diff --git a/python/spot/__init__.py b/python/spot/__init__.py
index dcaba0910..a78277c5b 100644
--- a/python/spot/__init__.py
+++ b/python/spot/__init__.py
@@ -166,7 +166,10 @@ class twa:
 class formula:
     def __init__(self, str):
         """Parse the given string to create a formula."""
-        self.this = parse_formula(str)
+        if type(str) == formula:
+            self.this = str
+        else:
+            self.this = parse_formula(str)
 
     def show_ast(self):
         """Display the syntax tree of the formula."""
diff --git a/tests/python/formulas.ipynb b/tests/python/formulas.ipynb
index ed7e042de..4786cfd19 100644
--- a/tests/python/formulas.ipynb
+++ b/tests/python/formulas.ipynb
@@ -15,7 +15,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.6.4rc1"
+   "version": "3.6.4"
   },
   "name": ""
  },
@@ -127,6 +127,36 @@
      ],
      "prompt_number": 4
     },
+    {
+     "cell_type": "markdown",
+     "metadata": {},
+     "source": [
+      "Passing a `formula` to `spot.formula` simply returns the formula."
+     ]
+    },
+    {
+     "cell_type": "code",
+     "collapsed": false,
+     "input": [
+      "spot.formula(h)"
+     ],
+     "language": "python",
+     "metadata": {},
+     "outputs": [
+      {
+       "latex": [
+        "$c \\land (a \\lor b)$"
+       ],
+       "metadata": {},
+       "output_type": "pyout",
+       "prompt_number": 5,
+       "text": [
+        "c & (a | b)"
+       ]
+      }
+     ],
+     "prompt_number": 5
+    },
     {
      "cell_type": "markdown",
      "metadata": {},
@@ -147,13 +177,13 @@
       {
        "metadata": {},
        "output_type": "pyout",
-       "prompt_number": 5,
+       "prompt_number": 6,
        "text": [
         "'p1 U (p2 R (p3 & !p4))'"
        ]
       }
      ],
-     "prompt_number": 5
+     "prompt_number": 6
     },
     {
      "cell_type": "markdown",
@@ -186,7 +216,7 @@
        ]
       }
      ],
-     "prompt_number": 6
+     "prompt_number": 7
     },
     {
      "cell_type": "markdown",
@@ -223,7 +253,7 @@
        ]
       }
      ],
-     "prompt_number": 7
+     "prompt_number": 8
     },
     {
      "cell_type": "markdown",
@@ -288,7 +318,7 @@
        ]
       }
      ],
-     "prompt_number": 8
+     "prompt_number": 9
     },
     {
      "cell_type": "markdown",
@@ -309,13 +339,13 @@
       {
        "metadata": {},
        "output_type": "pyout",
-       "prompt_number": 9,
+       "prompt_number": 10,
        "text": [
         "True"
        ]
       }
      ],
-     "prompt_number": 9
+     "prompt_number": 10
     },
     {
      "cell_type": "code",
@@ -329,13 +359,13 @@
       {
        "metadata": {},
        "output_type": "pyout",
-       "prompt_number": 10,
+       "prompt_number": 11,
        "text": [
         "False"
        ]
       }
      ],
-     "prompt_number": 10
+     "prompt_number": 11
     },
     {
      "cell_type": "markdown",
@@ -356,13 +386,13 @@
       {
        "metadata": {},
        "output_type": "pyout",
-       "prompt_number": 11,
+       "prompt_number": 12,
        "text": [
         "True"
        ]
       }
      ],
-     "prompt_number": 11
+     "prompt_number": 12
     },
     {
      "cell_type": "code",
@@ -376,13 +406,13 @@
       {
        "metadata": {},
        "output_type": "pyout",
-       "prompt_number": 12,
+       "prompt_number": 13,
        "text": [
         "False"
        ]
       }
      ],
-     "prompt_number": 12
+     "prompt_number": 13
     },
     {
      "cell_type": "code",
@@ -396,13 +426,13 @@
       {
        "metadata": {},
        "output_type": "pyout",
-       "prompt_number": 13,
+       "prompt_number": 14,
        "text": [
         "True"
        ]
       }
      ],
-     "prompt_number": 13
+     "prompt_number": 14
     },
     {
      "cell_type": "markdown",
@@ -426,13 +456,13 @@
        ],
        "metadata": {},
        "output_type": "pyout",
-       "prompt_number": 14,
