Symbol table for comprehensions (list, dict, set) still includes temporary _[1] variable

[mjpieters]

BPO	32836
Nosy	@mjpieters, @ncoghlan, @nitishch, @miss-islington
PRs	bpo-32836: Remove temporary variable usage #5680 [3.7] bpo-32836: Remove obsolete code from symtable pass (GH-5680) #5913

^{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 2018-08-27.09:55:02.075>
created_at = <Date 2018-02-12.21:33:51.537>
labels = ['3.8', 'type-bug', '3.7']
title = 'Symbol table for comprehensions (list, dict, set) still includes temporary _[1] variable'
updated_at = <Date 2018-08-27.09:55:02.073>
user = 'https://github.com/mjpieters'

bugs.python.org fields:

activity = <Date 2018-08-27.09:55:02.073>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = True
closed_date = <Date 2018-08-27.09:55:02.075>
closer = 'serhiy.storchaka'
components = []
creation = <Date 2018-02-12.21:33:51.537>
creator = 'mjpieters'
dependencies = []
files = []
hgrepos = []
issue_num = 32836
keywords = ['patch']
message_count = 4.0
messages = ['312081', '312155', '312959', '313565']
nosy_count = 4.0
nosy_names = ['mjpieters', 'ncoghlan', 'nitishch', 'miss-islington']
pr_nums = ['5680', '5913']
priority = 'normal'
resolution = None
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue32836'
versions = ['Python 3.7', 'Python 3.8']

[mjpieters]

In Python 2.6, a list comprehension was implemented in the current scope using a temporary _[1] variable to hold the list object:

>>> import dis
>>> dis.dis(compile('[x for x in y]', '?', 'exec'))
  1           0 BUILD_LIST               0
              3 DUP_TOP
              4 STORE_NAME               0 (_[1])
              7 LOAD_NAME                1 (y)
             10 GET_ITER
        >>   11 FOR_ITER                13 (to 27)
             14 STORE_NAME               2 (x)
             17 LOAD_NAME                0 (_[1])
             20 LOAD_NAME                2 (x)
             23 LIST_APPEND
             24 JUMP_ABSOLUTE           11
        >>   27 DELETE_NAME              0 (_[1])
             30 POP_TOP
             31 LOAD_CONST               0 (None)
             34 RETURN_VALUE

Nick Cochlan moved comprehensions into a separate scope in bpo-1660500, and removed the need for a temporary variable in the process (the list / dict / set lives only on the stack).

However, the symbol table generates the _[1] name:

>>> import symtable
>>> symtable.symtable('[x for x in y]', '?', 'exec').get_children()[0].get_symbols()
[<symbol '.0'>, <symbol '_[1]'>, <symbol 'x'>]

Can this be dropped? I think all temporary variable handling can be ripped out.

[ncoghlan]

We still need to the ".0" style temporary variables that are used for argument names in the implicitly generated functions, but it's definitely plausible that we're not actually using the "_[1]" style hidden variables anywhere anymore.

[ncoghlan]

New changeset 3a087be by Nick Coghlan (Nitish Chandra) in branch 'master':
bpo-32836: Remove obsolete code from symtable pass (GH-5680)
3a087be

[miss-islington]

New changeset 5506d60 by Miss Islington (bot) in branch '3.7':
bpo-32836: Remove obsolete code from symtable pass (GH-5680)
5506d60