Matplotlib pylab savefig runtime error in python 3.2.3 - python-3.x

Question

I am trying to test whether matplotlib installed properly in python3. I am following this basic tutorial: http://www.scipy.org/Plotting_Tutorial
This question is similar to one asked previously but unfortunately did not report the final solution explicitly and this is a different operating system.
The operating system for this is Mac OS X 10.6.8. Running the script in Python 3.2 does not work, but running it in Python 2.7 does. Unfortunately I need 3.2.
When the savefig line is commented out, it works fine.
"""
Example: simple line plot.
Show how to make and save a simple line plot with labels, title and grid
"""
import numpy
import pylab
t = numpy.arange(0.0, 1.0+0.01, 0.01)
s = numpy.cos(2*2*numpy.pi*t)
pylab.plot(t, s)
pylab.xlabel('time (s)')
pylab.ylabel('voltage (mV)')
pylab.title('About as simple as it gets, folks')
pylab.grid(True)
pylab.savefig('/Users/USERNAME/Documents/simple_plot.png', format='png')
pylab.show()
The code above returned the same error regardless of whether the path was set explicitly. I tried the following in addition to the above:
pylab.savefig('simple_plot.png', format='png')
pylab.savefig('simple_plot')
I tried setting the path explicitly as the other question said (as in the longer example above), but that resulted in the following errors. Note: mplex.py is the name of the script.
libpng warning: Application was compiled with png.h from libpng-1.2.44
libpng warning: Application is running with png.c from libpng-1.4.11
libpng warning: Incompatible libpng version in application and library
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "mplex.py", line 16, in <module>
pylab.savefig('/Users/USERNAME/Documents/simple_plot.png', format='png')
File "/opt/local/Library/Frameworks/Python.framework/Versions/3.2/lib/python3.2/site-packages/matplotlib/pyplot.py", line 474, in savefig
return fig.savefig(*args, **kwargs)
File "/opt/local/Library/Frameworks/Python.framework/Versions/3.2/lib/python3.2/site-packages/matplotlib/figure.py", line 1225, in savefig
self.canvas.print_figure(*args, **kwargs)
File "/opt/local/Library/Frameworks/Python.framework/Versions/3.2/lib/python3.2/site-packages/matplotlib/backend_bases.py", line 2075, in print_figure
**kwargs)
File "/opt/local/Library/Frameworks/Python.framework/Versions/3.2/lib/python3.2/site-packages/matplotlib/backend_bases.py", line 1846, in print_png
return agg.print_png(*args, **kwargs)
File "/opt/local/Library/Frameworks/Python.framework/Versions/3.2/lib/python3.2/site-packages/matplotlib/backends/backend_agg.py", line 497, in print_png
filename_or_obj, self.figure.dpi)
RuntimeError: Could not create write struct
Ideas? Is there another way to explicitly set the destination aside from what I tried in the savefig line?

Answer 1

I'm not sure what package manager you're using, and I'm on Python 2.7 (not 3.2.3), but I'm using Fink and had a similar problem (with different libpng version numbers).
In iPython --pylab, I was seeing this:
In [3]: savefig('foo.png')
libpng warning: Application is running with png.c from libpng-1.4.11
libpng warning: Incompatible libpng version in application and library
libpng warning: Application was compiled with png.h from libpng-1.5.11
Checking my Fink installations of libpng, I saw this:
$ fink list libpng
Information about 6268 packages read in 0 seconds.
i libpng14 1.4.11-1 PNG image format handling library
i libpng14-shlibs 1.4.11-1 Shared libraries for libpng14 package
libpng15 1.5.10-1 PNG image format handling library
libpng15-32bit 1.5.10-1 PNG library (32-bit)
libpng15-32bit-shli 1.5.10-1 Shared libraries for libpng15 package
i libpng15-shlibs 1.5.10-1 Shared libraries for libpng15 package
libpng3 1:1.2.49-1 PNG image format handling library
libpng3-shlibs 1:1.2.49-1 Shared libraries for libpng3 package
i system-pkgconfig-li 1.5.11-1 [virtual pkgconfig package representing libpng]
i system-pkgconfig-li 1.5.11-1 [virtual pkgconfig package representing libpng15]
After running "fink install libpng15", my libpng package versions looked like this:
$ fink list libpng
Information about 6268 packages read in 1 seconds.
libpng14 1.4.11-1 PNG image format handling library
i libpng14-shlibs 1.4.11-1 Shared libraries for libpng14 package
i libpng15 1.5.10-1 PNG image format handling library
libpng15-32bit 1.5.10-1 PNG library (32-bit)
libpng15-32bit-shli 1.5.10-1 Shared libraries for libpng15 package
i libpng15-shlibs 1.5.10-1 Shared libraries for libpng15 package
libpng3 1:1.2.49-1 PNG image format handling library
libpng3-shlibs 1:1.2.49-1 Shared libraries for libpng3 package
i system-pkgconfig-li 1.5.11-1 [virtual pkgconfig package representing libpng]
i system-pkgconfig-li 1.5.11-1 [virtual pkgconfig package representing libpng15]
As you can see, libpng14 is no longer installed and libpng15 is installed.
Next, "pip uninstall matplotlib" and "pip install matplotlib" yields a matplotlib build summary like this:
$ pip install matplotlib
Downloading/unpacking matplotlib
Downloading matplotlib-1.2.0.tar.gz (36.9Mb): 36.9Mb downloaded
Running setup.py egg_info for package matplotlib
basedirlist is: ['/usr/local/', '/usr', '/usr/X11', '/opt/local']
============================================================================
BUILDING MATPLOTLIB
matplotlib: 1.2.0
python: 2.7.3 (default, Jul 27 2012, 16:40:57) [GCC 4.2.1
Compatible Apple Clang 3.1
(tags/Apple/clang-318.0.61)]
platform: darwin
REQUIRED DEPENDENCIES
numpy: 1.6.2
freetype2: 15.0.9
OPTIONAL BACKEND DEPENDENCIES
libpng: 1.5.10
Tkinter: Tkinter: 81008, Tk: 8.5, Tcl: 8.5
Gtk+: no
* Building for Gtk+ requires pygtk; you must be able
* to "import gtk" in your build/install environment
Mac OS X native: yes
Qt: no
Qt4: no
PySide: no
Cairo: no
OPTIONAL DATE/TIMEZONE DEPENDENCIES
dateutil: 1.5
pytz: 2012b
OPTIONAL USETEX DEPENDENCIES
dvipng: no
ghostscript: 9.05
latex: no
You can see that the build will now pick up libpng 1.5.10. Previously, it was picking up 1.4.11 for the build.

Answer 2

Avoid using the standard console and the error will not appear by double clicking the yourcode.py or yourcode.pyw file.