Should the set_alpha method of a graphics context accept a string (instead of a number) as an argument? Currently, some backends (agg, ps, pdf) accept strings such as "0.2", whereas others (svg, cairo, Mac OS X) do not. Usually that is not a problem, since in almost all cases set_alpha is called with a number as the argument. However, the example axes_zoom_effect.py, which was recently added to examples/pylab_examples, has these lines:
which causes a call to gc.set_alpha of the form gc.set_alpha("0.2"). Backends that do not allow strings choke on that.
Whereas in general it may perhaps be useful to allow such strings, it adds complexity to the code, and I don't see a good use case for it.