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rasbt 2014-05-02 14:52:17 -04:00
parent 81610f64f6
commit afacae6cc4
2 changed files with 35 additions and 121 deletions
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136
.ipynb_checkpoints/not_so_obvious_python_stuff-checkpoint.ipynb
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@ -1,7 +1,7 @@
{ {
"metadata": { "metadata": {
"name": "", "name": "",
"signature": "sha256:92f953fd8004f1dd1e9969ce61096f5a0b43190de3f3c7c8f85642b667754fae" "signature": "sha256:70671a8e3f32d21b27867ecdc6d80f3b5c10f00339a2fae3860c4db8930dc7c6"
}, },
"nbformat": 3, "nbformat": 3,
"nbformat_minor": 0, "nbformat_minor": 0,
@ -13,7 +13,7 @@
"metadata": {}, "metadata": {},
"source": [ "source": [
"Sebastian Raschka \n", "Sebastian Raschka \n",
"last updated: 04/27/2014 ([Changelog](#changelog))\n", "last updated: 05/02/2014 ([Changelog](#changelog))\n",
"\n", "\n",
"[Link to this IPython Notebook on GitHub](https://github.com/rasbt/python_reference/blob/master/not_so_obvious_python_stuff.ipynb)\n", "[Link to this IPython Notebook on GitHub](https://github.com/rasbt/python_reference/blob/master/not_so_obvious_python_stuff.ipynb)\n",
"\n", "\n",
@ -3151,7 +3151,7 @@
] ]
} }
], ],
"prompt_number": 1 "prompt_number": 55
}, },
{ {
"cell_type": "markdown", "cell_type": "markdown",
@ -3179,12 +3179,12 @@
"stream": "stdout", "stream": "stdout",
"text": [ "text": [
"type of kwargs: <class 'dict'>\n", "type of kwargs: <class 'dict'>\n",
"kwargs contents: {'b': 2, 'a': 1, 'c': 3, 'd': 4}\n", "kwargs contents: {'d': 4, 'a': 1, 'c': 3, 'b': 2}\n",
"value of argument a: 1\n" "value of argument a: 1\n"
] ]
} }
], ],
"prompt_number": 2 "prompt_number": 56
}, },
{ {
"cell_type": "markdown", "cell_type": "markdown",
@ -3214,7 +3214,7 @@
] ]
} }
], ],
"prompt_number": 3 "prompt_number": 57
}, },
{ {
"cell_type": "markdown", "cell_type": "markdown",
@ -3237,69 +3237,14 @@
"metadata": {}, "metadata": {},
"source": [ "source": [
"Usually, it is the `__init__` method when we think of instanciating a new object from a class. However, it is the static method `__new__` (it is not a class method!) that creates and returns a new instance before `__init__()` is called. \n", "Usually, it is the `__init__` method when we think of instanciating a new object from a class. However, it is the static method `__new__` (it is not a class method!) that creates and returns a new instance before `__init__()` is called. \n",
"More specifically, this is what is returned: \n",
"`return super(<currentclass>, cls).__new__(subcls, *args, **kwargs)` \n",
"\n", "\n",
"For more information about the `__new__` method, please see the [documentation](https://www.python.org/download/releases/2.2/descrintro/#__new__).\n",
"\n", "\n",
"`__new__` is a so-called static method that returns \n", "As a little experiment, let us screw with `__new__` so that it returns `None` and see if `__init__` will be executed:"
"`return super(<currentclass>, cls).__new__(subcls, *args, **kwargs)`"
] ]
}, },
{
"cell_type": "code",
"collapsed": false,
"input": [
"print(help(object.__new__))"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"Help on built-in function __new__:\n",
"\n",
"__new__(...)\n",
" T.__new__(S, ...) -> a new object with type S, a subtype of T\n",
"\n",
"None\n"
]
}
],
"prompt_number": 17
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"print(dir(object))\n",
"print(help(object.__new__))\n",
"print(help(object.__init__))"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"['__class__', '__delattr__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__le__', '__lt__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__']\n",
"Help on built-in function __new__:\n",
"\n",
"__new__(...)\n",
" T.__new__(S, ...) -> a new object with type S, a subtype of T\n",
"\n",
"None\n",
"Help on wrapper_descriptor:\n",
"\n",
"__init__(...)\n",
" x.__init__(...) initializes x; see help(type(x)) for signature\n",
"\n",
"None\n"
]
}
],
"prompt_number": 18
},
{ {
"cell_type": "code", "cell_type": "code",
"collapsed": false, "collapsed": false,
@ -3327,22 +3272,32 @@
] ]
} }
], ],
"prompt_number": 33 "prompt_number": 53
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"As we can see in the code above, `__init__` requires the returned instance from `__new__` in order to called. So, here we just created a `NoneType` object. \n",
