finally blocks

.ipynb_checkpoints/not_so_obvious_python_stuff-checkpoint.ipynb

@@ -1,7 +1,7 @@
 {
  "metadata": {
   "name": "",
-  "signature": "sha256:1f7175fddf00a2283246009f1da253d4c1f87bc6ac35f6c2eea1b216e927254e"
+  "signature": "sha256:df40791be1e08052b49de55275b6025defd84acad805842af17f14781fca9db3"
  },
  "nbformat": 3,
  "nbformat_minor": 0,
@@ -78,7 +78,8 @@
       "- [Reusing global variable names and UnboundLocalErrors](#unboundlocalerror)\n",
       "- [Creating copies of mutable objects](#copy_mutable)\n",
       "- [Key differences between Python 2 and 3](#python_differences)\n",
-      "- [Function annotations - What are those `->`'s in my Python code?](#function_annotation)"
+      "- [Function annotations - What are those `->`'s in my Python code?](#function_annotation)\n",
+      "- [Abortive statements in `finally` blocks](#finally_blocks)"
      ]
     },
     {
@@ -336,6 +337,13 @@
       "\n"
      ]
     },
+    {
+     "cell_type": "markdown",
+     "metadata": {},
+     "source": [
+      "[[back to top](#sections)]"
+     ]
+    },
     {
      "cell_type": "markdown",
      "metadata": {},
@@ -350,13 +358,6 @@
       "Here is a [nice article](http://python.net/%7Egoodger/projects/pycon/2007/idiomatic/handout.html#other-languages-have-variables) explaining it using \"boxes\" (for people with C background) and \"name tags\" (in the case of Python):"
      ]
     },
-    {
-     "cell_type": "markdown",
-     "metadata": {},
-     "source": [
-      "[[back to top](#sections)]"
-     ]
-    },
     {
      "cell_type": "code",
      "collapsed": false,
@@ -492,14 +493,14 @@
      "cell_type": "markdown",
      "metadata": {},
      "source": [
-      "List modification of the original list does affect shallow copies, but not deep copies if the list contains compound objects."
+      "[[back to top](#sections)]"
      ]
     },
     {
      "cell_type": "markdown",
      "metadata": {},
      "source": [
-      "[[back to top](#sections)]"
+      "List modification of the original list does affect shallow copies, but not deep copies if the list contains compound objects."
      ]
     },
     {
@@ -2675,6 +2676,109 @@
       "..."
      ]
     },
+    {
+     "cell_type": "markdown",
+     "metadata": {},
+     "source": [
+      "<br>\n",
+      "<br>\n",
+      "<a name=\"finally_blocks\"></a>"
+     ]
+    },
+    {
+     "cell_type": "markdown",
+     "metadata": {},
+     "source": [
+      "# Abortive statements in `finally` blocks"
+     ]
+    },
+    {
+     "cell_type": "markdown",
+     "metadata": {},
+     "source": [
+      "Python's `try-except-finally` blocks are very handy for catching and handling errors. The `finally` block is always executed whether an `exception` has been raised or not as illustrated in the following example."
+     ]
+    },
+    {
+     "cell_type": "code",
+     "collapsed": false,
+     "input": [
+      "def try_finally1():\n",
+      "    try:\n",
+      "        print('in try:')\n",
+      "        print('do some stuff')\n",
+      "        float('abc')\n",
+      "    except ValueError:\n",
+      "        print('an error occurred')\n",
+      "    else:\n",
+      "        print('no error occurred')\n",
+      "    finally:\n",
+      "        print('always execute finally')\n",
+      "        \n",
+      "try_finally1()"
+     ],
+     "language": "python",
+     "metadata": {},
+     "outputs": [
+      {
+       "output_type": "stream",
+       "stream": "stdout",
+       "text": [
+        "in try:\n",
+        "do some stuff\n",
+        "an error occurred\n",
+        "always execute finally\n"
+       ]
+      }
+     ],
+     "prompt_number": 24
+    },
+    {
+     "cell_type": "markdown",
+     "metadata": {},
+     "source": [
+      "<br>\n",
+      "<br>\n",
+      "But can you also guess what will be printed in the next code cell?"
+     ]
+    },
+    {
+     "cell_type": "code",
+     "collapsed": false,
+     "input": [
+      "def try_finally2():\n",
+      "    try:\n",
+      "        print(\"do some stuff in try block\")\n",
+      "        return \"return from try block\"\n",
+      "    finally:\n",
+      "        print(\"do some stuff in finally block\")\n",
+      "        return \"always execute finally\"\n",
+      "    \n",
+      "print(try_finally2())"
+     ],
+     "language": "python",
+     "metadata": {},
+     "outputs": [
+      {
+       "output_type": "stream",
+       "stream": "stdout",
+       "text": [
+        "do some stuff in try block\n",
+        "do some stuff in finally block\n",
+        "always execute finally\n"
+       ]
+      }
+     ],
+     "prompt_number": 21
+    },
+    {
+     "cell_type": "markdown",
+     "metadata": {},
+     "source": [
+      "<br>\n",
+      "Here, the abortive `return` statement in the `finally` block simply overrules the `return` in the `try` block, since **`finally` is guaranteed to always be executed.** So, be careful using abortive statements in `finally` blocks!"
+     ]
+    },
     {
      "cell_type": "code",
      "collapsed": false,

