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Advanced Python Workshop - Nov 2013 http://advancedpython.hasgeek.com/
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Readme.txt
Notes from Advanced Python Workshop by Anand Chitipothu conducted on Nov 14-16, 2013
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{
"metadata": {
"name": ""
},
"nbformat": 3,
"nbformat_minor": 0,
"worksheets": [
{
"cells": [
{
"cell_type": "markdown",
"metadata": {},
"source": [
"# Advanced Python Workshop - Day 3\n",
"Nov 16, 2013<br/>\n",
"http://advancedpython.hasgeek.com/"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"x = 1"
],
"language": "python",
"metadata": {},
"outputs": [],
"prompt_number": 2
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"print type(x)"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"<type 'int'>\n"
]
}
],
"prompt_number": 4
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"int.__bases__"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 5,
"text": [
"(object,)"
]
}
],
"prompt_number": 5
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"print type(int)"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"<type 'type'>\n"
]
}
],
"prompt_number": 6
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"print type(type)"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"<type 'type'>\n"
]
}
],
"prompt_number": 7
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"type.__bases__"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 8,
"text": [
"(object,)"
]
}
],
"prompt_number": 8
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"print type(object)"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"<type 'type'>\n"
]
}
],
"prompt_number": 9
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"object.__bases__"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 10,
"text": [
"()"
]
}
],
"prompt_number": 10
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"class Point(object):\n",
" x = 0\n",
" y = 0"
],
"language": "python",
"metadata": {},
"outputs": [],
"prompt_number": 11
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"print Point"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"<class '__main__.Point'>\n"
]
}
],
"prompt_number": 12
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"type(Point)"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 13,
"text": [
"type"
]
}
],
"prompt_number": 13
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"isinstance(Point, type)"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 14,
"text": [
"True"
]
}
],
"prompt_number": 14
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"p = Point()"
],
"language": "python",
"metadata": {},
"outputs": [],
"prompt_number": 15
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"Just like we can create an instance of Point class, we can also create an instance of type. In fact the class statement is a syntactic-sugar of creating an instance of type."
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"Point2 = type(\"Point2\", (object,), {\"x\": 0, \"y\": 0})"
],
"language": "python",
"metadata": {},
"outputs": [],
"prompt_number": 16
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"Point2"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 17,
"text": [
"__main__.Point2"
]
}
],
"prompt_number": 17
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"Point"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 18,
"text": [
"__main__.Point"
]
}
],
"prompt_number": 18
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"Point.x"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 19,
"text": [
"0"
]
}
],
"prompt_number": 19
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"Point2.x"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 21,
"text": [
"0"
]
}
],
"prompt_number": 21
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"Point.__dict__"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 22,
"text": [
"<dictproxy {'__dict__': <attribute '__dict__' of 'Point' objects>,\n",
" '__doc__': None,\n",
" '__module__': '__main__',\n",
" '__weakref__': <attribute '__weakref__' of 'Point' objects>,\n",
" 'x': 0,\n",
" 'y': 0}>"
]
}
],
"prompt_number": 22
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"Point2.__dict__"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 23,
"text": [
"<dictproxy {'__dict__': <attribute '__dict__' of 'Point2' objects>,\n",
" '__doc__': None,\n",
" '__module__': '__main__',\n",
" '__weakref__': <attribute '__weakref__' of 'Point2' objects>,\n",
" 'x': 0,\n",
" 'y': 0}>"
]
}
],
"prompt_number": 23
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"class type2(type):\n",
" pass"
],
"language": "python",
"metadata": {},
"outputs": [],
"prompt_number": 24
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"Point3 = type2(\"Point3\", (object,), {\"x\":0, \"y\": 0})"
],
"language": "python",
"metadata": {},
"outputs": [],
"prompt_number": 25
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"Point3"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 26,
"text": [
"__main__.Point3"
]
}
],
"prompt_number": 26
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"type(Point3)"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 27,
"text": [
"__main__.type2"
]
}
],
"prompt_number": 27
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"Point3.__class__"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 28,
"text": [
"__main__.type2"
]
}
],
"prompt_number": 28
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"class type3(type):\n",
" xxx = 1234"
],
"language": "python",
"metadata": {},
"outputs": [],
"prompt_number": 29
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"Point3 = type3(\"Point3\", (object,), {})"
],
"language": "python",
"metadata": {},
"outputs": [],
"prompt_number": 30
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"Point3.xxx"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 31,
"text": [
"1234"
]
}
],
"prompt_number": 31
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"type(type3)"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 32,
"text": [
"type"
]
}
],
"prompt_number": 32
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"How can we create an instance of type3 using the class statement?"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"class Point3a(object):\n",
" pass\n",
"\n",
"print type(Point3a)"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"<type 'type'>\n"
]
}
],
"prompt_number": 34
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"class Point3b(object):\n",
" __metaclass__ = type2\n",
" pass\n",
"\n",
"print type(Point3b)"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"<class '__main__.type2'>\n"
]
}
],
"prompt_number": 35
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"This is same as\n",
"\n",
"<pre>\n",
"Point3b = type2(\"Point3b\", (object,), {})\n",
"</pre>"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"The metaclass is retained in all sub classes as well."
