The documentation states (http://matplotlib.org/users/pgf.html):

registering it for handling pdf output:
from matplotlib.backends.backend_pgf import FigureCanvasPgf
matplotlib.backend_bases.register_backend('pdf', FigureCanvasPgf)

Since commit 8159c81 the register_backend function now requires a description argument.
I see two possibilities:

1. update the documentation and add a note to the API changes document
2. give the description argument a default value (empty string or None meaning format)

I have many files using the above code from the documentation.