Simplify AST for slices

[serhiy-storchaka]

BPO	34822
Nosy	@gvanrossum, @brettcannon, @nascheme, @ncoghlan, @benjaminp, @serhiy-storchaka, @vedgar, @pablogsal, @thautwarm, @isidentical
PRs	bpo-34822: Simplify AST for subscription. #9605

^{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 2020-03-10.16:54:21.854>
created_at = <Date 2018-09-27.16:17:40.175>
labels = ['interpreter-core', 'type-feature', 'library', '3.9']
title = 'Simplify AST for slices'
updated_at = <Date 2020-03-10.16:54:21.854>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2020-03-10.16:54:21.854>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = True
closed_date = <Date 2020-03-10.16:54:21.854>
closer = 'serhiy.storchaka'
components = ['Interpreter Core', 'Library (Lib)']
creation = <Date 2018-09-27.16:17:40.175>
creator = 'serhiy.storchaka'
dependencies = []
files = []
hgrepos = []
issue_num = 34822
keywords = ['patch']
message_count = 15.0
messages = ['326570', '326991', '361997', '362016', '362224', '362234', '363583', '363592', '363593', '363597', '363598', '363648', '363650', '363756', '363829']
nosy_count = 10.0
nosy_names = ['gvanrossum', 'brett.cannon', 'nascheme', 'ncoghlan', 'benjamin.peterson', 'serhiy.storchaka', 'veky', 'pablogsal', 'thautwarm', 'BTaskaya']
pr_nums = ['9605']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue34822'
versions = ['Python 3.9']

[serhiy-storchaka]

Currently an AST for expressions can have non-terminal nodes of two types: expr and slice. Every of them can be a one of several kinds. A slice can be of kind Index (just an expression), Slice (optional expressions separated by ":") or ExtSlice (slices separated by ",").

For example, the expression "d[a, ..., b:c]" is represented as:

Subscript(
    Name('d', Load()),
    ExtSlice(
        [
            Index(Name('a', Load())),
            Index(Constant(Ellipsis)),
            Slice(Name('b', Load()), Name('c', Load()), None)
        ]
    ),
    Load()
)

and the expression "d[a, ..., b]" is represented as:

Subscript(
    Name('d', Load()),
    Index(
        Tuple(
            [
                Name('a', Load()),
                Constant(Ellipsis),
                Name('b', Load())
            ],
            Load()
        )
    ),
    Load()
)

(note that ExtSlice is used only if there are slices in subexpressions).

I suggest to get rid of the separate slice type. The Slice kind will be a kind of the expr type instead of the slice type. The ExtSlice kind will be always replaced with a Tuple, even if subexpressions contain slices. Nodes of the Index kind will be replaced with expr nodes to which they are referred. For example, the expression "d[a, ..., b:c]" will be represented as:

Subscript(
    Name('d', Load()),
    Tuple(
        [
            Name('a', Load()),
            Constant(Ellipsis),
            Slice(Name('b', Load()), Name('c', Load()), None)
        ],
        Load()
    ),
    Load()
)

This will simplify the code for handling AST, especially the C code. The following PR removes around 400 lines of code (a half of them are generated, but others are handwritten). I hope that this regularization will help to write a general code for walking AST for expressions and remove more duplicated code in ast_opt.c, ast_unparse.c, and symtable.c. This even can help to solve a problem with crashes in handling too deep AST if implement the recursion without using the C stack (but this is dreams).

This change is more breaking than a change in bpo-32892. What will continue to work:

• The code for creating an AST when pass values as arguments: Index(value) will return just value, ExtSlice(slices) will return Tuple(slices, Load()).

• NodeVisitor based processing. Methods visit_Index() and visit_ExtSlice() will be never invoked. The semantic of visit_Slice() will be not changed. visit_Tuple() will be invoked instead of visit_ExtSlice() for extended slices.

• isinstance() and issubclass() checks for Slice. Subclassing of Slice.

What will no longer work (with the current implementation):

• The code that creates empty AST nodes and sets their attributes. node = Index(); node.value = value will no longer work.

