New AST node Else() should be introduced

[IgorBronshteyn]

BPO	21992
Nosy	@birkenfeld, @vstinner, @nedbat, @benjaminp, @methane, @PCManticore, @markshannon, @serhiy-storchaka, @zvyn, @ilevkivskyi, @pablogsal, @isidentical, @potomak

^{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 2019-12-01.20:00:11.175>
created_at = <Date 2014-07-16.16:13:48.305>
labels = ['interpreter-core', 'type-feature']
title = 'New AST node Else() should be introduced'
updated_at = <Date 2019-12-01.20:00:11.174>
user = 'https://bugs.python.org/IgorBronshteyn'

bugs.python.org fields:

activity = <Date 2019-12-01.20:00:11.174>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = True
closed_date = <Date 2019-12-01.20:00:11.175>
closer = 'serhiy.storchaka'
components = ['Interpreter Core']
creation = <Date 2014-07-16.16:13:48.305>
creator = 'Igor.Bronshteyn'
dependencies = []
files = []
hgrepos = []
issue_num = 21992
keywords = []
message_count = 6.0
messages = ['223229', '237453', '237469', '347785', '347926', '357687']
nosy_count = 14.0
nosy_names = ['georg.brandl', 'vstinner', 'nedbat', 'benjamin.peterson', 'methane', 'Claudiu.Popa', 'Mark.Shannon', 'serhiy.storchaka', 'zvyn', 'Igor.Bronshteyn', 'levkivskyi', 'pablogsal', 'BTaskaya', 'potomak']
pr_nums = []
priority = 'normal'
resolution = 'rejected'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue21992'
versions = ['Python 3.5']

[IgorBronshteyn]

Quoting Collin Winter:
"I'd like there to be an Else() node for If.orelse, While.orelse, etc. My motivation is that I need the lineno and col_offset values of the "else" statement for a code-coverage utility; as it is, I have to find the last lineno of the if/while/etc suite and the first lineno of the "else" suite and then search between those two for the "else" statement."

from: http://bugs.python.org/issue1659410
Lack of position for "else" token hampers implementing of even simple static analysis checker that calculates NCNB (non-comment non-blank) lines (e.g. in a function).

[BreamoreBoy]

The referenced bpo-1659410 has been closed as "too late". Does that also apply here or is this still considered to be a valid enhancement request?

[markshannon]

I would say yes, it is too late. Any change to the AST is a breaking change.

The current AST is full of missing line numbers, incorrect column offsets and other inconsistencies. To fix them all would be a major undertaking with no obvious benefit to the core interpreter.

If you are interested in implementing a parser with proper location information, I would suggest discussing it on the code-quality mailing list:
https://mail.python.org/mailman/listinfo/code-quality

[zvyn]

Is there a reason for this still being open? /me trying to find issues to work on using [random issue]

[serhiy-storchaka]

In general case the AST does not preserve detailed information about all syntactic elements. For example in the AST for a + b does not include lineno and col_offset for "+".

>>> ast.dump(ast.parse('a + b'), include_attributes=True)
"Module(body=[Expr(value=BinOp(left=Name(id='a', ctx=Load(), lineno=1, col_offset=0, end_lineno=1, end_col_offset=1), op=Add(), right=Name(id='b', ctx=Load(), lineno=1, col_offset=4, end_lineno=1, end_col_offset=5), lineno=1, col_offset=0, end_lineno=1, end_col_offset=5), lineno=1, col_offset=0, end_lineno=1, end_col_offset=5)], type_ignores=[])"

But in most cases you can determine it from the position of the surrounded nodes. "+" lies between the end of BinOp.left and the beginning of BinOp.right, i.e. between columns 1 and 4 at line 1. What is left is to count the number of whitespace character before and after "+" to determine its exact position. The same method you can use to find the position of the "else" keyword.

It is easier in 3.8 since the AST contains now also the position of the end of the node (end_lineno and end_col_offset).

[isidentical]

IMHO there are no need to keep this issue open according to opinions of core developers, so can this be closed or it needs more discussion (which i prefer to do on mailing lists)?