multipage pdf + pgf?

I'm using

import matplotlib as mpl
mpl.use ('pdf')
from matplotlib.backends.backend_pdf import PdfPages
  self.pdf = PdfPages(file_name)
  self.pdf.savefig (self.fig)

This works fine, but now I want to try pgf.

If I add:
from matplotlib.backends.backend_pgf import FigureCanvasPgf
mpl.backend_bases.register_backend('pdf', FigureCanvasPgf)

at the beginning, then it chokes on the savefig (self.fig):

Traceback (most recent call last):
  File "./", line 362, in <module>
    the_plot.finish (args, opt, time, res)
  File "./", line 157, in finish
    self.pdf.savefig (self.fig)
  File "/home/nbecker/.local/lib/python2.7/site-
packages/matplotlib/backends/", line 2297, in savefig
    figure.savefig(self, format='pdf', **kwargs)
  File "/home/nbecker/.local/lib/python2.7/site-packages/matplotlib/",
line 1421, in savefig
    self.canvas.print_figure(*args, **kwargs)
  File "/home/nbecker/.local/lib/python2.7/site-
packages/matplotlib/", line 2220, in print_figure
  File "/home/nbecker/.local/lib/python2.7/site-
packages/matplotlib/", line 2060, in _print_method
    return print_method(*args, **kwargs)
  File "/home/nbecker/.local/lib/python2.7/site-
packages/matplotlib/backends/", line 869, in print_pdf
    raise ValueError("filename must be a path or a file-like object")
ValueError: filename must be a path or a file-like object

I know I had this working some time in the past, what's the trick to making
multi-page pdf with pgf?