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"class Foo(Point3b): pass\n",
"\n",
"print type(Foo)"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"<class '__main__.type2'>\n"
]
}
],
"prompt_number": 36
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"class LoggingMetaClass(type):\n",
" def __init__(self, *a, **kw):\n",
" type.__init__(self, *a, **kw)\n",
" print self, \"defined now\"\n",
"\n",
"print \"before Node class\" \n",
"class Node(object):\n",
" __metaclass__ = LoggingMetaClass\n",
"print \"after Node class\" \n",
" \n",
"class Div(Node): pass \n",
"class Pre(Node): pass \n",
"class H1(Node): pass \n",
"class H2(Node): pass \n",
" "
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"before Node class\n",
"<class '__main__.Node'> defined now\n",
"after Node class\n",
"<class '__main__.Div'> defined now\n",
"<class '__main__.Pre'> defined now\n",
"<class '__main__.H1'> defined now\n",
"<class '__main__.H2'> defined now\n"
]
}
],
"prompt_number": 39
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"**Example: web framework**"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"Can we have an API which looks like this:\n",
"\n",
" class hello(page):\n",
" def GET(self):\n",
" print \"hello\"\n",
" \n",
" class goodbye(page):\n",
" def GET(self):\n",
" print \"goodbye\"\n",
" \n",
" if __name__ == \"__main__\": \n",
" request(\"/hello\")\n",
" request(\"/goodbye\") \n",
" request(\"/no-such-page\") \n",
" "
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"Can we use a metaclass to create path to class registry?"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"%%file meta_web.py\n",
"\n",
"path_mapping = []\n",
"\n",
"class MetaPage(type):\n",
" def __init__(self, *a, **kw):\n",
" type.__init__(self, *a, **kw)\n",
" path = \"/\" + self.__name__\n",
" path_mapping.append((path, self))\n",
" print \"added\", path, \"to mapping\"\n",
"\n",
"class page(object):\n",
" __metaclass__ = MetaPage\n",
"\n",
"def request(path):\n",
" for p, cls in path_mapping:\n",
" if p == path:\n",
" return cls().GET()\n",
" print \"404 Not Found\"\n"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"Overwriting meta_web.py\n"
]
}
],
"prompt_number": 44
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"%%file meta_app1.py\n",
"from meta_web import page, request\n",
"\n",
"class hello(page):\n",
" def GET(self):\n",
" print \"hello\"\n",
" \n",
"class goodbye(page):\n",
" def GET(self):\n",
" print \"goodbye\"\n",
" \n",
"if __name__ == \"__main__\": \n",
" request(\"/hello\")\n",
" request(\"/goodbye\") \n",
" request(\"/no-such-page\") \n",
" "
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"Overwriting meta_app1.py\n"
]
}
],
"prompt_number": 45
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"!python meta_app1.py"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"added /page to mapping\r\n",
"added /hello to mapping\r\n",
"added /goodbye to mapping\r\n",
"hello\r\n",
"goodbye\r\n",
"404 Not Found\r\n"
]
}
],
"prompt_number": 46
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"**Problem:** Improve the framework to use the path specified in the `path` attribute of the class instead of the class name. If the path attribute is not specified, it should infer the path from the class name.\n",
"\n",
" class index(page):\n",
" path = \"/\"\n",
" def GET(self):\n",
" print \"Home Page\"\n",
" \n",
" class login(page):\n",
" path = \"/account/login\"\n",
" def GET(self):\n",
" print \"Login Form\""
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"How does `class Meta` in dango works? \n",
"\n",
"That is nothing to do with metaclasses.\n",
"\n",
"Lets try to change the above problem to use a class Meta.\n",
"\n",
" class index(page):\n",
" class Meta:\n",
" path = \"/\"\n",
" log_level = \"DEBUG\"\n",
" \n",
" def GET(self):\n",
" print \"Home Page\"\n",
"\n",
"The `class Meta` is just used as a namespace to hold bunch of atrributes together. The name `Meta` is what the library/framework expects.\n",
"\n",
"Putting it in another way:\n",
"\n",
" class index(page): \n",
" Meta = type(\"Meta\", (object,), {\"path\": \"/\", \"log_level\": \"DEBUG\"})\n",
