Conditional programming

Conditional programming, or branching, is something you do every day, every moment. It's about evaluating conditions: if the light is green, then I can cross; if it's raining, then I'm taking the umbrella; and if I'm late for work, then I'll call my manager.

The main tool is the if statement, which comes in different forms and colors, but basically it evaluates an expression and, based on the result, chooses which part of the code to execute. As usual, let's look at an example:

# conditional.1.py
late = True 
if late: 
    print('I need to call my manager!') 

This is possibly the simplest example: when fed to the if statement, late acts as a conditional expression, which is evaluated in a Boolean context (exactly like if we were calling bool(late)). If the result of the evaluation is True, then we enter the body of the code immediately after the if statement. Notice that the print instruction is indented: this means it belongs to a scope defined by the if clause. Execution of this code yields:

$ python conditional.1.py
I need to call my manager!

Since late is True, the print statement was executed. Let's expand on this example:

# conditional.2.py
late = False 
if late: 
    print('I need to call my manager!')  #1 
else: 
    print('no need to call my manager...')  #2 

This time I set late = False, so when I execute the code, the result is different:

$ python conditional.2.py
no need to call my manager...

Depending on the result of evaluating the late expression, we can either enter block #1 or block #2, but not both. Block #1 is executed when late evaluates to True, while block #2 is executed when late evaluates to False. Try assigning False/True values to the late name, and see how the output for this code changes accordingly.

The preceding example also introduces the else clause, which becomes very handy when we want to provide an alternative set of instructions to be executed when an expression evaluates to False within an if clause. The else clause is optional, as is evident by comparing the preceding two examples.