[Matplotlib-users] multipage pdf + pgf?

[Neal B. <ndb...@gm...>]

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)
  plt.close()
...
pdf.close()

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 "./plot_stuff2.py", line 362, in <module>
    the_plot.finish (args, opt, time, res)
  File "./plot_stuff2.py", line 157, in finish
    self.pdf.savefig (self.fig)
  File "/home/nbecker/.local/lib/python2.7/site-
packages/matplotlib/backends/backend_pdf.py", line 2297, in savefig
    figure.savefig(self, format='pdf', **kwargs)
  File "/home/nbecker/.local/lib/python2.7/site-packages/matplotlib/figure.py", 
line 1421, in savefig
    self.canvas.print_figure(*args, **kwargs)
  File "/home/nbecker/.local/lib/python2.7/site-
packages/matplotlib/backend_bases.py", line 2220, in print_figure
    **kwargs)
  File "/home/nbecker/.local/lib/python2.7/site-
packages/matplotlib/backend_bases.py", line 2060, in _print_method
    return print_method(*args, **kwargs)
  File "/home/nbecker/.local/lib/python2.7/site-
packages/matplotlib/backends/backend_pgf.py", 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?