Pylint depends on Graphviz & crashes if missing / Improve report-graph usage

[pylint-bot]

Originally reported by: Emile Anclin (BitBucket: anclin, GitHub: @Anclin?)

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Pylint's report graphs depends on Graphviz since it uses logilab.common.graph.DotBackend which just checks the extension of the output file.

But it took me a long road to figure out how to even to trigger this problem. (It is deactivated either by "-E" or "-rn", which is not totally obvious from the documention; and I used these options because I was only interested in the dependency graph -- which makes sense to me).
I found someone on stackoverflow complaining about the same issue; so
I used his .pylintrc file and triggered another bug:

#!bash

IOError: [Errno 2] No such file or directory: '/home/<my_username>/~/~/gr.dot'

ihmo it proves that it is seldom used, and I would move it to Pyreverse ;)
(I was preparing to pull-request a small patch for #168, but then I remembered Pylint's graph generation...)

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• Bitbucket: https://bitbucket.org/logilab/pylint/issue/228

[pylint-bot]

Original comment by Emile Anclin (BitBucket: anclin, GitHub: @Anclin?):

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So here is the simple way to trigger the crash (without .pylintrc):

#!bash

pylint --import-graph=my_output_file.svg /path/to/somefile.py
[...]
    make_graph(filename, dep_info, sect, '')
  File "/home/e1000/src/pylint/checkers/imports.py", line 117, in make_graph
    dependencies_graph(filename, dep_info)
  File "/home/e1000/src/pylint/checkers/imports.py", line 110, in dependencies_graph
    printer.generate(filename)
  File "/home/e1000/src/logilab/common/graph.py", line 123, in generate
    shell=use_shell)
  File "/usr/lib/python2.7/subprocess.py", line 493, in call
    return Popen(*popenargs, **kwargs).wait()
  File "/usr/lib/python2.7/subprocess.py", line 679, in __init__
    errread, errwrite)
  File "/usr/lib/python2.7/subprocess.py", line 1259, in _execute_child
    raise child_exception
OSError: [Errno 2] No such file or directory

[Pierre-Sassoulas]

This crash does not happen anymore in the latest version.