[Python-Dev] No longer enable Py_TRACE_REFS by default in debug build (original) (raw)

Steve Dower steve.dower at python.org
Wed Apr 10 16:50:01 EDT 2019

On 10Apr2019 1227, Nathaniel Smith wrote:

On Wed, Apr 10, 2019, 04:04 Victor Stinner <vstinner at redhat.com_ _<mailto:vstinner at redhat.com>> wrote: I don't think that I ever used sys.getobjects(), whereas many projects use gc.getobjects() which is also available in release builds (not only in debug builds).

Can anyone explain what pydebug builds are... for? Confession: I've never used them myself, and don't know why I would want to. (I have to assume that most of Steve's Windows downloads are from folks who thought they were downloading a python debugger.)

They're for debugging :)

In general, debug builds are meant for faster inner-loop development. They generally do incremental builds properly and much faster by omitting most optimisations, which also enables source mapping to be more accurate when debugging. Assertions are typically enabled so that you are notified when a precondition is first identified rather than when it causes the crash (compiling these out later means you don't pay a runtime cost once you've got the inputs correct - generally these are used for developer-controlled values, rather than user-provided ones).

So the idea is that you can quickly edit, build, debug, fix your code in a debug configuration, and then use a release configuration for the actual released build. Full release builds may take 2-3x longer than full debug builds, given the extra effort they make at optimisation, and very often can't do minimal incremental builds at all (so they may be 10-100x slower if you only modified one source file). But because the builds behave functionally equivalently, you can iterate with the faster configuration and get more done.

(Disclaimer: I do most of my work on Windows where this has been properly developed. What I hear from non-Windows developers is that other tools can't actually handle this kind of workflow properly. Sorry.)

The reason we ship debug Python binaries is because debug builds use a different C Runtime, so if you do a debug build of an extension module you're working on it won't actually work with a non-debug build of CPython.

While it's possible that people misread "Download debug binaries" (the text in the installer) and think that it's an actual debugger, I'd suggest that your total lack of context here means you should avoid making assumptions about users you know nothing about.

Cheers, Steve

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