Karthikeyan Singaravelan <[email protected]> added the comment:
I don't know if this is the correct issue for questions/clarifications but 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.
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<https://bugs.python.org/issue35224>
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