not_so_obvious_python_stuff.ipynb

@@ -1,7 +1,7 @@
 {
  "metadata": {
   "name": "",
-  "signature": "sha256:1f7175fddf00a2283246009f1da253d4c1f87bc6ac35f6c2eea1b216e927254e"
+  "signature": "sha256:df40791be1e08052b49de55275b6025defd84acad805842af17f14781fca9db3"
  },
  "nbformat": 3,
  "nbformat_minor": 0,
@@ -78,7 +78,8 @@
       "- [Reusing global variable names and UnboundLocalErrors](#unboundlocalerror)\n",
       "- [Creating copies of mutable objects](#copy_mutable)\n",
       "- [Key differences between Python 2 and 3](#python_differences)\n",
-      "- [Function annotations - What are those `->`'s in my Python code?](#function_annotation)"
+      "- [Function annotations - What are those `->`'s in my Python code?](#function_annotation)\n",
+      "- [Abortive statements in `finally` blocks](#finally_blocks)"
      ]
     },
     {
@@ -336,6 +337,13 @@
       "\n"
      ]
     },
+    {
+     "cell_type": "markdown",
+     "metadata": {},
+     "source": [
+      "[[back to top](#sections)]"
+     ]
+    },
     {
      "cell_type": "markdown",
      "metadata": {},
@@ -350,13 +358,6 @@
       "Here is a [nice article](http://python.net/%7Egoodger/projects/pycon/2007/idiomatic/handout.html#other-languages-have-variables) explaining it using \"boxes\" (for people with C background) and \"name tags\" (in the case of Python):"
      ]
     },
-    {
-     "cell_type": "markdown",
-     "metadata": {},
-     "source": [
-      "[[back to top](#sections)]"
-     ]
-    },
     {
      "cell_type": "code",
      "collapsed": false,
@@ -492,14 +493,14 @@
      "cell_type": "markdown",
      "metadata": {},
      "source": [
-      "List modification of the original list does affect shallow copies, but not deep copies if the list contains compound objects."
+      "[[back to top](#sections)]"
      ]
     },
     {
      "cell_type": "markdown",
      "metadata": {},
      "source": [
-      "[[back to top](#sections)]"
+      "List modification of the original list does affect shallow copies, but not deep copies if the list contains compound objects."
      ]
     },
     {
@@ -2675,6 +2676,109 @@
       "..."
      ]
     },
+    {
+     "cell_type": "markdown",
+     "metadata": {},
+     "source": [
+      "<br>\n",
+      "<br>\n",
+      "<a name=\"finally_blocks\"></a>"
+     ]
+    },
+    {
+     "cell_type": "markdown",
+     "metadata": {},
+     "source": [
+      "# Abortive statements in `finally` blocks"
+     ]
+    },
+    {
+     "cell_type": "markdown",
+     "metadata": {},
+     "source": [
+      "Python's `try-except-finally` blocks are very handy for catching and handling errors. The `finally` block is always executed whether an `exception` has been raised or not as illustrated in the following example."
+     ]
+    },
+    {
+     "cell_type": "code",
+     "collapsed": false,
+     "input": [
+      "def try_finally1():\n",
+      "    try:\n",
+      "        print('in try:')\n",
+      "        print('do some stuff')\n",
+      "        float('abc')\n",
+      "    except ValueError:\n",
+      "        print('an error occurred')\n",
+      "    else:\n",
+      "        print('no error occurred')\n",
+      "    finally:\n",
+      "        print('always execute finally')\n",
+      "        \n",
+      "try_finally1()"
+     ],
+     "language": "python",
+     "metadata": {},
+     "outputs": [
+      {
+       "output_type": "stream",
+       "stream": "stdout",
+       "text": [
+        "in try:\n",
+        "do some stuff\n",
+        "an error occurred\n",
+        "always execute finally\n"
+       ]
+      }
+     ],
+     "prompt_number": 24
+    },
+    {
+     "cell_type": "markdown",
+     "metadata": {},
+     "source": [
+      "<br>\n",
+      "<br>\n",
+      "But can you also guess what will be printed in the next code cell?"
+     ]
+    },
+    {
+     "cell_type": "code",
+     "collapsed": false,
+     "input": [
+      "def try_finally2():\n",
+      "    try:\n",
+      "        print(\"do some stuff in try block\")\n",
+      "        return \"return from try block\"\n",
+      "    finally:\n",
+      "        print(\"do some stuff in finally block\")\n",
+      "        return \"always execute finally\"\n",
+      "    \n",
+      "print(try_finally2())"
+     ],
+     "language": "python",
+     "metadata": {},
+     "outputs": [
+      {
+       "output_type": "stream",
+       "stream": "stdout",
+       "text": [
+        "do some stuff in try block\n",
+        "do some stuff in finally block\n",
+        "always execute finally\n"
+       ]
+      }
+     ],
+     "prompt_number": 21
+    },
+    {
+     "cell_type": "markdown",
+     "metadata": {},
+     "source": [
+      "<br>\n",
+      "Here, the abortive `return` statement in the `finally` block simply overrules the `return` in the `try` block, since **`finally` is guaranteed to always be executed.** So, be careful using abortive statements in `finally` blocks!"
+     ]
+    },
     {
      "cell_type": "code",
      "collapsed": false,