A conditional statement, often called an “if”-statement, consists of a test and one or more actions. The test is a so-called “boolean expression.” The actions are executed when the test evaluates to True. For instance, an app on a smartphone might give a warning if the battery level is lower than 5%. This means that the app needs to check if a certain variable battery_level is lower than the value \(5\), i.e., if battery_level < 5 evaluates to True. If the variable battery_level holds the value \(17\), then battery_level < 5 evaluates to False.

Booleans

True and False are so-called “boolean values” that are predefined in Python. True and False are actually the only boolean values, and anything that is not False, is True.

You might wonder what the data type of True and False is. The answer is that they are of the type bool. However, in Python every value can be interpreted as a boolean value, regardless of its data type. I.e., when you test a condition, and your test is of a value that is not True or False, it will still be interpreted as either True or False.

The following values are interpreted as False:

• The special value False

• The special value None (which you encountered in the chapter “Simple functions”

• Every numerical value that is zero, e.g., 0 and 0.0

• Every empty sequence, e.g., an empty string ("")

• Any function or method call that returns one of these listed values (this includes functions that return nothing)

Every other value is interpreted as True.

Any expression that is evaluated as True or False is called a “boolean expression.”

Comparisons

The most common boolean expressions are comparisons. A comparison consists of two values, and a comparison operator in between. Comparison operators are:

A common mistake is to use a single \(=\) as a comparison operator, as the single \(=\) is the assignment operator. In general, Python will produce a syntax or runtime error if you try to use a single \(=\) to make a comparison.

You can use the comparison operators to compare both numbers and strings. Comparison for strings is an alphabetical comparison, whereby all capitals come before all lower case letters (and digits come before both of them). More on this will follow in the chapter “Strings”.

Here are some examples of the results of comparisons:

print( "1.", 2 < 5 )
print( "2.", 2 <= 5 )
print( "3.", 3 > 3 )
print( "4.", 3 >= 3 )
print( "5.", 3 == 3.0 )
print( "6.", 3 == "3" )
print( "7.", "syntax" == "syntax" )
print( "8.", "syntax" == "semantics" )
print( "9.", "syntax" == " syntax" )
print( "10.", "Python" != "rubbish" )
print( "11.", "Python" > "Perl" )
print( "12.", "banana" < "orange" )
print( "13.", "banana" < "Orange" )

Make sure that you run these evaluations, check their outcome, and understand them!

Do you understand why 3 < 13 is True, but "3" < "13" is False? Think about it!

You can assign the outcome of a boolean expression to a variable if you like:

greater = 5 > 2
print( greater )
greater = 5 < 2
print( greater )
print( type( greater ) )

Write some code that allows you to test if \(1/2\) is greater than, equal to, or less than \(0.5\). Do the same for \(1/3\) and \(0.33\). Then do the same for \((1/3)*3\) and \(1\).

Comparisons of data types that cannot be compared, in general lead to runtime errors.

# This code gives a runtime error.
print( 3 < "3" )

in operator

Python has a special operator called the “membership test operator,” which is usually abbreviated to the “in operator” as it is written as in. The in operator tests if the value to the left side of the operator is found in a “collection” to the right side of the operator.

At this time, I have discussed only one “collection,” which is the string. A string is a collection of characters. You can test if a particular character or a sequence of characters is part of the string using the in operator. The opposite of the in operator is the not in operator, which gives True when in gives False, and which gives False when in gives True. For example:

print( "y" in "Python" )
print( "x" in "Python" )
print( "p" in "Python" )
print( "th" in "Python" )
print( "to" in "Python" )
print( "y" not in "Python" )

Again, make sure that you understand these evaluations!

Write some code that allows you to test for each vowel whether it occurs in your name. You may ignore capitals.

Logical operators

Boolean expressions can be combined with logical operators. There are three logical operators, and, or, and not.

and and or are placed between two boolean expressions. When and is between two boolean expressions, the result is True if and only if both expressions evaluate to True; otherwise it is False.

When or is between two boolean expressions, the result is True when one or both of the expressions evaluate to True; it is only False if both expressions evaluate to False.

not is placed in front of a boolean expression to switch it from True to False or vice versa.

For example:

t = True
f = False
print( t and t )
print( t and f )
print( f and t )
print( f and f )
print( t or t )
print( t or f )
print( f or t )
print( f or f )
print( not t )
print( not f )

You have to be careful with logical operators, because combinations of ands and ors might lead to unexpected results. To ensure that they are evaluated in the order that you intend, you can use parentheses. For example, rather than writing a and b or c you should write (a and b) or c or you should write a and (b or c) (depending on the order in which you want to evaluate the logical operators), so that it is immediately clear from your code which evaluation you want the code to do. Even if you know the order in which the logical operators are processed by Python, someone else who reads your code might not.

For the code below, give values True or False to each of the variables a, b, and c, so that the two expressions evaluate to different values.

a = # True or False?
b = # True or False?
c = # True or False?

print( (a and b) or c )
print( a and (b or c) )

If all the logical operators in a boolean expression are and, or they all are or, the use of parentheses is not needed, since there is only one possible evaluation of the expression.

Boolean expressions are processed from left to right, and Python will stop the processing of an expression when it already knows whether it will end in True or False. Take, for instance, the following code:

x = 1
y = 0
print( (x == 0) or (y == 0) or (x / y == 1) )

When you divide by zero, Python gives a runtime error, so the expression x / y == 1 crashes the program if y is zero. And y actually is zero. However, when Python processed the whole boolean expression, at the point where it tested y == 0 it determined that the expression as a whole is True, because if any of the expressions that are connected by an or to the expression as a whole is True, then the whole expression is True. So there was no need for Python to determine the value of x / y == 1, and it did not even attempt to evaluate it. Of course, the test y == 0 must be to the left of x / y == 1, so that Python will test y == 0 first.

Note: While you can make truly complex boolean expressions using logical operators, I recommend that you keep your expressions simple if possible. Simple boolean expressions make code readable.