"Let us override the `__new__`, now and let us confirm that `__init__` is called now to instantiate the new object\":"
]
}, },
{ {
"cell_type": "code", "cell_type": "code",
"collapsed": false, "collapsed": false,
"input": [ "input": [
"class a_class(object):\n", "class a_class(object):\n",
" def __new__(clss, *args, **kwargs):\n", " def __new__(cls, *args, **kwargs):\n",
" print('excecuted __new__')\n", " print('excecuted __new__')\n",
" inst = super(a_class, clss).__new__(clss, *args, **kwargs)\n", " inst = super(a_class, cls).__new__(cls)\n",
" return inst\n", " return inst\n",
" def __init__(self):\n", " def __init__(self, an_arg):\n",
" print('excecuted __init__')\n", " print('excecuted __init__')\n",
" self.an_arg = an_arg\n",
" \n", " \n",
"a_object = a_class()\n", "a_object = a_class(1)\n",
"print('Type of a_object:', type(a_object))" "print('Type of a_object:', type(a_object))\n",
"print('a_object.an_arg: ', a_object.an_arg)"
], ],
"language": "python", "language": "python",
"metadata": {}, "metadata": {},
@ -3353,37 +3308,12 @@
"text": [ "text": [
"excecuted __new__\n", "excecuted __new__\n",
"excecuted __init__\n", "excecuted __init__\n",
"Type of a_object: <class '__main__.a_class'>\n" "Type of a_object: <class '__main__.a_class'>\n",
"a_object.an_arg: 1\n"
] ]
} }
], ],
"prompt_number": 40 "prompt_number": 54
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"class a_class(object):\n",
" def __init__(self, an_arg):\n",
" print('excecuted __init__')\n",
" self.an_arg = an_arg\n",
" \n",
"a_object = a_class(1)\n",
"print('Type of a_object:', type(a_object))"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"excecuted __init__\n",
"Type of a_object <class '__main__.a_class'>\n"
]
}
],
"prompt_number": 24
}, },
{ {
"cell_type": "markdown", "cell_type": "markdown",
@ -3424,14 +3354,6 @@
"- minor fixes of typos \n", "- minor fixes of typos \n",
"- new section: \"Only the first clause of generators is evaluated immediately\"" "- new section: \"Only the first clause of generators is evaluated immediately\""
] ]
},
{
"cell_type": "code",
"collapsed": false,
"input": [],
"language": "python",
"metadata": {},
"outputs": []
} }
], ],
"metadata": {} "metadata": {}

20
not_so_obvious_python_stuff.ipynb
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@ -1,7 +1,7 @@
{ {
"metadata": { "metadata": {
"name": "", "name": "",
"signature": "sha256:8d4ed5310005c7fea9f7cfda8b7624b8b3cb390a566950a3bc179c516f89b410" "signature": "sha256:70671a8e3f32d21b27867ecdc6d80f3b5c10f00339a2fae3860c4db8930dc7c6"
}, },
"nbformat": 3, "nbformat": 3,
"nbformat_minor": 0, "nbformat_minor": 0,
@ -13,7 +13,7 @@
"metadata": {}, "metadata": {},
"source": [ "source": [
"Sebastian Raschka \n", "Sebastian Raschka \n",
"last updated: 04/27/2014 ([Changelog](#changelog))\n", "last updated: 05/02/2014 ([Changelog](#changelog))\n",
"\n", "\n",
"[Link to this IPython Notebook on GitHub](https://github.com/rasbt/python_reference/blob/master/not_so_obvious_python_stuff.ipynb)\n", "[Link to this IPython Notebook on GitHub](https://github.com/rasbt/python_reference/blob/master/not_so_obvious_python_stuff.ipynb)\n",
"\n", "\n",
@ -3151,7 +3151,7 @@
] ]
} }
], ],
"prompt_number": 1 "prompt_number": 55
}, },
{ {
"cell_type": "markdown", "cell_type": "markdown",
@ -3179,12 +3179,12 @@
"stream": "stdout", "stream": "stdout",
"text": [ "text": [
"type of kwargs: <class 'dict'>\n", "type of kwargs: <class 'dict'>\n",
"kwargs contents: {'b': 2, 'a': 1, 'c': 3, 'd': 4}\n", "kwargs contents: {'d': 4, 'a': 1, 'c': 3, 'b': 2}\n",
"value of argument a: 1\n" "value of argument a: 1\n"
] ]
} }
], ],
"prompt_number": 2 "prompt_number": 56
}, },
{ {
"cell_type": "markdown", "cell_type": "markdown",
@ -3214,7 +3214,7 @@
] ]
} }
], ],
"prompt_number": 3 "prompt_number": 57
}, },
{ {
"cell_type": "markdown", "cell_type": "markdown",
@ -3354,14 +3354,6 @@
"- minor fixes of typos \n", "- minor fixes of typos \n",
"- new section: \"Only the first clause of generators is evaluated immediately\"" "- new section: \"Only the first clause of generators is evaluated immediately\""
] ]
},
{
"cell_type": "code",
"collapsed": false,
"input": [],
"language": "python",
"metadata": {},
"outputs": []
} }
], ],
"metadata": {} "metadata": {}