Most of the time in programming, our programs execute one line at a time. So for example we might see something like this:
name = input('Enter your name => ') # This statement runs first print('Hello ', name) # Now this statement runs next
Sometimes our programs hit a fork in the road and they need to make a decision about the direction they are going to take. Python gives us conditional statements to help make such decision.
A conditional statement is a condition that changes the course of the program’s execution based on a true or false evaluation. Let’s start with a code example to help make this more clear.
gender = 'F' name = input('Enter your name => ') if gender == 'F': print('Hello Ms. ', name) # This code only executes for women # (gender == 'F') else: print ('Hello ', name) # This code executes for anyone else # (gender != 'F')
This code snippet demonstrates a conditional statement. We begin with a variable named
gender that is set to the value ‘F’ for female. Ideally, if we know the gender of a person, we should offer the correct greeting for that person.
if gender == 'F': is the conditional statement. What this statement is doing is using a true/false expression to decide if we should print ‘Hello Ms.’ instead of just the standard ‘Hello’ greeting. If our gender variable is ‘F’, we will print the ‘Hello Ms.’, otherwise we print the ‘Hello’ greeting.
We can handle multiple conditions with the
elif keyword. Let’s build on our example:
gender = 'F' name = input('Enter your name => ') if gender == 'F': # First check if the gender is F print('Hello Ms. ', name) # This code only executes for women # (gender == 'F') elif gender == 'M': # Now check if gender is M print ('Hello Mr. ', name) # This time print a greeting for men # (gender == 'M') else: print ('Hello ', name) # This code executes for anyone else # (gender != 'F')
elif keyword is a way to stack up multiple conditions and define an action for each condition. Now I should point out at this time that
else are completely optional. So it is ok to do this:
gender = 'F' if gender == 'F': print('Found female') print ('Done checking for m/f') # At this point, the code continues # executing normally
We can also check for false conditions by using the
gendar = 'F' if gender != 'M': # Execute the code inside of the if block if # if the gender is not male print('Found female') print('Done checking for m/f') # Return to normal execution