PEP 572: Create a section on alternate proposals

pep-0572.rst

@@ -135,6 +135,53 @@ Open questions
    adding to the confusion.
 
 
+Alternative syntax proposals
+============================
+
+Proposals of this nature have come up frequently on python-ideas. Below are
+a number of alternative syntaxes, some of them specific to comprehensions,
+which have been rejected in favour of the one given above.
+
+1. ``where``, ``let``, ``given``::
+
+       stuff = [(y, y) where y = f(x) for x in range(5)]
+       stuff = [(y, y) let y = f(x) for x in range(5)]
+       stuff = [(y, y) given y = f(x) for x in range(5)]
+
+   This brings the subexpression to a location in between the 'for' loop and
+   the expression. It introduces an additional language keyword, which creates
+   conflicts. Of the three, ``where`` reads the most cleanly, but also has the
+   greatest potential for conflict (eg SQLAlchemy and numpy have ``where``
+   methods, as does ``tkinter.dnd.Icon`` in the standard library).
+
+2. ``with``::
+
+       stuff = [(y, y) with y = f(x) for x in range(5)]
+
+   As above, but reusing the `with` keyword. Doesn't read too badly, and needs
+   no additional language keyword. Is restricted to comprehensions, though,
+   and cannot as easily be transformed into "longhand" for-loop syntax. Has
+   the C problem that an equals sign in an expression can now create a name
+   binding, rather than performing a comparison.
+
+3. ``with... as``::
+
+       stuff = [(y, y) with f(x) as y for x in range(5)]
+
+   As per option 2, but using ``as`` in place of the equals sign. Aligns
+   syntactically with other uses of ``as`` for name binding, but a simple
+   transformation to for-loop longhand would create drastically different
+   semantics; the meaning of ``with`` inside a comprehension would be
+   completely different from the meaning as a stand-alone statement.
+
+4. ``EXPR as NAME`` without parentheses::
+
+       stuff = [[f(x) as y, y] for x in range(5)]
+
+   Omitting the parentheses from this PEP's proposed syntax introduces many
+   syntactic ambiguities.
+
+
 References
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