What is the best or proper way to allow debugging of generated code?

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For various reasons, in one project I generate executable code by means of generating AST from various source files the compiling that to bytecode (though the question could also work for cases where the bytecode is generated directly I guess).

From some experimentation, it looks like the debugger more or less just uses the lineno information embedded in the AST alongside the filename passed to compile in order to provide a representation for the debugger's purposes, however this assumes the code being executed comes from a single on-disk file.

That is not necessarily the case for my project, the executable code can be pieced together from multiple sources, and some or all of these sources may have been fetched over the network, or been retrieved from non-disk storage (e.g. database).

And so my Y questions, which may be the wrong ones (hence the background):

  • is it possible to provide a memory buffer of some sort, or is it necessary to generate a singular on-disk representation of the "virtual source"?
  • how well would the debugger deal with jumping around between the different bits and pieces if the virtual source can't or should not be linearised[0]
  • and just in case, is the assumption of Python only supporting a single contiguous source file correct or can it actually be fed multiple sources somehow?

[0] for instance a web-style literate program would be debugged in its original form, jumping between the code sections, not in the so-called "tangled" form

abstract-syntax-tree python debugging
1 Answers

Some of this can be handled by the trepan3k debugger. For other things various hooks are in place.

First of all it can debug based on bytecode alone. But of course stepping instructions won't be possible if the line number table doesn't exist. And for that reason if for no other, I would add a "line number" for each logical stopping point, such as at the beginning of statements. The numbers don't have to be line numbers, they could just count from 1 or be indexes into some other table. This is more or less how go's Pos type position works.

The debugger will let you set a breakpoint on a function, but that function has to exist and when you start any python program most of the functions you define don't exist. So the typically way to do this is to modify the source to call the debugger at some point. In trepan3k the lingo for this is:

from trepan.api import debug; debug()

Do that in a place where the other functions you want to break on and that have been defined.

And the functions can be specified as methods on existing variables, e.g. self.my_function()

One of the advanced features of this debugger is that will decompile the bytecode to produce source code. There is a command called deparse which will show you the context around where you are currently stopped.

Deparsing bytecode though is a bit difficult so depending on which kind of bytecode you get the results may vary.

As for the virtual source problem, well that situation is somewhat tolerated in the debugger, since that kind of thing has to go on when there is no source. And to facilitate this and remote debugging (where the file locations locally and remotely can be different), we allow for filename remapping.

Another library pyficache is used to for this remapping; it has the ability I believe remap contiguous lines of one file into lines in another file. And I think you could use this over and over again. However so far there hasn't been need for this. And that code is pretty old. So someone would have to beef up trepan3k here.

Lastly, related to trepan3k is a trepan-xpy which is a CPython bytecode debugger which can step bytecode instructions even when the line number table is empty.

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