Generating a PNG with matplotlib when DISPLAY is undefined

Question

I am trying to use networkx with Python. When I run this program it get this error. Is there anything missing?

#!/usr/bin/env python  import networkx as nx import matplotlib import matplotlib.pyplot import matplotlib.pyplot as plt  G=nx.Graph() G.add_node(1) G.add_nodes_from([2,3,4,5,6,7,8,9,10]) #nx.draw_graphviz(G) #nx_write_dot(G, 'node.png') nx.draw(G) plt.savefig("/var/www/node.png")   Traceback (most recent call last):   File "graph.py", line 13, in <module>     nx.draw(G)   File "/usr/lib/pymodules/python2.5/networkx/drawing/nx_pylab.py", line 124, in draw     cf=pylab.gcf()   File "/usr/lib/pymodules/python2.5/matplotlib/pyplot.py", line 276, in gcf     return figure()   File "/usr/lib/pymodules/python2.5/matplotlib/pyplot.py", line 254, in figure     **kwargs)   File "/usr/lib/pymodules/python2.5/matplotlib/backends/backend_tkagg.py", line 90, in new_figure_manager     window = Tk.Tk()   File "/usr/lib/python2.5/lib-tk/Tkinter.py", line 1650, in __init__     self.tk = _tkinter.create(screenName, baseName, className, interactive, wantobjects, useTk, sync, use) _tkinter.TclError: no display name and no $DISPLAY environment variable 

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I get a different error now:

#!/usr/bin/env python  import networkx as nx import matplotlib import matplotlib.pyplot import matplotlib.pyplot as plt  matplotlib.use('Agg')  G=nx.Graph() G.add_node(1) G.add_nodes_from([2,3,4,5,6,7,8,9,10]) #nx.draw_graphviz(G) #nx_write_dot(G, 'node.png') nx.draw(G) plt.savefig("/var/www/node.png") 

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/usr/lib/pymodules/python2.5/matplotlib/__init__.py:835: 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.    if warn: warnings.warn(_use_error_msg) Traceback (most recent call last):   File "graph.py", line 15, in <module>     nx.draw(G)   File "/usr/lib/python2.5/site-packages/networkx-1.2.dev-py2.5.egg/networkx/drawing/nx_pylab.py", line 124, in draw     cf=pylab.gcf()   File "/usr/lib/pymodules/python2.5/matplotlib/pyplot.py", line 276, in gcf     return figure()   File "/usr/lib/pymodules/python2.5/matplotlib/pyplot.py", line 254, in figure     **kwargs)   File "/usr/lib/pymodules/python2.5/matplotlib/backends/backend_tkagg.py", line 90, in new_figure_manager     window = Tk.Tk()   File "/usr/lib/python2.5/lib-tk/Tkinter.py", line 1650, in __init__     self.tk = _tkinter.create(screenName, baseName, className, interactive, wantobjects, useTk, sync, use) _tkinter.TclError: no display name and no $DISPLAY environment variable 

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I get a different error now:

#!/usr/bin/env python  import networkx as nx import matplotlib import matplotlib.pyplot import matplotlib.pyplot as plt  matplotlib.use('Agg')  G=nx.Graph() G.add_node(1) G.add_nodes_from([2,3,4,5,6,7,8,9,10]) #nx.draw_graphviz(G) #nx_write_dot(G, 'node.png') nx.draw(G) plt.savefig("/var/www/node.png") 

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/usr/lib/pymodules/python2.5/matplotlib/__init__.py:835: 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.    if warn: warnings.warn(_use_error_msg) Traceback (most recent call last):   File "graph.py", line 15, in <module>     nx.draw(G)   File "/usr/lib/python2.5/site-packages/networkx-1.2.dev-py2.5.egg/networkx/drawing/nx_pylab.py", line 124, in draw     cf=pylab.gcf()   File "/usr/lib/pymodules/python2.5/matplotlib/pyplot.py", line 276, in gcf     return figure()   File "/usr/lib/pymodules/python2.5/matplotlib/pyplot.py", line 254, in figure     **kwargs)   File "/usr/lib/pymodules/python2.5/matplotlib/backends/backend_tkagg.py", line 90, in new_figure_manager     window = Tk.Tk()   File "/usr/lib/python2.5/lib-tk/Tkinter.py", line 1650, in __init__     self.tk = _tkinter.create(screenName, baseName, className, interactive, wantobjects, useTk, sync, use) _tkinter.TclError: no display name and no $DISPLAY environment variable 

Answer 1

The main problem is that (on your system) matplotlib chooses an x-using backend by default. I just had the same problem on one of my servers. The solution for me was to add the following code in a place that gets read before any other pylab/matplotlib/pyplot import:

import matplotlib # Force matplotlib to not use any Xwindows backend. matplotlib.use('Agg') 

The alternative is to set it in your .matplotlibrc

Answer 2

Just as a complement of Reinout's answer.

The permanent way to solve this kind of problem is to edit .matplotlibrc file. Find it via

>>> import matplotlib
>>> matplotlib.matplotlib_fname() # This is the file location in Ubuntu '/etc/matplotlibrc' 

Then modify the backend in that file to backend : Agg. That is it.