[Python-ideas] f-string "debug" conversion

Nathaniel Smith njs at pobox.com
Wed Oct 3 01:40:39 EDT 2018

On Tue, Oct 2, 2018 at 8:44 PM, David Teresi <dkteresi at gmail.com> wrote:
> print(f'{value!d}') is a lot of symbols and boilerplate to type out just
> a debugging statement that will be deleted later. Especially now that
> breakpoint() exists, I can't really see myself using this.
> I also don't see the use case of it being within an f-string, because I've
> never had to interpolate a debug string within some other string or format
> it in a fancy way. You said it yourself, taking advantage of other
> features isn't very useful in this case.
> If other people can find a use for it, I'd suggest making it ita own
> function -- debug(value) or something similar.

There was some discussion of this back in April:


I think the way I'd do it would be:

Step 1: Take the current "lnotab" that lets us map bytecode offsets -> line
numbers, and extend it with more detailed information, so that we can map
e.g. a CALL operation to the exact start and end positions of that call
expression in the source. This is free at runtime, and would allow more
detailed tracebacks (see [1] for an example), and more detailed coverage
information. It would definitely take some work to thread the necessary
information through the compiler infrastructure, but I think this would be
a worthwhile feature even without the debug() use case.

Step 2: Add a 'debug' helper function that exploits the detailed
information to reconstruct its call, by peeking into the calling frame and
finding the source for the call. Of course this would be a strange and ugly
thing to do for a regular function, but for a debugging helper it's
reasonable. So e.g. if you had the code:

  total = debug(x) + debug(y / 10)

The output might be:

  debug:myfile.py:10: 'x' is 3
  debug:myfile.py:10: 'y / 10' is 7

Or if you have a clever UI, like in an IDE or ipython, maybe it overrides
the debug() operator to print something like:

  total = debug(*x*) + debug(y / 10)
                                     ^ *3*

  total = debug(x) + debug(*y / 10*)
                                                              ^^^^^^^ *7*

(for anyone for whom the rendering is borked: on my screen the "x" on the
first line and the "y / 10" on the second line are highlighted in a
different font, and the carets draw an underline beneath them.)


[1] https://mail.python.org/pipermail/python-ideas/2018-April/050137.html

Nathaniel J. Smith -- https://vorpus.org
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