Improve syntax error for invalid comparisons such as:

>>> if x = 3:
  File "<stdin>", line 1
    if x = 3:
         ^
SyntaxError: invalid syntax

to print:

>>> if x = 3:
  File "<stdin>", line 1
    if x = 3:
         ^
SyntaxError: cannot assign to name. Maybe you meant '==' or ':=' instead of '='?

instead

"cannot assign to name" looks wrong. Usually we can assign to name, unless it is a keyword. And the problem is not that we cannot assign to name (we can), but that is is an invalid syntax, assignment is a statement, not an expression.

Seems it handles only simplest cases with "=" in "if". It does not handle "while x = 3" and "if y and x = 3". Could it be possible to make it more general.

I tried to add

    | a=NAME '=' {
        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(
            a, "invalid syntax. Maybe you meant '==' or ':=' instead of '='?") }
    | a=bitwise_or '=' {
        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(
            a, "invalid syntax. Maybe you meant '==' instead of '='?") }

in invalid_named_expression, but it does not have any effect.

These are good points, let me investigate a bit

> in invalid_named_expression, but it does not have any effect.

That is because the rule for named_expressions are:

 named_expression[expr_ty]:
     | a=NAME ':=' ~ b=expression
     | expression !':='
     | invalid_named_expression

and the second alternative (| expression !':=') succeeds with an assignment, not letting the invalid_named_expression run

I have updated the PR with:

* Works for while statements (we can make it work for other kinds of statements but is a bit tricky because those have different grammar paths. We could do this in other PRs.

* I changed the error message for when the LHS is a name. In other cases we still want the special error message. For example:

>>> f() = 1
  File "<stdin>", line 1
    f() = 1
        ^
SyntaxError: cannot assign to function call. Maybe you meant '==' instead of '='?

If we still want something different we could say:

"cannot assign to ... here". The "here" makes it clear that in other places could be possible to do an assignment (for instance with names or attributes).

New changeset b86ed8e3bb41ede77eeab4a8bb4e2b91a8065283 by Pablo Galindo in branch 'master':
bpo-43797: Improve syntax error for invalid comparisons (#25317)
https://github.com/python/cpython/commit/b86ed8e3bb41ede77eeab4a8bb4e2b91a8065283

New changeset 30ed93bfec5dfa7ee05982e2df8fd810f3f49305 by Pablo Galindo in branch 'master':
bpo-43797: Handle correctly invalid assignments inside function calls and generators (GH-25390)
https://github.com/python/cpython/commit/30ed93bfec5dfa7ee05982e2df8fd810f3f49305