Error should be flagged if Walrus operator is used for multiple assigment

[JohnPie]

BPO	42295
Nosy	@isidentical, @johnpie
Files	walrustest.py: Python code

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

Show more details

GitHub fields:

assignee = None
closed_at = <Date 2020-11-09.12:44:05.710>
created_at = <Date 2020-11-09.10:48:14.577>
labels = ['build', 'invalid', '3.9']
title = 'Error should be flagged if Walrus operator is used for multiple assigment'
updated_at = <Date 2020-11-09.12:44:05.710>
user = 'https://github.com/JohnPie'

bugs.python.org fields:

activity = <Date 2020-11-09.12:44:05.710>
actor = 'BTaskaya'
assignee = 'none'
closed = True
closed_date = <Date 2020-11-09.12:44:05.710>
closer = 'BTaskaya'
components = []
creation = <Date 2020-11-09.10:48:14.577>
creator = 'JohnPie'
dependencies = []
files = ['49583']
hgrepos = []
issue_num = 42295
keywords = []
message_count = 2.0
messages = ['380580', '380581']
nosy_count = 2.0
nosy_names = ['BTaskaya', 'JohnPie']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'compile error'
url = 'https://bugs.python.org/issue42295'
versions = ['Python 3.9']

[JohnPie]

#The 'Walrus' operator does not support multiple assignment but does not #flag an attempt to make a multiple assigment as an error
#This results in unexpected behavior at execution time:

a, b = 100, 200

print (a, b)
#result is
#100 200

if (a, b := 3, 4): #this should be flagged as an error but is not

print ("found true")
else:
print ("found false")

print (a, b)
#result is
#100 3 but if multiple assigment were allowed this would be '3, 4'

----------------------------------------------------------------------

Python 3.9.0 (tags/v3.9.0:9cf6752, Oct  5 2020, 15:34:40) [MSC v.1927 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license()" for more information.
>>> 
= RESTART: C:\Users\John PC 2017\AppData\Local\Programs\Python\Python38-32\walrustest.py
100 200
found true
100 3
>>>

[isidentical]

Due to the precedence of the walrus operator, it is not actually a multiple assignment but rather a tuple of 3 elements with one being the value of the assignment expression.

 $ python -m ast  
(a, b := 3, 4)
Module(
   body=[
      Expr(
         value=Tuple(
            elts=[
               Name(id='a', ctx=Load()),
               NamedExpr(
                  target=Name(id='b', ctx=Store()),
                  value=Constant(value=3)),
               Constant(value=4)],
            ctx=Load()))],
   type_ignores=[])

In this case, it creates a tuple with loading the name a from the current scope, using the value of the 3 and also assigning 3 to the b, and loading constant 4.

So basically (a, b := 3, 4) is actually (a, (b := 3), 4)