Walrus operator in Python interpreter

When I use the walrus operator as below in the Python(3.9.6) interpreter,

>>> walrus:=True

I get a syntax error:

  File "<stdin>", line 1
    walrus := True
                  ^
SyntaxError: invalid syntax

How is this different from the following?

>>> print(walrus := True)

Answer

It’s different because the Python core developers were very ambivalent about violating the Zen of Python guideline “There should be one– and preferably only one –obvious way to do it”, and chose to make it inconvenient to replace most uses of plain = with := without adding additional parentheses to the expression.

Rather than allowing := to replace = in all contexts, they specifically prohibited unparenthesized top-level use of the walrus:

Unparenthesized assignment expressions are prohibited at the top level of an expression statement.

y := f(x)  # INVALID
(y := f(x))  # Valid, though not recommended

This rule is included to simplify the choice for the user between an assignment statement and an assignment expression – there is no syntactic position where both are valid.

In many cases where := is prohibited, you can make it valid by adding otherwise unnecessary parentheses around the expression, so:

(walrus:=True)

works just fine, but it’s enough of a pain that the assumption is that most people will stick to the simpler and more Pythonic:

walrus = True

in that scenario.