Understanding Function Syntax

The Waiting Prompt

When you define a function in an interactive interpreter, the interpreter cannot finish processing the definition after the first line. The function header announces that a body is coming, so the interpreter changes its prompt. Instead of returning to the normal >>> prompt, it displays . . . . This is not an error message. It is a signal that the definition is incomplete and that the interpreter is waiting for the function body.

Reading the Ellipses

The . . . prompt tells you that the interpreter is still collecting the function definition. After the function header, enter the function body at the ellipsis prompt. Every line that belongs to the body must be indented. Typically, indentation uses four spaces or one tab. If the body has more than one line, each line that belongs to it must use the same indentation level.

>>> def greet(): ... print("Hello") ...

Closing the Definition

After entering the final indented body line, the interpreter still needs a clear signal that the function body has ended. In interactive mode, provide that signal by entering an empty line: press Enter to move to a new line, then press Enter again without typing code or indentation. The blank line tells the interpreter that the definition is complete, so it returns to the normal >>> prompt.

>>> def greet():
...     print("Hello")
...
>>> greet()
Hello

Hello

Interactive Sessions and Scripts

The empty-line requirement belongs to interactive mode. In an interactive session, the interpreter receives one line at a time, so it needs a signal that no more lines belong to the current function definition. The . . . prompt shows that the interpreter is waiting, and the empty line signals completion. In a script file, the interpreter can read the file structure and recognize that the function has ended when indentation returns to the base level. A script therefore does not need a separate empty line to terminate the function.

REPL Practice

Frequent Prompt Mistakes

• Treating the . . . prompt as an error.

  The ellipses indicate that the function definition is incomplete and that the interpreter is waiting for the body.

  Fix: Enter the function body at the . . . prompt.

• Entering a body line without indentation.

  Indentation tells the interpreter which lines belong to the function body.

  Fix: Indent each body line, typically by four spaces or one tab.

• Typing a new statement instead of an empty line after the body.

  The interpreter is still collecting the function definition, so the definition has not yet been terminated.

  Fix: Press Enter again without typing code or indentation, then wait for the >>> prompt.

• Expecting a script file to require the same blank-line action.

  The empty-line requirement is specific to interactive mode. A script's indentation and file structure show where the function ends.

  Fix: Use the empty line when entering the definition interactively; do not treat it as a required script-file terminator.

Key Takeaways

1. The . . . prompt means that an interactive function definition is incomplete and is waiting for its body.
2. Function body lines must be indented so the interpreter knows they belong to the function.
3. Press Enter twice after the final body line, with the second line completely empty, to finish the definition interactively.
4. Interactive mode needs this blank-line signal because it receives input one line at a time.
5. A script file can determine the function's end from its complete structure and indentation changes, so it does not require the interactive blank-line action.