Creating an REPL is easy in Python

By Vasudev Ram


Read Eval Print Loop

It's easy to create a simple Python REPL (a Read-Eval-Print Loop) in Python - with code.interact(), that is, with the interact function from the code module in Python's standard library.

I wrote and interacted with a small Python program to illustrate it.

It sets a couple of variables, then calls code.interact(), at which point I interact with the program via the REPL session that interact() creates.

I can see and set the variables set earlier by the program, because I pass the locals() dict to interact().

I can define a function in the session, which can access the variables set before the session.

And any changes the session makes to the variables, via my assignments, persist in the program after the interactive session ends (upon typing Ctrl-Z) and the program continues.

All this is shown in the code and output below:

from __future__ import print_function
#--------------------------------------------------------------
# Reference:
# https://docs.python.org/2.7/library/code.html
# https://docs.python.org/3.6/library/code.html
# code_interact.py
# Copyright 2016 Vasudev Ram
# Web site: https://vasudevram.github.io
# Blog: http://jugad2.blogspot.com
# Product store: https://gumroad.com/vasudevram
#--------------------------------------------------------------

import code

a = 1
b = "hello"
print("Before code.interact, a = {}, b = {}".format(a, b))

banner="code.interact session, type Ctrl-Z to exit."
code.interact( banner=banner, local=locals())

print("After code.interact, a = {}, b = {}".format(a, b))

The output of the program:

$ python code_interact.py
Before code.interact, a = 1, b = hello
code.interact session, type Ctrl-Z to exit.
>>> greeting = "hello "
>>> times = 2
>>> a
1
>>> b
'hello'
>>> def repeat_string(s, n):
...     return s * n
...
>>> repeat_string(greeting, times)
'hello hello '
>>> repeat_string(greeting, a)
'hello '
>>> repeat_string(b, times)
'hellohello'
>>> a += 2
>>> a
3
>>> repeat_string(greeting, a)
'hello hello hello '
>>> repeat_string(b, a)
'hellohellohello'
>>>
After code.interact, a = 3, b = hello
$

While on the topic of REPLs, check out these earlier posts:

repl.it, online REPL for many languages, and empythoned.

Codingbat, Progress Graphs and Michael Jordan

codepad.org, executable multi-language pastebin, in Python

- Vasudev Ram - Online Python training and consulting

Get updates on my software products / ebooks / courses.

Jump to posts: Python   DLang   xtopdf

Subscribe to my blog by email

My ActiveState recipes

FlyWheel - Managed WordPress Hosting

Share |

Python's code.interact() - programmatically emulate the interpreter

By Vasudev Ram


The Python module named code (from the standard library) has a neat and potentially useful feature - it allows you to programmatically embed an REPL in a Python application.

It is more powerful than the Python REPL one-liner that I blogged about recently. For example, that REPL only allows Python expressions. This one allows Python statements, function definitions and invocations, and class definitions and method invocations. I tried all of those and they worked. Also, a syntax error does not cause an exit from this REPL, whereas the earlier one-liner exits on error.

Saw it via this tweet by Dan North (@tastapod).

To try it from your OS command prompt, use:

python -c "import code; code.interact(local=locals())"

This creates an instance of code.InteractiveConsole, a class from the code module, and runs it. The behavior closely emulates the interactive Python interpreter. Though I showed the use of it from the OS command prompt, it is more meant to be used from within a Python application, where it can potentially be quite useful, particularly if you can expose some of your application's API to the method. But it should probably be used with caution, in secure settings only.

- Vasudev Ram - Dancing Bison Enterprises

Share |