Theory: The result of logical operations

In this lesson, we will learn the rules for converting an argument and how to work with compound expressions and double negation.

Rules of conversion

Take a look at this example:

print(0 or 1)

1

The OR operator interrupts its execution from left to right and returns the result of the first argument, which we can convert to True. When we have no such arguments, the program returns the last one on the right.

Here we see an example with the AND operator:

print(0 and 1)

0

The AND operator interrupts execution from left to right and returns the result of the first argument, which it can convert to `False'. When we have no such argument, the program returns the last one on the right.

There are two conversion rules in Python:

• We should convert to False the 0, 0.0, '', and None. Those values are considered falsy. It includes other types of data that we will study in Hexlet
• We should convert to True everything else

These rules are used in development, for example, to define a default value:

value = name or ''
# Some examples
234 or '' # 234
'hexlet' or '' # 'hexlet'
None or '' # ''

When name takes one of the false values, we assign an empty string to the value variable. In this case, we can treat value as a string in the following code.

However, there is a potential bug. Suppose name contains:

• A falsy value
• The variable value to which we can assign values like 0, false, or none

In this case, the above code will not work correctly:

# The value is there, but it is falsy
# So, we do not select it on the OR condition
False or '' # ''
0 or '' # ''
None or '' # ''

Compound expressions

If you combine logical expressions, you can get some pretty interesting ways of solving problems with code.

Suppose we need to implement some code with a variable that gets:

• The yes string if the number is even
• The no string if it is odd

We can do this by using the knowledge gained above:

# The number is even
result = 10 % 2 == 0 and 'yes' or 'no' # 'yes'
# Or print directly to the screen
print(10 % 2 == 0 and 'yes' or 'no') # => 'yes'
# The number is odd
print(11 % 2 == 0 and 'yes' or 'no') # => 'no'

These expressions work by order and priority. The assignment has the lowest priority, so it comes last. The comparison operator == has a higher priority than the logical operators and and or, so it comes earlier.

Next, the code is executed from left to right, since the priority of and is higher than that of or. Let's look at this step by step:

# For an even number:
# Step 1
10 % 2 == 0 # True
# Step 2
True and 'yes' # The result is true
# The code passes the `or` check and immediately returns `yes`
# So, we do not execute the right part

# For an odd number:
# step 1
11 % 2 == 0 # False
# step 2
False and 'yes' # The result is `false`
# step 3
False or 'no' # The code returns `no`

Also, we can use it with any expression at the beginning:

print(somefunc() and 'yes' or 'no')

Double negations

Let us remember what the negation operation looks like:

answer = True
print(not answer)  # => False

With double negation, the final value is equal to the initial value:

answer = True
print(not not answer)  # => True

The not operator always returns a Boolean value, regardless of the type of argument passed. It does not replace the value with its opposite. Therefore, double negation will also return a boolean True or False:

answer = 'python'
print(not answer) # => False
print(not not answer) # => True

Selection errors

Suppose we need to check whether a value equals one or another. For example, the variable value must contain one of two values: first or second. Beginners sometimes write this expression this way:

value == ('first' or 'second')

However, this kind of code will lead to the wrong result. We must remember the priority of operations. The first thing to evaluate is everything in parentheses like this:

'first' or 'second'

python
Python 3.8.2 (default, Apr 12 2020, 15:53:37)
>>> 'first' or 'second'
'first'
>>>

Now we will substitute the original expression with the partly evaluated one:

value == 'first'

It is not what we expected. Now we go back to the beginning and write the check correctly:

# The priority of '==' is higher than the priority of 'or'
# So, there is no need for brackets
value == 'first' or value == 'second'