PEP 572: Add a section detailing how SLNBs are different from regular

pep-0572.rst

@@ -92,6 +92,25 @@ Some of these examples should be considered *bad code* and rejected by code
 review and/or linters; they are not, however, illegal.
 
 
+Differences from regular assignment statements
+----------------------------------------------
+
+Using ``(EXPR as NAME)`` is similar to ``NAME = EXPR``, but has a number of
+important distinctions.
+
+* Assignment is a statement; an SLNB is an expression whose value is the same
+  as the object bound to the new name.
+* SLNBs disappear at the end of their enclosing statement, at which point the
+  name again refers to whatever it previously would have.  SLNBs can thus
+  shadow other names without conflict (although deliberately doing so will
+  often be a sign of bad code).
+* SLNBs cannot be closed over by nested functions, and are completely ignored
+  for this purpose.
+* SLNBs do not appear in ``locals()`` or ``globals()``.
+* An SLNB cannot be the target of any form of assignment, including augmented.
+  Attempting to do so will remove the SLNB and assign to the fully-scoped name.
+
+
 Example usage
 =============