cannot change font to Helvetica in Matplotlib in Python on Mac OS X 10.6

Question

I am trying to change the matplotlib font to helvetica, which I'd like to use in a PDF plot. I try the following:

import matplotlib
matplotlib.use('PDF')
import matplotlib.pylab as plt
from matplotlib import rc
plt.rcParams['ps.useafm'] = True
rc('font',**{'family':'sans-serif','sans-serif':['Helvetica']})
plt.rcParams['pdf.fonttype'] = 42

This does not work -- when I run my code with --verbose-debug, I get the error:

backend WXAgg version 2.8.10.1
/Library/Frameworks/Python.framework/Versions/6.2/lib/python2.6/site-packages/matplotlib/__init__.py:833: UserWarning:  This call to matplotlib.use() has no effect
because the the backend has already been chosen;
matplotlib.use() must be called *before* pylab, matplotlib.pyplot,
or matplotlib.backends is imported for the first time.
findfont: Could not match :family=sans-serif:style=normal:variant=normal:weight=normal:stretch=normal:size=medium. Returning /Library/Frameworks/Python.framework/Versions/6.2/lib/python2.6/site-packages/matplotlib/mpl-data/fonts/ttf/Vera.ttf
Assigning font /F1 = /Library/Frameworks/Python.framework/Versions/6.2/lib/python2.6/site-packages/matplotlib/mpl-data/fonts/ttf/Vera.ttf
Embedding font /Library/Frameworks/Python.framework/Versions/6.2/lib/python2.6/site-packages/matplotlib/mpl-data/fonts/ttf/Vera.ttf
Writing TrueType font

So apparently it cannot find Helvetica. I am not sure why. I have Helvetica in the afm directory of mpl-data, and when matplotlib initiates it reads it and outputs:

createFontDict: /Library/Frameworks/Python.framework/Versions/6.2/lib/python2.6/site-packages/matplotlib/mpl-data/fonts/afm/Helvetica.afm

Do I need a special .ttf Helvetica font in addition? If so, how can I get it? I know I have Helvetica on my system since I see it in Illustrator and many other programs.

I am using Enthought Python distribution as follows:

$ python
Enthought Python Distribution -- http://www.enthought.com
Version: 6.2-2 (32-bit)

Python 2.6.5 |EPD 6.2-2 (32-bit)| (r265:79063, May 28 2010, 15:13:03) 
[GCC 4.0.1 (Apple Inc. build 5488)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import matplotlib
>>> matplotlib.__version__
'0.99.3'

Any ideas how this can be fixed?

thanks.

Solution

Step-by-step solution for OS X 10.11 El Capitan and Python 3 (based on this post).

1. Install fondu: brew install fondu

2. Find out matplotlib location:

   python3 -c "import matplotlib ; print(matplotlib.matplotlib_fname())" 
   

   For me it's /usr/local/lib/python3.5/site-packages/matplotlib/mpl-data/matplotlibrc

3. Make a copy of Helvetica:

   mkdir ~/Desktop/font_copies
   cp /System/Library/Fonts/Helvetica.dfont ~/Desktop/font_copies
   

4. Convert the Helvetica copy we've made from dfont to ttf:

   cd /usr/local/lib/python3.5/site-packages/matplotlib/mpl-data/fonts/ttf/
   fondu -show ~/Desktop/font_copies/Helvetica.dfont
   

5. Remove font cache: rm ~/.matplotlib/fontList.py3k.cache

Done! Now you can use Helvetica:

import matplotlib.pyplot as plt
plt.rc('font', family='Helvetica')