Julia Evans

Debugging by starting a REPL at a breakpoint is fun

Hello! I was talking to a Python programmer friend yesterday about debugging, and I mentioned that I really like debugging using a REPL. He said he’d never tried it and that it sounded fun, so I thought I’d write a quick post about it.

This debugging method doesn’t work in a lot of languages, but it does work in Python and Ruby and kiiiiiind of in C (via gdb).

what’s a REPL?

REPL stands for “read eval print loop”. A REPL is a program that:

  1. reads some input from you like print(f"2 + 2 = {2+2}") (read)
  2. evaluates the input (eval)
  3. print out the result (print)
  4. and then goes back to step 1 (loop)

Here’s an example of me using the IPython REPL to run a print statement. (also it demonstrates f-strings, my favourite Python 3 feature)

$ ipython3
Python 3.9.5 (default, May 24 2021, 12:50:35) 
Type 'copyright', 'credits' or 'license' for more information
IPython 7.24.1 -- An enhanced Interactive Python. Type '?' for help.

In [1]: print(f"2 + 2 = {2+2}")
2 + 2 = 4

In [2]: 

you can start a REPL at a breakpoint

There are 2 ways to use a REPL when debugging.

Way 1: Open an empty REPL (like IPython, pry, or a browser Javascript console) to test out something.

This is great but it’s not what I’m talking about in this post.

Way 2: Set a breakpoint in your program, and start a REPL at that breakpoint.

This is the one we’re going to be talking about. I like doing this because it gives me both:

  1. all the variables in scope at the breakpoint, so I can print them out interactively
  2. easy access to all the functions in my program, so I can call them to try to find issues

how to get a REPL in Python: ipdb.set_trace()

Here’s a program called test.py that sets a breakpoint on line 5 using import ipdb; ipdb.set_trace().

import requests

def make_request():
    result = requests.get("https://google.com")
    import ipdb; ipdb.set_trace()


And here’s what it looks like when you run it: you get a REPL where you can inspect the result variable or do anything else you want.

python3 test.py
> /home/bork/work/homepage/test.py(5)make_request()
      4     result = requests.get("https://google.com")
----> 5     import ipdb; ipdb.set_trace()

ipdb> result.headers
{'Date': 'Thu, 16 Sep 2021 13:11:19 GMT', 'Expires': '-1', 'Cache-Control': 'private, max-age=0', 'Content-Type': 'text/html; charset=ISO-8859-1', 'P3P': 'CP="This is not a P3P policy! See g.co/p3phelp for more info."', 'Content-Encoding': 'gzip', 'Server': 'gws', 'X-XSS-Protection': '0', 'X-Frame-Options': 'SAMEORIGIN', 'Set-Cookie': '1P_JAR=2021-09-16-13; expires=Sat, 16-Oct-2021 13:11:19 GMT; path=/; domain=.google.com; Secure, NID=223=FXhKNT7mgxX7Fjhh6Z6uej9z13xYKdm9ZuAU540WDoIwYMj9AZzWTgjsVX-KJF6GErxfMijl-uudmjrJH1wgH3c1JjudPcmDMJovNuuAiJqukh1dAao_vUiqL8ge8pSIXRx89vAyYy3BDRrpJHbEF33Hbgt2ce4_yCZPtDyokMk; expires=Fri, 18-Mar-2022 13:11:19 GMT; path=/; domain=.google.com; HttpOnly', 'Alt-Svc': 'h3=":443"; ma=2592000,h3-29=":443"; ma=2592000,h3-T051=":443"; ma=2592000,h3-Q050=":443"; ma=2592000,h3-Q046=":443"; ma=2592000,h3-Q043=":443"; ma=2592000,quic=":443"; ma=2592000; v="46,43"', 'Transfer-Encoding': 'chunked'}

You have to install ipdb to make this work, but I think it’s worth it – import pdb; pdb.set_trace() will work too (and is built into Python) but ipdb is much nicer. I just learned that you can also use breakpoint() in Python 3 to get a breakpoint, but that puts you in pdb too which I don’t like.

how to get a REPL in Ruby: binding.pry

Here’s the same thing in Ruby – I wrote a test.rb program:

require 'net/http'
require 'pry'

def make_request()
  result = Net::HTTP.get_response('example.com', '/')


and here’s what it looks like when I run it:

$ ruby test.rb
From: /home/bork/work/homepage/test.rb:6 Object#make_request:

    4: def make_request()
    5:   result = Net::HTTP.get_response('example.com', '/')
 => 6:   binding.pry
    7: end

[1] pry(main)> result.code
=> "200"

you can also do get a REPL in the middle of an HTTP request

Rails also lets you start a REPL in the middle of a HTTP request and poke around and see what’s happening. I assume you can do this in Flask and Django too – I’ve only really done this in Sinatra (in Ruby).

GDB is sort of like a REPL for C

I was talking to another friend about REPLs, and we agreed that GDB is a little bit like a REPL for C.

Now, obviously this is sort of not true – C is a compiled language, and you can’t just type in arbitrary C expressions in GDB and have them work.

But you can do a surprising number of things like:

  • call functions
  • inspect structs if your program has debugging symbols (p var->field->subfield)

This stuff only works in gdb because the gdb developers put in a lot of work doing Very Weird Things to make it easier to get a REPL-like experience. I wrote a blog post a few years called how does gdb call functions? about how surprising it is that gdb can call functions, and how it does that.

This is the only way I use gdb when looking at C programs – I never set watchpoints or do anything fancy, I just set a couple of breakpoints in the program and then poke around at those points.

where this method works

languages where this works:

  • Python
  • Ruby
  • probably PHP, but I don’t know
  • C, sort of, in a weird way (though you might disagree :))
  • in Javascript: it seems like you can use debugger; to get a REPL through either node inspect or the browser console. There seem to be some limitations on what you can do (like node won’t let me use await in its REPL), but I haven’t done enough JS to fully understand this.
  • In Java, apparently IntelliJ lets you evaluate arbitrary expressions at a breakpoint, which isn’t quite a REPL but is cool

languages where this doesn’t work:

  • most compiled languages

REPL debugging is easy for me to remember how to do

There are (at least) 4 different ways of debugging:

  1. Lots of print statements
  2. a debugger
  3. getting a REPL at a breakpoint
  4. inspect your program with external tools like strace

I think part of the reason I like this type of REPL debugging more than using a more traditional debugger is – it’s so easy to remember how to do it! I can just set a breakpoint, and then run code to try to figure out what’s wrong.

With debuggers, I always forget how to use the debugger (probably partly because I switch programming languages a lot) and I get confused about what features it has and how they work, so I never use it.

Quadratic algorithms are slow (and hashmaps are fast) Teaching by filling in knowledge gaps