The way you define f is a Python function, not a symbolic expression. In particular, the name of the input does not matter. So, there is no point to deal with f(y) which requires that y is defined (as a symbol).
What you can do is the following:
sage: plot(f,(0,1))
Launched png viewer for Graphics object consisting of 1 graphics primitive
Asked: 2015-07-09 11:34:34 +0100
Seen: 1,977 times
Last updated: Jul 09 '15
Copyright Sage, 2010. Some rights reserved under creative commons license. Content on this site is licensed under a Creative Commons Attribution Share Alike 3.0 license.