Python conditionals

Python conditionals#

Conditional statements give you a way to change the flow of a Python program depending on the result of a boolean expression.

Bolean expressions#

Let’s first see what are boolean expressions. Boolean expressions are Python expressions that return either True of False. The simplest ones are:

True

True

False

False

But here are some more:

1 > 2

False

25.0 <= 25.1

True

5 == 5

True

Or with variables:

x = 'This is a string.'
len(x) > 10

True

Here are a few things with strings:

x == 'This is a string.'

True

x == 'This is a st'

False

'This' < 'This is'

True

So, what can you put in a boolean expression? Well, anything that can be evaluated to either True or False. This is quite general, but very often we use the following building components:

  • >: greater than

  • >=: greater than or equal to

  • ==: equal to

  • < less than

  • <= less than or equal to

  • not gives you the opposite of whatever boolean expression follows

  • or True if any of the boolean expressions to the left or to the right are True

  • and True if both of the boolean expressions to the left and to the right are True

You absolutely have to remember all these!!!

The simplest if-statement#

Now, let’s look at the simplest conditional statement. It’s syntax is as follows:

if <boolean_expression>:
    # Expressions that run if the bolean_expression is True

That’s it. Let’s try it:

x = 'Sort string'
if len(x) <= 20:
    print('The string x has less than 20 characters.')

The string x has less than 20 characters.

The following does not print anything for the default x because the boolean expression is False:

if not len(x) <= 20:
    print('The string x does not have less than 20 characters.')

And here is a somewhat more complicated boolean expression:

if len(x) >= 10 and len(x) <= 30:
    print('The string x have between 10 and 30 characters.')

The string x have between 10 and 30 characters.

Questions#

  • Change the string x in the code block above so that nothing is printed in the first if and something is printed in the second if.

Be careful of white spaces#

Note

In Python, the empty spaces below if are very important when you have multiple expressions.

Here is an example that works:

if len(x) <= 20:
    print('The string x has less than 20 characters.')
    print('And this is an additional line.')

The string x has less than 20 characters.
And this is an additional line.

Here is an example that doesn’t work because the white spaces are not aligned:

if len(x) <= 20:
    print('The string x has less than 20 characters.')
  print('And this is an additional line.')

  File "<tokenize>", line 3
    print('And this is an additional line.')
    ^
IndentationError: unindent does not match any outer indentation level

By the way, I am using four spaces below if’s. It doesn’t matter how many spaces you use as soon as you are consistent. For example, here is the same code with just one space:

if len(x) <= 20:
 print('The string x has less than 20 characters.')
 print('And this is an additional line.')

The string x has less than 20 characters.
And this is an additional line.

The best practice (and what Jupyter is trying to do by default) is to use a tab or 4 white spaces.

The if-else statement#

Some times you want to test for a boolean expression and run something else if it is false. You can do this with the if-else statement:

if <boolean_expression>:
    # Expressions that run if the bolean_expression is True
else:
    # Expressions n to run if the boolean_expression is False

Let’s try it:

x = 'Sort string'
if len(x) <= 20:
    print('The string x has less than 20 characters.')
else:
    print('The string x does not have less than 20 characters.')

The string x has less than 20 characters.

Questions#

  • Change x in the code block above so that the expressions after else run.

The if-elif-else statement#

Sometimes you want to test for multiple boolean expressions. You can do this using the if-elif-else statement:

if <boolean_expression>:
    # Expressions that run if the bolean_expression is True
elif <other_boolean_expression>:
    # Expressions that run if the other_bolean_expression is True
    # if boolean_expression is False
else:
    # Expressions n to run otherwise

An example:

x = 'Sort string'
if len(x) <= 20:
    print('The string x has less than 20 characters.')
elif len(x) <= 30:
    print('The string x has between than 21 and 30 characters.')
else:
    print('The string x has more than 30 characters.')

The string x has less than 20 characters.

Note that you can have as many elif’s as you want and that else is always optional.

Questions#

  • Change the string x in the above code block so that the elif expression runs.

  • Change the string x in the above code block so that the else expression runs.

  • Add one more elif to test if len(x) <= 40. Modify the print() statements accordingly.

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