" \n",
" def GET(self):\n",
" print \"Home Page\"\n",
"\n",
"How to access `Meta`?\n",
"\n",
" m = index.Meta()\n",
"\n",
"It is just like accessing a class attribute `Point.x`.\n",
"\n",
"\n",
"It would give same effect as using a dict, but writing a class looks simpler.\n",
"\n",
" class index(page): \n",
" meta = {\"path\": \"/\", \"log_level\": \"DEBUG\"}\n",
" \n",
"is probably more verbose than writing a class.\n"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"**Example: final class a.la. java"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"class FinalMeta(type):\n",
" def __init__(self, name, bases, attrs):\n",
" # raise error if the class is begin extended from a final class, \n",
" if any(isinstance(b, FinalMeta) for b in bases):\n",
" raise Exception(\"You can't extend from a final class.\")\n",
" type.__init__(self, name, bases, attrs)\n",
" print \"created class\", self\n",
" \n",
"class FinalFoo(object):\n",
" __metaclass__ = FinalMeta\n",
" \n",
"class FinalBar(FinalFoo):\n",
" pass\n",
" "
],
"language": "python",
"metadata": {},
"outputs": [
{
"ename": "Exception",
"evalue": "You can't extend from a final class.",
"output_type": "pyerr",
"traceback": [
"\u001b[0;31m---------------------------------------------------------------------------\u001b[0m\n\u001b[0;31mException\u001b[0m Traceback (most recent call last)",
"\u001b[0;32m<ipython-input-55-7dc13b888a25>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m()\u001b[0m\n\u001b[1;32m 10\u001b[0m \u001b[0m__metaclass__\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0mFinalMeta\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m 11\u001b[0m \u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0;32m---> 12\u001b[0;31m \u001b[0;32mclass\u001b[0m \u001b[0mFinalBar\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0mFinalFoo\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m:\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m\u001b[1;32m 13\u001b[0m \u001b[0;32mpass\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m 14\u001b[0m \u001b[0;34m\u001b[0m\u001b[0m\n",
"\u001b[0;32m<ipython-input-55-7dc13b888a25>\u001b[0m in \u001b[0;36m__init__\u001b[0;34m(self, name, bases, attrs)\u001b[0m\n\u001b[1;32m 3\u001b[0m \u001b[0;31m# raise error if the class is begin extended from a final class,\u001b[0m\u001b[0;34m\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m 4\u001b[0m \u001b[0;32mif\u001b[0m \u001b[0many\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0misinstance\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0mb\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0mFinalMeta\u001b[0m\u001b[0;34m)\u001b[0m \u001b[0;32mfor\u001b[0m \u001b[0mb\u001b[0m \u001b[0;32min\u001b[0m \u001b[0mbases\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m:\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0;32m----> 5\u001b[0;31m \u001b[0;32mraise\u001b[0m \u001b[0mException\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0;34m\"You can't extend from a final class.\"\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m\u001b[1;32m 6\u001b[0m \u001b[0mtype\u001b[0m\u001b[0;34m.\u001b[0m\u001b[0m__init__\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0mself\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0mname\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0mbases\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0mattrs\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m 7\u001b[0m \u001b[0;32mprint\u001b[0m \u001b[0;34m\"created class\"\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0mself\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n",
"\u001b[0;31mException\u001b[0m: You can't extend from a final class."
]
},
{
"output_type": "stream",
"stream": "stdout",
"text": [
"created class <class '__main__.FinalFoo'>\n"
]
}
],
"prompt_number": 55
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## `__new__` "
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"The `__init__` method is called to initialize the object after it is created. The `__new__` method is called to create the object itself."
