Why does "if not a := say_empty()" raise a SyntaxError?

Question

PEP 572 introduces the assignment operator ("walrus operator").

I tried to negate a condition:

def say_empty():
    return ''

if not a := say_empty():
    print("empty")
else:
    print("not empty")

This raises a SyntaxError

    if not a := say_empty():
       ^
SyntaxError: cannot use assignment expressions with operator

I am wondering why this limitation was put in place.

PEP 572 explains why using the assignment in iterations is problematic (and raises SyntaxError), but I did not find anything about boolean ones.

khelwood · Accepted Answer · 2021-02-20 08:03:46Z

27

Operator precedence indicates that := has a lower precedence than not. So not a := is read as trying to assign to not a, hence the syntax error.

You can use parentheses to clarify the meaning:

if not (a := say_empty()):
    ...

edited Feb 20, 2021 at 8:03

answered Feb 20, 2021 at 7:54

khelwood

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