The Python REPL

Start and stop the REPL

Type python3 in a terminal to start the REPL. On Windows you can also type py. The prompt >>> shows that Python waits for your input.

Type exit() or quit() to stop the REPL. You can also press Ctrl-D on Linux and macOS. On Windows, press Ctrl-Z and then Enter.

The REPL runs one statement at a time. It prints the result of each expression immediately. This makes it useful for quick tests.

Keep the last result with underscore

The name _ holds the result of the last expression. You can use it in the next line. This saves typing when you chain calculations.

The value of _ changes after each expression. It does not keep older results. Store a value in a named variable if you need it later.

Get help inside the REPL

The function help() opens the built-in help system. Call help() with no argument to read the general help text. Call help(name) to read the help for an object.

The function dir() lists the names in a scope. Call dir() with no argument to list the current names. Call dir(object) to list the attributes of an object.

Press q to leave the help viewer. The REPL prompt returns after you quit the viewer.

Import a module and call it

You can import any module at the prompt. Then call its functions in the next line. The example below imports the math module and computes a square root.

The call math.sqrt(16) returns 4.0. The result is a float, not an int. The REPL prints it at once.

An import stays active only while the REPL runs. A new session starts with no modules loaded.

import math
result = math.sqrt(16)
print(result)
print(math.pi)

Run code from the command line

The option -c runs one line of code and then exits. Use quotes around the code. The example below prints a sum and stops.

The command python3 file.py runs a script file. The REPL does not start in this mode. Python exits when the script ends.

The option -i runs a script and then opens the REPL. The REPL keeps the names that the script defined. This helps you inspect the state after the script runs.

python3 -c "print(2 + 3)"
python3 -i file.py
python3 file.py

History, Ctrl-R, and state

The REPL keeps a history of your input lines. Press the Up arrow to recall an earlier line. Press Enter to run it again.

Press Ctrl-R to search the history. Type part of an old line and the REPL finds a match. Press Ctrl-R again to move to an older match.

The REPL keeps state only in memory. A new session starts empty, so you must run your imports again. A common mistake is to call a module after a restart without the import. The fix is to run the import line first.

When a script fails, the next step is pdb. Run python3 -m pdb file.py to start the debugger. The debugger stops at the first line and lets you step through the code.

Common questions

How do I exit the Python REPL?

Type exit() or quit(), or press Ctrl-D on Linux and macOS. On Windows, press Ctrl-Z and then Enter.

What is the underscore variable?

The name _ holds the result of the last expression in the REPL. It changes after each new expression.

Why does the REPL lose my variables?

The REPL keeps state only in memory. A new session starts empty, so you must run your imports and assignments again.