Python If . Else
Python supports the usual logical conditions from mathematics:
- Equals: a == b
- Not Equals: a != b
- Less than: a < b
- Less than or equal to: a
- Greater than: a > b
- Greater than or equal to: a >= b
These conditions can be used in several ways, most commonly in «if statements» and loops.
An «if statement» is written by using the if keyword.
Example
In this example we use two variables, a and b , which are used as part of the if statement to test whether b is greater than a . As a is 33 , and b is 200 , we know that 200 is greater than 33, and so we print to screen that «b is greater than a».
Indentation
Python relies on indentation (whitespace at the beginning of a line) to define scope in the code. Other programming languages often use curly-brackets for this purpose.
Example
If statement, without indentation (will raise an error):
Elif
The elif keyword is Python’s way of saying «if the previous conditions were not true, then try this condition».
Example
In this example a is equal to b , so the first condition is not true, but the elif condition is true, so we print to screen that «a and b are equal».
Else
The else keyword catches anything which isn’t caught by the preceding conditions.
Example
a = 200
b = 33
if b > a:
print(«b is greater than a»)
elif a == b:
print(«a and b are equal»)
else:
print(«a is greater than b»)
In this example a is greater than b , so the first condition is not true, also the elif condition is not true, so we go to the else condition and print to screen that «a is greater than b».
You can also have an else without the elif :
Example
Short Hand If
If you have only one statement to execute, you can put it on the same line as the if statement.
Example
Short Hand If . Else
If you have only one statement to execute, one for if, and one for else, you can put it all on the same line:
Example
One line if else statement:
This technique is known as Ternary Operators, or Conditional Expressions.
You can also have multiple else statements on the same line:
Example
One line if else statement, with 3 conditions:
And
The and keyword is a logical operator, and is used to combine conditional statements:
Example
Test if a is greater than b , AND if c is greater than a :
Or
The or keyword is a logical operator, and is used to combine conditional statements:
Example
Test if a is greater than b , OR if a is greater than c :
Not
The not keyword is a logical operator, and is used to reverse the result of the conditional statement:
Example
Test if a is NOT greater than b :
Nested If
You can have if statements inside if statements, this is called nested if statements.
Example
if x > 10:
print(«Above ten,»)
if x > 20:
print(«and also above 20!»)
else:
print(«but not above 20.»)
The pass Statement
if statements cannot be empty, but if you for some reason have an if statement with no content, put in the pass statement to avoid getting an error.
Python IF. ELIF. ELSE Statements
An else statement can be combined with an if statement. An else statement contains the block of code that executes if the conditional expression in the if statement resolves to 0 or a FALSE value.
The else statement is an optional statement and there could be at most only one else statement following if.
Syntax
The syntax of the if. else statement is −
if expression: statement(s) else: statement(s)
Flow Diagram
Example
#!/usr/bin/python var1 = 100 if var1: print "1 - Got a true expression value" print var1 else: print "1 - Got a false expression value" print var1 var2 = 0 if var2: print "2 - Got a true expression value" print var2 else: print "2 - Got a false expression value" print var2 print "Good bye!"
When the above code is executed, it produces the following result −
1 - Got a true expression value 100 2 - Got a false expression value 0 Good bye!
The elif Statement
The elif statement allows you to check multiple expressions for TRUE and execute a block of code as soon as one of the conditions evaluates to TRUE.
Similar to the else, the elif statement is optional. However, unlike else, for which there can be at most one statement, there can be an arbitrary number of elif statements following an if.
syntax
if expression1: statement(s) elif expression2: statement(s) elif expression3: statement(s) else: statement(s)
Core Python does not provide switch or case statements as in other languages, but we can use if..elif. statements to simulate switch case as follows −
Example
#!/usr/bin/python var = 100 if var == 200: print "1 - Got a true expression value" print var elif var == 150: print "2 - Got a true expression value" print var elif var == 100: print "3 - Got a true expression value" print var else: print "4 - Got a false expression value" print var print "Good bye!"
When the above code is executed, it produces the following result −
3 - Got a true expression value 100 Good bye!