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"class Point(object):\n",
" def __new__(cls, *a, **kw):\n",
" print \"Point.__new__\", cls, a, kw \n",
" return object.__new__(cls, *a, **kw)\n",
"\n",
" def __init__(*a, **kw):\n",
" print \"Point.__init__\", a, kw \n",
"\n",
"p = Point(1, 2)\n",
"print p"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"Point.__new__ <class '__main__.Point'> (1, 2) {}\n",
"Point.__init__ (<__main__.Point object at 0x101993f50>, 1, 2) {}\n",
"<__main__.Point object at 0x101993f50>\n"
]
}
],
"prompt_number": 66
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"**Example: Singleton**"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"class Singleton(object):\n",
" def __new__(cls, *a, **kw):\n",
" try:\n",
" i = cls.INSTANCE\n",
" print \"reusing old instance of\", cls\n",
" return i\n",
" except AttributeError:\n",
" print \"creating new instance of\", cls\n",
" cls.INSTANCE = object.__new__(cls, *a, **kw)\n",
" return cls.INSTANCE\n",
"\n",
"class Zero(Singleton):\n",
" pass\n",
"\n",
"z0 = Zero()\n",
"z1 = Zero()\n",
"\n",
"print id(z0), id(z1)"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"creating new instance of <class '__main__.Zero'>\n",
"reusing old instance of <class '__main__.Zero'>\n",
"4321867472 4321867472\n"
]
}
],
"prompt_number": 73
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"**Problem** Write a class Cached, which memoizes class creation and reuses existing objects when called with same arguments.\n",
"\n",
" class Point(Cached):\n",
" def __init__(self, x, y):\n",
" self.x = x\n",
" self.y = y\n",
" \n",
" \n",
" p1 = Point(1, 2)\n",
" p2 = Point(1, 2)\n",
" print p1 is p2 # True"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"Possible solution:\n",
"\n",
" class Cached(object):\n",
" @memoize\n",
" def __new__(cls, *a):\n",
" return object.__new__(cls, *a)\n",
" \n",
" class Point(Cached):\n",
" def __init__(self, x, y):\n",
" self.x = x\n",
" self.y = y\n",
" \n",
" \n",
" p1 = Point(1, 2)\n",
" p2 = Point(1, 2)\n",
" print p1 is p2 # True \n"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"**staticmethod and classmethod**"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"import json\n",
"\n",
"class Point:\n",
" def __init__(self, x, y):\n",
" self.x = x\n",
" self.y = y\n",
" \n",
" def to_dict(self):\n",
" return {\"x\": self.x, \"y\": self.y}\n",
" \n",
" def to_json(self):\n",
" return json.dumps(self.to_dict())\n",
"\n",
"p1 = Point(1, 2)\n",
"print p1.to_json()"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"{\"y\": 2, \"x\": 1}\n"
]
}
],
"prompt_number": 74
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"What about `from_json`?"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"import json\n",
"\n",
"class Point(object):\n",
" def __init__(self, x, y):\n",
" self.x = x\n",
" self.y = y\n",
" \n",
" def __repr__(self):\n",
" return \"Point(%d, %d)\" % (self.x, self.y)\n",
" \n",
" def to_dict(self):\n",
" return {\"x\": self.x, \"y\": self.y}\n",
" \n",
" def to_json(self):\n",
" return json.dumps(self.to_dict())\n",
" \n",
" @staticmethod\n",
" def from_json(jsontext):\n",
" d = json.loads(jsontext)\n",
" return Point(d['x'], d['y'])\n",
"\n",
"p1 = Point(1, 2)\n",
"print p1\n",
"jsontext = p1.to_json()\n",
"\n",
"# create point from jsontext\n",
"p2 = Point.from_json(jsontext)\n",
"print p2"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"Point(1, 2)\n",
"Point(1, 2)\n"
]
}
],
"prompt_number": 78
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"Why can't I put it outside the class?\n",
"\n",
"What if you have multiple classes?"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"class Size(object):\n",
" a = 1\n",
" def __init__(self, w, h):\n",
" self.w = w\n",
" self.h = h\n",
"\n",
" def __repr__(self):\n",
" return \"Size(%d, %d)\" % (self.w, self.h)\n",
" \n",
" def to_json(self):\n",
" return json.dumps({\"w\": self.w, \"h\": self.h})\n",
" \n",
" @staticmethod\n",
" def from_json(jsontext):\n",
" d = json.loads(jsontext)\n",
" return Size(d['w'], d['h'])\n",
" \n",
"\n",
"s1 = Size(10, 20)\n",
"text = s1.to_json()\n",
"s2 = Size.from_json(text)\n",
"print s1, s2"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"Size(10, 20) Size(10, 20)\n"
