Dis module doesn't know how to disassemble async generator or coroutine objects

[syncosmic]

BPO	31183
Nosy	@ncoghlan, @serhiy-storchaka, @Mariatta, @syncosmic
PRs	fix bpo-31183: allowing dis to disassemble async generator and coroutine objects #3077

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2017-08-19.11:07:30.550>
created_at = <Date 2017-08-11.15:21:31.458>
labels = ['extension-modules', 'type-feature', '3.7']
title = "`Dis` module doesn't know how to disassemble async generator or coroutine objects"
updated_at = <Date 2017-08-19.11:09:37.757>
user = 'https://github.com/syncosmic'

bugs.python.org fields:

activity = <Date 2017-08-19.11:09:37.757>
actor = 'ncoghlan'
assignee = 'none'
closed = True
closed_date = <Date 2017-08-19.11:07:30.550>
closer = 'ncoghlan'
components = ['Extension Modules']
creation = <Date 2017-08-11.15:21:31.458>
creator = 'syncosmic'
dependencies = []
files = []
hgrepos = []
issue_num = 31183
keywords = []
message_count = 8.0
messages = ['300167', '300184', '300188', '300194', '300200', '300207', '300470', '300579']
nosy_count = 4.0
nosy_names = ['ncoghlan', 'serhiy.storchaka', 'Mariatta', 'syncosmic']
pr_nums = ['3077']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue31183'
versions = ['Python 3.7']

[syncosmic]

bpo-21947 informed the dis module about the gi_code attribute, which stores code objects for generator objects. This allows inspection of generator objects, not just functions which return them. However, asynchronous generator objects use ag_code and coroutine objects use cr_code, so dis raises a TypeError on them. I'm making a pull request to extend the generator behavior to async generators and coroutines.

Credit to Luciano Ramalho: I tripped over this at his (great) concurrency workshop at PyBay 2017 and he identified exactly what was happening and suggested a patch.

[Mariatta]

Adding Serhiy and Nick who worked on these parts of CPython.

[serhiy-storchaka]

Is this a bug fix or a new feature? Adding support of generators in bpo-21947 was a new feature and was not backported to older versions.

[syncosmic]

Hm--either/both? I encountered it as a bug ("dis works on my generator object and my async generator function, why does it break on my async generator object?") but strictly speaking it's a new feature (just as bpo-21947 was). To my mind, adding this level of indirection (finding the code object) for generators was a bit of a design decision but once it's in for generators it should be in for async generators and coroutines by analogy.

[ncoghlan]

While I agree the line is blurry, extending automatic disassembly to cover new types is currently still considered a new feature in the dis module rather than a bug in the addition of those new types to the language.

Whether or not that's the right policy, or if we should consider tweaking it so that oversights in dis and inspect are treated as bugs in the addition of affected types, would be a question for python-dev (fortunately, the new cherry-pick based workflow means the current PR review can proceed independently of that question).

[syncosmic]

Makes sense. I'll update the PR in a bit, and if anyone is hitting this repeatedly and thinks it should be added to 3.6 they can advocate for a policy change in the maintaining-consistency-with-previous-new-features corner case.

Thanks, all!

[ncoghlan]

New changeset fe2b56a by Nick Coghlan (syncosmic) in branch 'master':
bpo-31183: dis now handles coroutines & async generators (GH-3077)
fe2b56a

[ncoghlan]

Thanks for the patch!

bpo-31230 is a follow-up issue looking at whether or not we should define a general "__code__" attribute as a general purpose introspection protocol for disassembly support (amongst other generalisations of state introspection support for async operations).