• The code that reads attributes of just created Index and ExtSlice nodes. Index(value) will return just value instead of a new object whose attribute "value" is a specified value. A list of subexpressions of ExtSlice(slices) will be accessible as the "elts" attribute instead of "dims" (because it is a Tuple).

• isinstance() and issubclass() checks for Index and ExtSlice will always return False.

• Subclassing of Index and ExtSlice. Instantiating subclasses of Index and ExtSlice will return the same result as for Index and ExtSlice, i.e. not instance of these classes.

• The code that produces a Python code from AST will need to handle indexing with tuples specially (see Tools/parser/unparse.py) because d[(a, b)] is valid syntax (although parenthesis are redundant), but d[(a, b:c)] is not.

Some limitations are caused by the desire for simplicity and can be removed. For example it is possible to add the "dims" alias of "elts" to Tuple, and make subclasses working as before. It is more hard and inefficient to keep isinstance() checks and attribute access for Index. If some breakage for Index is not avoidable, I'm not sure that it is worth to spent efforts for imitating the old behavior for ExtSlice.

[nascheme]

Hello Serhiy,

I've not reviewed the patch but I trust that if you say it simplifies things, that's so. Perhaps the important question is if it is okay to change the AST in backwards incompatible ways within 3.x releases. As a library author who could be affected, I think it is okay. The AST is mostly an implementation detail and should not required to maintain compatibility. We expose it via the 'ast' module for low level tools that want to manipulate it. It is up to those users to handle backwards incompatible changes.

That said, we shouldn't just change it for trivial reasons. That just causes work for 3rd party libraries. Removing 400 lines of code seems like sufficient reason.

[isidentical]

Serhiy, any plans to bump this patch to 3.9 and continue / merge? In general the benefits looks great, but on the other hand this might cause some breakage which I guess as @nascheme is OK in AST.

[serhiy-storchaka]

Updated the PR.

I can obviously be biased about my changes, so I need an approval of other core developer to merge them.

I created several PRs to popular third-party projects which work with AST to support both old and new AST schemes.

[vedgar]

I wrote some AST analyzers, and this would have simplified my code. So I welcome it. :-)

However, it means people might be tempted to write

a[b:c, d]    as    a[(b:c, d)]

(or at least expect it to work -- same as a[b, c] can now be written as a[(b, c)]). We aren't going to allow this?

[serhiy-storchaka]

No, this PR does not change the Python syntax. It only changes the AST representation.

[gvanrossum]

Haven’t looked at the code but I welcome the simplification.

[ncoghlan]

The one thing in the PR that makes me slightly wary is the point Vedran raised: in the old AST _Unparser code, the fact that index tuples containing slices should be printed without parentheses was encapsulated in the ExtSlice node type, but with Index and ExtSlice removed, that behaviour is now instead implemented by duplicating the visit_Tuple logic directly in visit_Subscript, purely to omit the surrounding parens.

So I agree that the parse tree simplification is an improvement, but I'm wondering if it might make sense to add an "is_subscript" attribute to expression nodes.

That way the Tuple vs ExtSlice and Expr vs Index distinctions would be preserved, but the representation of those distinctions would change in a way that meant that most consuming code didn't need to care that the distinctions existed (ExtSlice would become a Tuple with is_subscript set to True, and similarly, Index would become Expr instances with is_subscript set to True).

It's been so long since I changed the AST, though, that I'm not sure how painful adding that attribute would be.

[serhiy-storchaka]

It was added to produce nicer output.

Currently:

>> print(ast.unparse(ast.parse('a[i, j]')))

a[(i, j)]

With PR 9605:

>> print(ast.unparse(ast.parse('a[i, j]')))

a[i, j]

The current code is not consistent with outputting parenthesis:

>> print(ast.unparse(ast.parse('a[i:j, k]')))

a[i:j, k]

It also produces the same output for a[i:j] and a[i:j,] which have different AST and compiled to different bytecode (this is a bug).

>> print(ast.unparse(ast.parse('a[i:j,]')))

a[i:j]