Docs: update cheatsheet to include exec

Author: edschofield

docs/compatible_idioms.rst

@@ -952,6 +952,33 @@ file()
     f = open(pathname, 'rb')   # if f.read() should return bytes
     # or
     f = open(pathname, 'rt')   # if f.read() should return unicode text
+exec
+~~~~
+
+.. code:: python
+
+    # Python 2 only:
+    exec 'x = 10'
+
+    # Python 2 and 3:
+    exec('x = 10')
+.. code:: python
+
+    # Python 2 only:
+    g = globals()
+    exec 'x = 10' in g
+
+    # Python 2 and 3:
+    g = globals()
+    exec('x = 10', g)
+.. code:: python
+
+    # Python 2 only:
+    l = locals()
+    exec 'x = 10' in g, l
+
+    # Python 2 and 3:
+    exec('x = 10', g, l)
 execfile()
 ~~~~~~~~~~
 

docs/notebooks/Writing Python 2-3 compatible code.ipynb

@@ -2064,6 +2064,68 @@
     "f = open(pathname, 'rt')   # if f.read() should return unicode text"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "### exec"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": true
+   },
+   "outputs": [],
+   "source": [
+    "# Python 2 only:\n",
+    "exec 'x = 10'\n",
+    "\n",
+    "# Python 2 and 3:\n",
+    "exec('x = 10')"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": true
+   },
+   "outputs": [],
+   "source": [
+    "# Python 2 only:\n",
+    "g = globals()\n",
+    "exec 'x = 10' in g\n",
+    "\n",
+    "# Python 2 and 3:\n",
+    "g = globals()\n",
+    "exec('x = 10', g)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": true
+   },
+   "outputs": [],
+   "source": [
+    "# Python 2 only:\n",
+    "l = locals()\n",
+    "exec 'x = 10' in g, l\n",
+    "\n",
+    "# Python 2 and 3:\n",
+    "exec('x = 10', g, l)"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "But note that Py3's `exec()` is less powerful (and less dangerous) than Py2's `exec` statement."
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},