Parsing does not raise error for code assigning to a function call

[DRMacIver]

LibCST will happily parse the following code:

import libcst
libcst.parse_statement('max() = 1')

However this is a syntax error, as you can't assign to function calls, and ideally would be raised as such.

(Tested on libcst 0.4.9)

[jakkdl]

So, I'm looking at whether to place this validation in BaseAssignTargetExpression, AssignTarget, and/or Assign/AnnAssign/AugAssign.

LibCST/libcst/_nodes/expression.py

Lines 286 to 301 in ea19578

	class BaseAssignTargetExpression(BaseExpression, ABC):
	"""
	An expression that's valid on the left side of an assignment. That assignment may
	be part an :class:`Assign` node, or it may be part of a number of other control
	structures that perform an assignment, such as a :class:`For` loop.
	Python's grammar defines all expression as valid in this position, but the AST
	compiler further restricts the allowed types, which is what this type attempts to
	express.
	This is similar to a :class:`BaseDelTargetExpression`, but it also includes
	:class:`StarredElement` as a valid node.
	The set of valid nodes are defined as part of `CPython's AST context computation
	<https://github.com/python/cpython/blob/v3.8.0a4/Python/ast.c#L1120>`_.
	"""

since 3.9 the python grammar does define the valid assignment targets

 assignment:
    | NAME ':' expression ['=' annotated_rhs ] 
    | ('(' single_target ')' 
         | single_subscript_attribute_target) ':' expression ['=' annotated_rhs ] 
    | (star_targets '=' )+ (yield_expr | star_expressions) !'=' [TYPE_COMMENT] 
    | single_target augassign ~ (yield_expr | star_expressions) 

so I thought I'd add a _validate to BaseAssignTargetExpression that checks whether it's a Name, Subscript or StarredElement. But then I noticed that a lot of other classes inherits from it, including Name, Annotation, Attribute - which all quickly starts breaking if I restrict what BaseAssignTargetExpression can be. Is there a reason why all of these don't just inherit from BaseExpression, or can I change them to do so? Or should I place the validation in AssignTarget instead?

[jakkdl]

@zsol any thoughts on the above?

[zsol]

The native parser correctly raises a syntax error here. So to work around you can set the LIBCST_PARSER_TYPE environment variable to native before calling libcst, or wait for the next release which will have this behavior by default due to #929

[jakkdl]

Oh, I totally thought I tested it with native - but apparently not. Sweet!

[zsol]

Gonna close this out as native parser is enabled by default in 1.0.0