parenthesis is mandatory for named expressions in while statement

[tirkarthi]

BPO	35877
Nosy	@gvanrossum, @emilyemorehouse, @tirkarthi
PRs	bpo-35877: Make parenthesis optional for named expression in while statement #11724 bpo-35877: Make parenthesis optional for named expression in while statement #11724 bpo-35877: Make parenthesis optional for named expression in while statement #11724 bpo-35877: Add test for while loop named expression without parentheses #11726 bpo-35877: Add test for while loop named expression without parentheses #11726 bpo-35877: Add test for while loop named expression without parentheses #11726

^{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 = 'https://github.com/emilyemorehouse'
closed_at = <Date 2019-02-01.22:28:36.404>
created_at = <Date 2019-02-01.10:55:55.277>
labels = ['interpreter-core', 'type-bug', '3.8']
title = 'parenthesis is mandatory for named expressions in while statement'
updated_at = <Date 2019-02-01.22:39:54.852>
user = 'https://github.com/tirkarthi'

bugs.python.org fields:

activity = <Date 2019-02-01.22:39:54.852>
actor = 'xtreak'
assignee = 'emilyemorehouse'
closed = True
closed_date = <Date 2019-02-01.22:28:36.404>
closer = 'emilyemorehouse'
components = ['Interpreter Core']
creation = <Date 2019-02-01.10:55:55.277>
creator = 'xtreak'
dependencies = []
files = []
hgrepos = []
issue_num = 35877
keywords = ['patch', 'patch', 'patch']
message_count = 9.0
messages = ['334665', '334688', '334692', '334699', '334704', '334724', '334725', '334726', '334727']
nosy_count = 3.0
nosy_names = ['gvanrossum', 'emilyemorehouse', 'xtreak']
pr_nums = ['11724', '11724', '11724', '11726', '11726', '11726']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue35877'
versions = ['Python 3.8']

[tirkarthi]

I thought to open a separate report itself in order to keep the original issue not cluttered with questions since it could be used for other docs. Sorry for my report there. Original report as per msg334341 .

It seems parens are mandatory while using named expressions in while statement which makes some of the examples invalid like https://www.python.org/dev/peps/pep-0572/#sysconfig-py . From my limited knowledge while statement Grammar was not modified at https://github.com/python/cpython/pull/10497/files#diff-cb0b9d6312c0d67f6d4aa1966766ceddR73 and no tests for while statement which made me assume it's intentional. I haven't followed the full discussion about PEP-572 so feel free to correct me if it's a conscious decision and in that case the PEP-572 can be updated.

# python info

➜ cpython git:(master) ./python.exe
Python 3.8.0a0 (heads/bpo35113-dirty:49329a217e, Jan 25 2019, 09:57:53)
[Clang 7.0.2 (clang-700.1.81)] on darwin
Type "help", "copyright", "credits" or "license" for more information.

>>

# Example as in PEP-572 to create a simple file that reads itself and prints lines that matches "foo"

➜ cpython git:(master) cat /tmp/foo.py
import re

with open("/tmp/foo.py") as f:
    while line := f.readline():
        if match := re.search(r"foo", line):
            print(match.string.strip("\n"))
➜  cpython git:(master) ./python.exe /tmp/foo.py
  File "/tmp/foo.py", line 4
    while line := f.readline():
               ^
SyntaxError: invalid syntax

# Wrapping named expression with parens for while makes this valid

➜ cpython git:(master) cat /tmp/foo.py
import re

with open("/tmp/foo.py") as f:
    while (line := f.readline()):
        if match := re.search(r"foo", line):
            print(match.string.strip("\n"))
➜  cpython git:(master) ./python.exe /tmp/foo.py
with open("/tmp/foo.py") as f:
        if match := re.search(r"foo", line):

As a user I think parens shouldn't be mandatory in while statement since if statement works fine. Parens can cause while statement to be superfluous in some cases and an extra case to remember while teaching.

[gvanrossum]

Emily, I think this would be as simple as making a tiny change to Grammar/Grammar and running make regen-grammar. Can you take care of that?

[tirkarthi]

Emily, I think this would be as simple as making a tiny change to Grammar/Grammar and running make regen-grammar. Can you take care of that?

Thanks, can confirm that this fixes the issue. I changed test to namedexpr_test for while statement in grammar and ran make regen-grammar and make. The reported program runs fine and Lib/test/test_named_expressions.py also passes.

➜ cpython git:(master) ✗ cat /tmp/foo.py
import re

with open("/tmp/foo.py") as f:
    while line := f.readline():
        if match := re.search(r"foo", line):
            print(match.string.strip("\n"))
➜  cpython git:(master) ✗ ./python.exe /tmp/foo.py
with open("/tmp/foo.py") as f:
        if match := re.search(r"foo", line):

Patch :

➜  cpython git:(master) ✗ git diff | cat
diff --git a/Grammar/Grammar b/Grammar/Grammar
index e65a688e4c..a425978059 100644
--- a/Grammar/Grammar
+++ b/Grammar/Grammar
@@ -72,7 +72,7 @@ assert_stmt: 'assert' test [',' test]
 compound_stmt: if_stmt | while_stmt | for_stmt | try_stmt | with_stmt | funcdef | classdef | decorated | async_stmt
 async_stmt: 'async' (funcdef | with_stmt | for_stmt)
 if_stmt: 'if' namedexpr_test ':' suite ('elif' namedexpr_test ':' suite)* ['else' ':' suite]
-while_stmt: 'while' test ':' suite ['else' ':' suite]
+while_stmt: 'while' namedexpr_test ':' suite ['else' ':' suite]
 for_stmt: 'for' exprlist 'in' testlist ':' [TYPE_COMMENT] suite ['else' ':' suite]
 try_stmt: ('try' ':' suite
            ((except_clause ':' suite)+
diff --git a/Python/graminit.c b/Python/graminit.c
index 6e0f19891b..5cdde2789c 100644
--- a/Python/graminit.c
+++ b/Python/graminit.c
@@ -971,7 +971,7 @@ static arc arcs_42_0[1] = {
     {103, 1},
 };
 static arc arcs_42_1[1] = {
-    {26, 2},
+    {99, 2},
 };
 static arc arcs_42_2[1] = {
     {27, 3},

[gvanrossum]

Thanks, Karthikeyan! Can you submit that as a PR?

[tirkarthi]

Thanks, Karthikeyan! Can you submit that as a PR?

Sure, I will submit the patch with tests for the same.

Thanks

[emilyemorehouse]

New changeset d4fceaa by Emily Morehouse (Xtreak) in branch 'master':
bpo-35877: Make parenthesis optional for named expression in while statement (GH-11724)
d4fceaa

[emilyemorehouse]

Thanks, Karthikeyan! I added an additional test to make sure this gets coverage. I'll close out this issue once tests pass and I can merge that.

[emilyemorehouse]

New changeset ac19081 by Emily Morehouse in branch 'master':
bpo-35877: Add test for while loop named expression without parentheses (GH-11726)
ac19081

[tirkarthi]

Thanks a lot Guido and Emily for guidance on this issue. Happy to see this for alpha release :)