Conditional Statements

Conditional statements in python allow code to do different things in different circumstances. These statements use the if, elif, and else keywords. The easiest way to show conditional statements in python is through a simple example:

# what will i do?
In [1]: raining = True

In [2]: if raining:  # start of the if statement
   ...:      print('clean the garage')  # what I will do if the condition is true
   ...: else: # start of the otherwise (else) clause
   ...:      print('weed the garden')  # what I will do if raining == False
   ...: 
clean the garage

In human speak this says that if it is raining (which it is), I’ll clean the garage otherwise (else), I’ll weed the garden. Python identifies that something is in the if clause (or the else clause) by the indentation of the code. After the if statement the code is indented by exactly 4 spaces be for the actions (print(‘clean the garage’)). While this may seem pedantic, at the end of the day it creates much clearer, human readable code that is less prone to typos and other errors.

Before we can get onto more complex conditional statements we need to talk about the conditional operators:

Operator	meaning	applies to
==	equals	most objects
!=	does not equal	most objects
<	less than	numbers
>	greater than	numbers
<=	less than or equal	numbers
>=	greater than or equal	numbers
all()	all elements True	iterable of boolean
any()	any element True	iterable of boolean

Armed with these comparisons and the elif keyword we can create much more complicated conditional statements. elif is an abbreviation for else if and allows the user to specify more conditions, For example:

In [3]: weather = 'sunny'

 # what will I do?
In [4]: if weather == 'raining':
   ...:      print('clean the garage')
   ...: elif weather == 'sunny':
   ...:      print('go to the beach')
   ...: elif weather == 'raining hellfire':
   ...:      pass  # the python keyword pass will move past the conditional statement without doing anything
   ...: else:
   ...:      print('weed the garden')
   ...: print('ok then')
   ...: 
go to the beach
ok then