word: throw an exception when printing an empty word

Fixes #146.

* spot/twaalgos/word.cc: Here.
* tests/python/word.ipynb: Add a test case.

spot/twaalgos/word.cc

@@ -93,6 +93,8 @@ namespace spot
   std::ostream&
   operator<<(std::ostream& os, const twa_word& w)
   {
+    if (w.cycle.empty())
+      throw std::runtime_error("a twa_word may not have an empty cycle");
     auto d = w.get_dict();
     if (!w.prefix.empty())
       for (auto& i: w.prefix)
@@ -100,7 +102,6 @@ namespace spot
 	  bdd_print_formula(os, d, i);
 	  os << "; ";
 	}
-    assert(!w.cycle.empty());
     bool notfirst = false;
     os << "cycle{";
     for (auto& i: w.cycle)

tests/python/word.ipynb

 {
  "metadata": {
   "name": "",
-  "signature": "sha256:a4f8f6e783c38abbf8cce75fb3eb8d9344ece79c11ad5c58556df2db3e40d6a2"
+  "signature": "sha256:261c16336ba5deefb7e9ebe705cc5c24f1cbba8622030874d2719f5045289c53"
  },
  "nbformat": 3,
  "nbformat_minor": 0,
@@ -139,7 +139,7 @@
         "</svg>\n"
        ],
        "text": [
-        "<spot.impl.twa_graph; proxy of <Swig Object of type 'std::shared_ptr< spot::twa_graph > *' at 0x7fcdb40da870> >"
+        "<spot.impl.twa_graph; proxy of <Swig Object of type 'std::shared_ptr< spot::twa_graph > *' at 0x7fa33c302870> >"
        ]
       }
      ],
@@ -454,7 +454,7 @@
         "</svg>\n"
        ],
        "text": [
-        "<spot.impl.twa_graph; proxy of <Swig Object of type 'std::shared_ptr< spot::twa_graph > *' at 0x7fcdb40da8d0> >"
+        "<spot.impl.twa_graph; proxy of <Swig Object of type 'std::shared_ptr< spot::twa_graph > *' at 0x7fa33c3028d0> >"
        ]
       }
      ],
@@ -546,6 +546,42 @@
       }
      ],
      "prompt_number": 15
+    },
+    {
+     "cell_type": "code",
+     "collapsed": false,
+     "input": [
+      "# Creating an empty word is OK...\n",
+      "w = spot.twa_word(spot._bdd_dict)"
+     ],
+     "language": "python",
+     "metadata": {},
+     "outputs": [],
+     "prompt_number": 16
+    },
+    {
+     "cell_type": "code",
+     "collapsed": false,
+     "input": [
+      "# ... as long as this word is not printed.\n",
+      "print(w)"
+     ],
+     "language": "python",
+     "metadata": {},
+     "outputs": [
+      {
+       "ename": "RuntimeError",
+       "evalue": "a twa_word may not have an empty cycle",
+       "output_type": "pyerr",
+       "traceback": [
+        "\u001b[0;31m---------------------------------------------------------------------------\u001b[0m\n\u001b[0;31mRuntimeError\u001b[0m                              Traceback (most recent call last)",
+        "\u001b[0;32m<ipython-input-17-971829bb8f5f>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m()\u001b[0m\n\u001b[1;32m      1\u001b[0m \u001b[0;31m# ... as long as this word is not printed.\u001b[0m\u001b[0;34m\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0;32m----> 2\u001b[0;31m \u001b[0mprint\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0mw\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m",
+        "\u001b[0;32m/home/adl/git/spot/python/spot/impl.py\u001b[0m in \u001b[0;36m__str__\u001b[0;34m(self)\u001b[0m\n\u001b[1;32m   3511\u001b[0m \u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m   3512\u001b[0m     \u001b[0;32mdef\u001b[0m \u001b[0m__str__\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0mself\u001b[0m\u001b[0;34m)\u001b[0m \u001b[0;34m->\u001b[0m \u001b[0;34m\"std::string\"\u001b[0m\u001b[0;34m:\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0;32m-> 3513\u001b[0;31m         \u001b[0;32mreturn\u001b[0m \u001b[0m_impl\u001b[0m\u001b[0;34m.\u001b[0m\u001b[0mtwa_word___str__\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0mself\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m\u001b[1;32m   3514\u001b[0m \u001b[0mtwa_word_swigregister\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0m_impl\u001b[0m\u001b[0;34m.\u001b[0m\u001b[0mtwa_word_swigregister\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m   3515\u001b[0m \u001b[0mtwa_word_swigregister\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0mtwa_word\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n",
+        "\u001b[0;31mRuntimeError\u001b[0m: a twa_word may not have an empty cycle"
+       ]
+      }
+     ],
+     "prompt_number": 17
     }
    ],
    "metadata": {}