Programming Fundamentals › Programming Basics

Boolean Flag Arguments

Passing true/false to switch a function's behavior, and why it hurts readability.

Also known as: boolean parameter, flag parameter, boolean flag argument, boolean trap

A flag argument is a boolean parameter that switches a function between two behaviors:

render(page, True)        # what does True mean?
send_email(user, False)

At the call site, True and False say nothing. Readers must open the function to learn what they do. And the function now does two things, chosen by the flag, which usually means it should be two functions.

Better options

Split into two functions:

render_for_print(page)
render_for_screen(page)

Name the argument at the call site (keyword arguments):

send_email(user, include_attachments=False)

sendEmail(user, { includeAttachments: false });   // an options object in JavaScript

Use an enum when there may be more than two modes:

render(page, mode=RenderMode.PRINT)

Why it hurts

• Readability: connect(host, True, False) is a puzzle.
• Hidden complexity: an if flag: inside the function means two code paths to test.
• Growth: a second flag makes four combinations, a third makes eight.

When it’s fine

A flag with a clear, self-evident meaning, passed by keyword (verbose=True), or a setter like setVisible(true), is OK. The problem is the unlabeled boolean that switches behavior. See boolean and single responsibility.