The interactive interpreter and REPL workflow

Understanding The interactive interpreter and REPL workflow

The interactive prompt is itself a small program running a loop: it reads what you type, compiles it in "single" mode, executes it, and then hands any resulting value to sys.displayhook, which prints repr(value) and stores it in the builtin name _. Everything surprising about the prompt follows from that last step. Typing 2 + 3 shows 5 with no print() call because an expression statement produces a value; typing x = 5 shows nothing because an assignment produces no value; typing print("hi") shows only hi because print returns None and displayhook deliberately skips None.

The second prompt, ..., exists because after every Enter the loop must decide whether it has a complete statement yet. It joins the lines you have typed so far with newlines and passes the result to codeop.compile_command, which answers "complete", "not finished yet", or "broken". A for header on its own is not finished, so the loop keeps reading; the empty line you press at the end of a block is what appends the trailing newline that makes the joined source compilable, which is the real reason blocks need a blank line. A genuinely invalid line such as 2 + is not treated as unfinished at all and raises SyntaxError immediately.

In practice the prompt is for questions that fit on one line and files are for anything you want tomorrow: does this slice include the endpoint, what does this method return on an empty string, what attributes does this object have. Because names persist for the whole session in a single namespace, you can build up state step by step, then promote what worked into a .py file. Starting python -i script.py runs the file and then drops you at the prompt with everything the file defined still alive, which saves retyping setup code while you poke at it with type(), dir() and help().

import builtins
import sys

# These calls are exactly what the prompt does after evaluating each line you type.
sys.displayhook(2 + 3)          # you typed: 2 + 3
sys.displayhook("tab\there")    # repr(), so escapes stay visible
sys.displayhook(None)           # print() returns None, so nothing is echoed
print("_ still holds", repr(builtins._))

Worked examples

from code import InteractiveConsole

session = ["x = 6", "x * 7", "if x > 5:", "    print('plenty')", ""]
console = InteractiveConsole()
prompt = ">>> "

for line in session:
    print(f"{prompt}{line}".rstrip())
    prompt = "... " if console.push(line) else ">>> "

from codeop import compile_command

def status(source):
    try:
        code = compile_command(source, "<input>", "single")
    except SyntaxError:
        return "invalid, reported immediately"
    return "incomplete, keep reading" if code is None else "complete, run it"

print(status("total = 2 + 3"))
print(status("for i in range(2):"))
print(status("for i in range(2):\n    print(i)"))
print(status("for i in range(2):\n    print(i)\n"))
print(status("2 +"))

def area(w, h):
    """Return the area of a w by h rectangle."""
    return w * h

print(type(area))
print(area.__doc__)
print([n for n in dir("repl") if n.startswith("is")])

Important notes

Common mistakes

Try it yourself