Combining Boolean Expressions with and, or, and not

From One Question to Several

A boolean expression is an expression that evaluates to True or False. These two values belong to Python's bool data type. A single comparison asks one question, such as whether one value is greater than another. Logical operators let you combine such questions into a larger boolean expression.

temperature > 0 and temperature < 30

The expression above contains two comparisons joined by and. To understand the final result, evaluate each comparison first, then apply the logical operator that joins them.

How Logical Operators Combine Results

The operator and produces True only when both boolean expressions on its sides are True. The operator or produces True when at least one of its two expressions is True. The operator not reverses a boolean result: not True becomes False, and not False becomes True. Each smaller expression should be understood as producing a boolean value before you determine the result of the whole expression.

Tracing a Combined Expression

True

Building the Comparisons

Every comparison expression has a left operand, a comparison operator, and a right operand. Python examines the two operands, applies the operator's rule, and produces True or False. These comparison results can then be combined with and, or, or not.

5 > 5
5 >= 5
5 < 5
5 <= 5

False
True
False
True

Assignment and Comparison

The single equals sign, =, assigns a value. The double equals sign, ==, compares values and produces a boolean result. These operators are not interchangeable. Writing = when you intend a comparison is a common and critical error.

Equality and Identity

The operators == and is both compare two operands, but they ask different questions. The == operator compares the values of the operands. The is operator checks whether the operands refer to the exact same object in memory. Two objects can contain equal values without being the same object.

first = [1, 2]
second = [1, 2]

first == second
first is second

True
False

In this example, first and second contain equal values, so == produces True. They are nevertheless separate objects, so is produces False. For simple values such as numbers and strings, the distinction often does not matter, but for more complex objects it can produce different results.

Mistakes to Avoid

• Using = when you mean ==.

  The single equals sign assigns a value; it does not compare two values.

  Fix: Use score == 5 when you want a boolean comparison.

• Treating > and >= as interchangeable.

  The > operator is exclusive and does not return True when the operands are equal.

  Fix: Use >= when equality should also count.

• Using is to test whether two values are equal.

  The is operator tests whether two operands refer to the same object, not merely whether their values are equal.

  Fix: Use == for value equality.

• Trying to decide the combined result before evaluating its component comparisons.

  A combined expression depends on the boolean result of each comparison.

  Fix: Evaluate each comparison first, then apply and, or, or not.

Practice Before Execution

Reliable Evaluation Routine

1. Separate the expression into its individual comparisons.
2. Identify the left operand, operator, and right operand in each comparison.
3. Check whether each comparison produces True or False.
4. Apply and, or, or not to those boolean results.
5. Check whether you used = for assignment or == for value comparison.
6. If you used is or is not, confirm that you are asking about object identity rather than value equality.

1. Boolean expressions produce True or False. Comparison expressions have a left operand, an operator, and a right operand. Use and when both conditions must be True, or when at least one condition may be True, and not to reverse a boolean result. Remember that = assigns, while == compares values. Python's eight comparison operators are ==, !=, >, <, >=, <=, is, and is not. Finally, == asks whether values are equal, while is asks whether two operands refer to the same object.