title: Matplotlib backend missing modules with underscore
body: <p>I've been using matplotlib for some time without problems. It's been a while since i needed the interactive plot functions (for which Tkaag was used). Since then i updated matplotlib a few times.</p>
  
  <p>I tried to use it today, but it spawned an error.</p>
  
  <pre><code>/usr/local/lib/python2.7/dist-packages/matplotlib/backends/tkagg.py in &lt;module&gt;()
  ----&gt; 1 import _tkagg
        2 import Tkinter as Tk
        3 
        4 def blit(photoimage, aggimage, bbox=None, colormode=1):
        5     tk = photoimage.tk
  
  ImportError: No module named _tkagg
  </code></pre>
  
  <p>I tried a different backend, added the </p>
  
  <pre><code>backend      : GTKAgg
  </code></pre>
  
  <p>to matplotlibrc. Just to get the same error for a different module.</p>
  
  <pre><code>ImportError: No module named _backend_gdk
  </code></pre>
  
  <p>When I browsed for backends in /usr/local/lib/python2.7/dist-packages/matplotlib/backends/ i've noticed that all the required modules with underscore are missing.</p>
  
  <pre><code>alan@linux /usr/local/lib/python2.7/dist-packages/matplotlib/backends $ ls
  backend_agg.py        backend_macosx.py    backend_template.pyc
  backend_agg.pyc       backend_macosx.pyc   backend_tkagg.py
  _backend_agg.so       backend_mixed.py     backend_tkagg.pyc
  backend_cairo.py      backend_mixed.pyc    backend_wxagg.py
  backend_cairo.pyc     backend_pdf.py       backend_wxagg.pyc
  backend_cocoaagg.py   backend_pdf.pyc      backend_wx.py
  backend_cocoaagg.pyc  backend_ps.py        backend_wx.pyc
  backend_emf.py        backend_ps.pyc       __init__.py
  backend_emf.pyc       backend_qt4agg.py    __init__.pyc
  backend_fltkagg.py    backend_qt4agg.pyc   Matplotlib.nib
  backend_fltkagg.pyc   backend_qt4.py       qt4_compat.py
  backend_gdk.py        backend_qt4.pyc      qt4_compat.pyc
  backend_gdk.pyc       backend_qtagg.py     qt4_editor
  backend_gtkagg.py     backend_qtagg.pyc    tkagg.py
  backend_gtkagg.pyc    backend_qt.py        tkagg.pyc
  backend_gtkcairo.py   backend_qt.pyc       windowing.py
  backend_gtkcairo.pyc  backend_svg.py       windowing.pyc
  backend_gtk.py        backend_svg.pyc
  backend_gtk.pyc       backend_template.py
  </code></pre>
  
  <p>My current matplotlib version:</p>
  
  <pre><code>matplotlib      - 1.1.1        - active development (/usr/local/lib/python2.7/dist-packages)
  </code></pre>
  
  <p>I've tried uninstalled and reinstalling matplotlib with:</p>
  
  <pre><code>pip uninstall matplotlib
  pip install matplotlib
  </code></pre>
  
  <p>and all went well.</p>
  
  <p>Tips on setting on getting interactive plotting running again?</p>
  
  <pre><code>    BUILDING MATPLOTLIB
  
                  matplotlib: 1.1.1
  
                      python: 2.7.3rc2 (default, Apr 22 2012, 22:35:38)  [GCC
  
                              4.6.3]
  
                    platform: linux2
  
  
  
      REQUIRED DEPENDENCIES
  
                       numpy: 1.6.2
  
                   freetype2: 14.1.8
  
  
  
      OPTIONAL BACKEND DEPENDENCIES
  
                      libpng: 1.2.49
  
                     Tkinter: no
  
                              * Using default library and include directories for
  
                              * Tcl and Tk because a Tk window failed to open.
  
                              * You may need to define DISPLAY for Tk to work so
  
                              * that setup can determine where your libraries are
  
                              * located. Tkinter present, but header files are not
  
                              * found. You may need to install development
  
                              * packages.
  
                  pkg-config: looking for pygtk-2.0 gtk+-2.0
  
                              * Package pygtk-2.0 was not found in the pkg-config
  
                              * search path. Perhaps you should add the directory
  
                              * containing `pygtk-2.0.pc' to the PKG_CONFIG_PATH
  
                              * environment variable No package 'pygtk-2.0' found
  
                              * Package gtk+-2.0 was not found in the pkg-config
  
                              * search path. Perhaps you should add the directory
  
                              * containing `gtk+-2.0.pc' to the PKG_CONFIG_PATH
  
                              * environment variable No package 'gtk+-2.0' found
  
                              * You may need to install 'dev' package(s) to
  
                              * provide header files.
  
                        Gtk+: no
  
                              * Could not find Gtk+ headers in any of
  
                              * '/usr/local/include', '/usr/include', '.'
  
             Mac OS X native: no
  
                          Qt: no
  
                         Qt4: Qt: 4.8.1, PyQt4: 4.9.1
  
                      PySide: no
  
                       Cairo: 1.8.8
  
  
  
      OPTIONAL DATE/TIMEZONE DEPENDENCIES
  
                    datetime: present, version unknown
  
                    dateutil: matplotlib will provide
  
                        pytz: matplotlib will provide
  
      adding pytz
  
  
  
      OPTIONAL USETEX DEPENDENCIES
  
                      dvipng: 1.14
  
                 ghostscript: 9.05
  
                       latex: 3.1415926
  
                     pdftops: 0.18.4
  </code></pre>