Comparison Operators in Python

The Decision Point

Programs often need to respond differently depending on whether a condition is true or false. In Python, an if-else statement provides two mutually exclusive branches: one branch runs when the condition is true, and the other runs when the condition is false. This is called alternative execution.

Conditions Become Decisions

A comparison condition evaluates to a Boolean result: true or false. That result controls which branch Python follows. For example, a condition can compare two values with an operator such as ==, <, or >. The important control-flow question is not which comparison operator was used, but whether the completed condition is true or false.

Two Mutually Exclusive Paths

After Python evaluates the condition, control moves into only the matching branch. The true branch runs when the condition is true. The else branch runs when the condition is false. Python does not run both branches during the same evaluation of the if-else statement.

score = 72

if score > 50:
    result = "pass"
else:
    result = "try again"

print(result)

pass

Tracing a Branch

temperature = 18 if temperature > 20: message = "warm" else: message = "cool" print(message) print("Finished")

Writing the Structure Correctly

An if-else statement requires a colon after if and after else. The statements belonging to each branch must be indented consistently. The indentation shows which statements belong to the if branch and which belong to the else branch.

if condition:
    statement_for_true_case
else:
    statement_for_false_case

Mistakes That Change the Path

• Leaving out the colon after if or else.

  The if and else parts of the structure require a colon.

  Fix: Write if score > 50: and include a colon after else as well.

• Using inconsistent indentation inside the branches.

  Both blocks must be indented consistently so Python can identify the statements belonging to each branch.

  Fix: Use consistent indentation for the statements under if and else.

• Tracing both branches instead of selecting one.

  The two branches are mutually exclusive. Exactly one branch runs.

  Fix: Evaluate the condition first, then follow only the branch matching the result.

• Forgetting that execution continues after the if-else block.

  After the selected branch finishes, Python continues with code written after the if-else statement.

  Fix: Trace the selected branch and then inspect the statements that follow the entire block.

Try the Trace

Key Takeaways

1. A comparison condition produces a true or false result used to make a decision.
2. An if-else statement creates two mutually exclusive branches.
3. Exactly one branch executes: the if branch for a true condition or the else branch for a false condition.
4. Trace an if-else statement by evaluating the condition first, following the selected branch, and then continuing after the block.
5. A colon is required after if and else, and both branches must use consistent indentation.