Add printer for Graphviz Dot format

This generates a graph of todo items, where the edges denote
dependencies between items. The tooltip shows the description (visible
in an SVG by using a decent browser).

Based on the idea that I implemented in topydo's predecessor:
Dot output by todo.txt-tools [1]. There, I implemented it as a
subsubcommand of `dep` to visualize dependencies. This implementation,
however, is part of the `ls` command, where you can use a filter
expression to limit the size of the graph.

The todo.txt-tools implementation also prints a box with a lookup table
of the numbers to descriptions. This is lacking at the moment.

To visualize the todo items of a project, run:

    topydo ls -x -f dot +Project | dot -Tsvg -o Project1.svg

(-x is suggested because parents with unfinished todo items are not
printed by default).

[1] https://github.com/bram85/todo.txt-tools/blob/2f9b2baa6382346c5cd9d2272c61ee0e342e25b6/actions/dep#L162

topydo/commands/ListCommand.py

@@ -62,6 +62,9 @@ class ListCommand(ExpressionCommand):
                     if self._poke_icalendar():
                         from topydo.lib.IcalPrinter import IcalPrinter
                         self.printer = IcalPrinter(self.todolist)
+                elif value == 'dot':
+                    from topydo.lib.DotPrinter import DotPrinter
+                    self.printer = DotPrinter(self.todolist)
                 else:
                     self.printer = None
             elif opt == '-n':

topydo/lib/DotPrinter.py

+# Topydo - A todo.txt client written in Python.
+# Copyright (C) 2015 Bram Schoenmakers <me@bramschoenmakers.nl>
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+"""
+Provides a printer that transforms a list of Todo items to a graph in Dot
+notation. Useful for displaying dependencies.
+"""
+
+from topydo.lib.PrettyPrinter import Printer
+
+
+class DotPrinter(Printer):
+    """
+    A printer that converts a list of Todo items to a string in Dot format.
+    """
+
+    def __init__(self, p_todolist):
+        super(DotPrinter, self).__init__()
+        self.todolist = p_todolist
+
+    def print_list(self, p_todos):
+        node_name = lambda t: str(self.todolist.number(t))
+        node_tooltip = lambda t: todo.text().replace('"', '\\"')
+
+        result = 'digraph {\n'
+
+        # print todos
+        for todo in p_todos:
+            result += '  {} [tooltip="{}"]\n'.format(
+                node_name(todo),
+                node_tooltip(todo),
+            )
+
+        # print edges
+        for todo in p_todos:
+            for child in self.todolist.children(todo, p_only_direct=True):
+                result += '  {} -> {}\n'.format(
+                    node_name(todo),
+                    node_name(child)
+                )
+
+        result += '}\n'
+        return result