 
  print('Age between 1 and 5') 
elif 6 <= age <= 17: 
  print('Age between 6 and 17') 
elif 18 <= age <= 30: 
  print('Age between 18 and 30') 
elif 30 < age: 
  print('Age more than 30') 
  print('Age not valid!')



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