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What is an if statement?

Conditional execution.
As it stands right now, we've seen Python as a series of statements that we execute one after another. However, we may wish to, on occasion, conditionally execute a piece of code. For example, if a variable a is set to 1, we want to print "Hello!", print "World!" if it is set to 2, or print "Bye" otherwise. We can do so through if statements.
if (a == 1):
print("Hello!")
elif (a == 2):
print("World!")
else:
print("Bye")
In an if-else block, Python will only ever execute one suite of statements.
Something fascinating happens in if-statements. Not only does Python not run the statements for which the condition is not True, it does not even execute them — errors are ignored. Consider the following:
a = 1
if (a == 1):
print("Hello!")
else:
print(1/0)
This code does not error, because Python never ends up reading the else-block — the condition is True, and the if suite is executed instead.

Evaluating if Statement Conditions

In our section on Data Types, we had talked about truth-y and false-y values. If conditions basd on such values check for their truthiness or falsiness by the same design. For example
if a:
print("Hello")
else:
print("World")
Setting a=1 would print Hello, while setting a=0 would print World.