ast.Tuple has wrong col_offset

[isaiah]

BPO	33520
Nosy	@bitdancer, @ambv, @isaiah

^{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 2018-05-15.20:08:58.752>
created_at = <Date 2018-05-15.15:16:22.321>
labels = ['3.7', 'invalid', 'type-bug', 'library']
title = 'ast.Tuple has wrong col_offset'
updated_at = <Date 2018-05-16.08:26:54.299>
user = 'https://github.com/isaiah'

bugs.python.org fields:

activity = <Date 2018-05-16.08:26:54.299>
actor = 'isaiah'
assignee = 'none'
closed = True
closed_date = <Date 2018-05-15.20:08:58.752>
closer = 'r.david.murray'
components = ['Library (Lib)']
creation = <Date 2018-05-15.15:16:22.321>
creator = 'isaiah'
dependencies = []
files = []
hgrepos = []
issue_num = 33520
keywords = []
message_count = 8.0
messages = ['316665', '316684', '316685', '316708', '316710', '316713', '316715', '316785']
nosy_count = 3.0
nosy_names = ['r.david.murray', 'lukasz.langa', 'isaiah']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue33520'
versions = ['Python 3.7']

[isaiah]

The col_offset of the ast.Tuple node is set to the column offset of the first element, shown in code:

>>> a = "{1,2,3}"
>>> b = ast.parse(a).body[0]                                                                                                                                                                  
>>> b.value.col_offset
0
>>> a = "[1,2,3]"                                                                                                                                                                             
>>> b = ast.parse(a).body[0]                                                                                                                                                                  
>>> b.value.col_offset                                                                                                                                                                        
0                                                                                                                                                                                             
>>> a = "(1,2,3)"                                                                                                                                                                             
>>> ast.parse(a).body[0].value.col_offset                                                                                                                                                     
1 
>>> a = "()"                                                                                                                                                                                  
>>> ast.parse(a).body[0].value.col_offset                                                                                                                                                     
0  

It's correct for dict, set, list, even empty tuple, Though this is not a serious bug, for other python implementations that uses the tests as language spec, this is annoying.

[ambv]

This is because technically parentheses aren't part of the tuple. They are just organizational and unnecessary for the tuple to be recognized by the parser.

Those two are equivalent:

  >>> ast.parse("(1,2,3)").body[0].value.col_offset
  1
  >>> ast.parse("(1)").body[0].value.col_offset
  1

You can see similar behavior within generator expressions in contexts where the parentheses are not semantically required:

  >>> ast.parse("c(i for i in range(10))").body[0].value.args[0].col_offset
  2
  >>> ast.parse("c((i for i in range(10)))").body[0].value.args[0].col_offset
  3

[ambv]

For comparison, a tuple without parentheses:

  >>> ast.parse("1,2,3").body[0].value.col_offset
  0

[isaiah]

Thanks for the reply, that's quite reasonable, especially take the generator expression case into consideration. However I found this is not consistent with empty tuple:

>>> a = "()"                                                                                                                                                                                  
>>> ast.parse(a).body[0].value.col_offset                                                                                                                                                     
0

It's true that the parenthesis is required to construct a tuple, but if the parenthesis is served as the starting point of the tuple, then the col_offset should be the opening parenthesis. i.e. in the following example, both should start from col 2:

  >>> ast.parse("c(i for i in range(10))").body[0].value.args[0].col_offset
  2
  >>> ast.parse("c((i for i in range(10)))").body[0].value.args[0].col_offset
  3

[isaiah]

It's true that the parenthesis is required to construct a tuple

Sorry, I mean the parenthesis is *not* required.

[bitdancer]

No, the parenthesis are never part of the tuple itself, even if you can't write syntactically correct code without them. They just syntactically group the expression list to isolate it from the surrounding context. It's the same principle as having an expression like (123). That's an integer that happens to be surrounded by parenthesis. The integer itself still starts at column 1.

[bitdancer]

Oh, and the empty tuple is a specific syntactic construct that really is the empty parenthesis, so that's consistent with the language definition.

[isaiah]

Fair enough, thanks for clarification.