+       "prompt_number": 15,
        "text": [
         "\"a > b\" & \"proc[2]@init\" & GF_foo_"
        ]
       }
      ],
-     "prompt_number": 14
+     "prompt_number": 15
     },
     {
      "cell_type": "code",
@@ -449,13 +479,13 @@
        ],
        "metadata": {},
        "output_type": "pyout",
-       "prompt_number": 15,
+       "prompt_number": 16,
        "text": [
         "a & b & GFc"
        ]
       }
      ],
-     "prompt_number": 15
+     "prompt_number": 16
     },
     {
      "cell_type": "code",
@@ -472,13 +502,13 @@
        ],
        "metadata": {},
        "output_type": "pyout",
-       "prompt_number": 16,
+       "prompt_number": 17,
        "text": [
         "p0 & p1 & GFp2"
        ]
       }
      ],
-     "prompt_number": 16
+     "prompt_number": 17
     },
     {
      "cell_type": "markdown",
@@ -506,7 +536,7 @@
       {
        "metadata": {},
        "output_type": "pyout",
-       "prompt_number": 17,
+       "prompt_number": 18,
        "svg": [
         "<svg height=\"260pt\" viewBox=\"0.00 0.00 269.00 260.00\" width=\"269pt\" xmlns=\"http://www.w3.org/2000/svg\" xmlns:xlink=\"http://www.w3.org/1999/xlink\">\n",
         "<g class=\"graph\" id=\"graph0\" transform=\"scale(1 1) rotate(0) translate(4 256)\">\n",
@@ -615,7 +645,7 @@
        ]
       }
      ],
-     "prompt_number": 17
+     "prompt_number": 18
     },
     {
      "cell_type": "markdown",
@@ -636,7 +666,7 @@
       {
        "metadata": {},
        "output_type": "pyout",
-       "prompt_number": 18,
+       "prompt_number": 19,
        "svg": [
         "<svg height=\"210\" version=\"1.1\" width=\"220\" xmlns=\"http://www.w3.org/2000/svg\">\n",
         "<polygon fill=\"cyan\" opacity=\".2\" points=\"20,0 200,120 200,210 20,210\"/>\n",
@@ -668,7 +698,7 @@
        ]
       }
      ],
-     "prompt_number": 18
+     "prompt_number": 19
     },
     {
      "cell_type": "code",
@@ -682,13 +712,13 @@
       {
        "metadata": {},
        "output_type": "pyout",
-       "prompt_number": 19,
+       "prompt_number": 20,
        "text": [
         "'recurrence'"
        ]
       }
      ],
-     "prompt_number": 19
+     "prompt_number": 20
     },
     {
      "cell_type": "code",
@@ -705,13 +735,13 @@
        ],
        "metadata": {},
        "output_type": "pyout",
-       "prompt_number": 20,
+       "prompt_number": 21,
        "text": [
         "F(a & X(!a & b))"
        ]
       }
      ],
-     "prompt_number": 20
+     "prompt_number": 21
     },
     {
      "cell_type": "markdown",
@@ -735,13 +765,13 @@
        ],
        "metadata": {},
        "output_type": "pyout",
-       "prompt_number": 21,
+       "prompt_number": 22,
        "text": [
         "F(a & ((a & (a U (!a & b)) & ((!b U !a) | (b U !a))) | (!a & (!a U (a & !a & b)) & ((!b U a) | (b U a))) | (b & (b U (!a & b & !b)) & ((!a U !b) | (a U !b))) | (!b & (!b U (!a & b)) & ((!a U b) | (a U b))) | (!a & b & (G!a | Ga) & (G!b | Gb))))"
        ]
       }
      ],
-     "prompt_number": 21
+     "prompt_number": 22
     },
     {
      "cell_type": "markdown",
@@ -766,13 +796,13 @@
        ],
        "metadata": {},
        "output_type": "pyout",
-       "prompt_number": 22,
+       "prompt_number": 23,
        "text": [
         "(0 R !(a <-> b)) -> (1 U (a <-> b))"
        ]
       }
      ],
-     "prompt_number": 22
+     "prompt_number": 23
     },
     {
      "cell_type": "code",
@@ -789,13 +819,13 @@
        ],
        "metadata": {},
        "output_type": "pyout",
-       "prompt_number": 23,
+       "prompt_number": 24,
        "text": [
         "(1 U ((a & b) | (!a & !b))) | !(0 R ((!a & b) | (a & !b)))"
        ]
       }
      ],
-     "prompt_number": 23
+     "prompt_number": 24
     },
     {
      "cell_type": "markdown",
@@ -833,7 +863,7 @@
        ]
       }
      ],
-     "prompt_number": 24
+     "prompt_number": 25
     }
    ],
    "metadata": {}