Add endline and endcolumn to every AST node

[ilevkivskyi]

BPO	33416
Nosy	@gvanrossum, @brettcannon, @rhettinger, @vstinner, @ambv, @aivarannamaa, @serhiy-storchaka, @ilevkivskyi, @miss-islington
PRs	bpo-33416: Add end positions to Python AST #11605 bpo-33416: Add end positions to Python AST #11605 bpo-33416: Add end positions to Python AST #11605 bpo-35224: PEP 572 Implementation #10497 bpo-33416: Document changes in PyNode_AddChild and PyParser_AddToken #14214 [3.8] bpo-33416: Document changes in PyNode_AddChild and PyParser_AddToken (GH-14214) #14215

^{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 2019-06-19.00:34:46.255>
created_at = <Date 2018-05-03.10:15:25.514>
labels = ['interpreter-core', 'type-feature', '3.8']
title = 'Add endline and endcolumn to every AST node'
updated_at = <Date 2019-06-19.00:34:46.254>
user = 'https://github.com/ilevkivskyi'

bugs.python.org fields:

activity = <Date 2019-06-19.00:34:46.254>
actor = 'levkivskyi'
assignee = 'none'
closed = True
closed_date = <Date 2019-06-19.00:34:46.255>
closer = 'levkivskyi'
components = ['Interpreter Core']
creation = <Date 2018-05-03.10:15:25.514>
creator = 'levkivskyi'
dependencies = []
files = []
hgrepos = []
issue_num = 33416
keywords = ['patch', 'patch', 'patch']
message_count = 15.0
messages = ['316117', '330180', '330207', '330287', '333575', '333583', '333587', '333608', '333617', '334205', '334228', '334300', '344507', '344519', '346015']
nosy_count = 9.0
nosy_names = ['gvanrossum', 'brett.cannon', 'rhettinger', 'vstinner', 'lukasz.langa', 'Aivar.Annamaa', 'serhiy.storchaka', 'levkivskyi', 'miss-islington']
pr_nums = ['11605', '11605', '11605', '10497', '14214', '14215']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue33416'
versions = ['Python 3.8']

[ilevkivskyi]

Some Python tools (in particular I am interested in type checkers) will benefit from knowing where a given expression ends to indicate/highlight location of an error in the source code. Other tools and IDEs may have also some other benefits. Currently such tools need to use some hacks and/or custom parser to find the end line and end column of an expression, while it would be more straightforward to just add this information to every AST node by CPythons own parser.

This will increase memory usage, but expectation is that this effect will be minor.

What do you think?

[serhiy-storchaka]

I like this. This can help to generate more meaningful error messages at compile and run time.

[serhiy-storchaka]

This may be a duplicate of bpo-22616.

See also bpo-16795, bpo-21295, bpo-33211, bpo-34876.

[ilevkivskyi]

bpo-22616 is a bit different (proposing line/column ranges instead of just endline/endcolumn). I am happy to close this one in favor of bpo-22616 if we agree that we will go with endline/endcolumn.

I can't guarantee, but likely I will work on this during winter holidays, unless there are some objections, or unless someone will do this before :-)

[ilevkivskyi]

FYI, I started working on this. I will have PR ready end of next week.

Serhiy, I don't think we should keep both this and bpo-22616 open. Which one would you prefer to close?

[rhettinger]

This idea seems reasonable. Most of the AST nodes have a span and it would be nice to know what that is. I'm sure we will find use cases though I doubt that many compiler syntax errors would benefit (since a syntax error means that we don't have a completely matched grammar rule).

BTW, does this information have to be added by the parser or could there be am AST module function that deduces the end locations from the start location of next sibling node or from the parent node? If so, it may still be possible get the benefit without slowing down or complicating the parser logic.

Do we know what other languages do (carry the full span info in the AST or deduce the span after the fact)? I don't know what the best practices are in this regard.

One other thought: We should be careful not to impair existing AST capabilities that support code rewriting and movement (i.e. refactoring tools). The copy_location() and fix_missing_locations() functions would need to be updated as well.

[serhiy-storchaka]

It is up to you Ivan.

The end location can not be deduces from the start location of next sibling node or from the parent node. For example, the AST for the expression "foo.bar.baz" does not contain information for the end location of "foo.bar".

[ilevkivskyi]

I'm sure we will find use cases though I doubt that many compiler syntax errors would benefit (since a syntax error means that we don't have a completely matched grammar rule).

This is mostly useful for code analysis tools and IDEs.

BTW, does this information have to be added by the parser or could there be am AST module function that deduces the end locations from the start location of next sibling node or from the parent node?

There may be some other ways to do it, but currently I am adding both end_lineno and end_col_offset to AST, and n_end_lineno and n_end_col_offset in CST. The latter are needed to easily take care of situations like a + (b ), and other corner cases. I could imagine it is possible to live with only end position information in AST to save some memory, but then the code will be much harder to maintain. We can discuss details in the PR.

Do we know what other languages do (carry the full span info in the AST or deduce the span after the fact)?

I didn't do real research, but quick browsing shows carrying the end position info is quite common. But more importantly as Serhiy noted, deducing might be just not possible in some situations.

It is up to you Ivan.

OK, I will close the other one as superseded by this issue.

[aivarannamaa]

I strongly support this feature, because my IDE (https://thonny.org) needs to highlight AST nodes in the source code.

There would be many interested parties if you count the stars of this project: https://github.com/gristlabs/asttokens

[ilevkivskyi]

New changeset 9932a22 by Ivan Levkivskyi in branch 'master':
bpo-33416: Add end positions to Python AST (GH-11605)
9932a22

[ilevkivskyi]

Buildbots are green, this can be now closed.