]
}
],
"prompt_number": 79
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"The code for both Point and Size look almost the same. Can we generalize that?"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"%%file fromjson.py\n",
"from __future__ import unicode_literals\n",
"import json\n",
"\n",
"class Serializable(object):\n",
" @classmethod\n",
" def from_json(cls, jsontext):\n",
" d = json.loads(jsontext)\n",
" return cls.from_dict(d)\n",
" \n",
" @classmethod\n",
" def from_dict(cls, d):\n",
" return cls(**d)\n",
" \n",
" def to_json(self):\n",
" return json.dumps(self.to_dict())\n",
" \n",
" def to_dict(self):\n",
" return self.__dict__\n",
" \n",
" \n",
"class Point(Serializable):\n",
" def __init__(self, x, y):\n",
" self.x = x\n",
" self.y = y\n",
" \n",
" def __repr__(self):\n",
" return \"Point(%d, %d)\" % (self.x, self.y)\n",
" \n",
"class Size(Serializable):\n",
" def __init__(self, w, h):\n",
" self.w = w\n",
" self.h = h\n",
" \n",
" def __repr__(self):\n",
" return \"Size(%d, %d)\" % (self.w, self.h)\n",
" \n",
"\n",
"p1 = Point(1, 2)\n",
"print p1\n",
"\n",
"p2 = Point.from_json(p1.to_json())\n",
"print p2\n",
"\n",
"s1 = Size(1, 2)\n",
"print s1\n",
"s2 = Size.from_json(s1.to_json())\n",
"print s2"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"Overwriting fromjson.py\n"
]
}
],
"prompt_number": 91
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"!python2.7 fromjson.py"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"Point(1, 2)\r\n",
"Point(1, 2)\r\n",
"Size(1, 2)\r\n",
"Size(1, 2)\r\n"
]
}
],
"prompt_number": 92
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"**property**"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"import re\n",
"class Person(object):\n",
" def __init__(self, fullname, phone):\n",
" self.fullname = fullname\n",
" self._phone = phone\n",
" \n",
" @property\n",
" def phone(self):\n",
" print \"get_phone\"\n",
" return self._phone\n",
" \n",
" @phone.setter\n",
" def phone(self, phone):\n",
" print \"set_phone\", phone\n",
" if not re.match(\"^[0-9 ]+$\", phone):\n",
" raise ValueError(\"Bad phone number: %r\" % phone)\n",
" self._phone = phone\n",
" \n",
" #phone = property(get_phone, set_phone)\n",
" \n",
" def __repr__(self):\n",
" return \"Person(%s, %s)\" % (self.fullname, self._phone)\n",
" \n",
"\n",
"p1 = Person(\"Joe\", \"12345567\")\n",
"print p1\n",
"\n",
"p1.phone = \"bad number\"\n",
"print p1"
],
"language": "python",
"metadata": {},
"outputs": [
{
"ename": "ValueError",
"evalue": "Bad phone number: u'bad number'",
"output_type": "pyerr",
"traceback": [
"\u001b[0;31m---------------------------------------------------------------------------\u001b[0m\n\u001b[0;31mValueError\u001b[0m Traceback (most recent call last)",
"\u001b[0;32m<ipython-input-112-4d63f40fcd67>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m()\u001b[0m\n\u001b[1;32m 26\u001b[0m \u001b[0;32mprint\u001b[0m \u001b[0mp1\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m 27\u001b[0m \u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0;32m---> 28\u001b[0;31m \u001b[0mp1\u001b[0m\u001b[0;34m.\u001b[0m\u001b[0mphone\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0;34m\"bad number\"\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m\u001b[1;32m 29\u001b[0m \u001b[0;32mprint\u001b[0m \u001b[0mp1\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n",
"\u001b[0;32m<ipython-input-112-4d63f40fcd67>\u001b[0m in \u001b[0;36mphone\u001b[0;34m(self, phone)\u001b[0m\n\u001b[1;32m 14\u001b[0m \u001b[0;32mprint\u001b[0m \u001b[0;34m\"set_phone\"\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0mphone\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m 15\u001b[0m \u001b[0;32mif\u001b[0m \u001b[0;32mnot\u001b[0m \u001b[0mre\u001b[0m\u001b[0;34m.\u001b[0m\u001b[0mmatch\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0;34m\"^[0-9 ]+$\"\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0mphone\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m:\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0;32m---> 16\u001b[0;31m \u001b[0;32mraise\u001b[0m \u001b[0mValueError\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0;34m\"Bad phone number: %r\"\u001b[0m \u001b[0;34m%\u001b[0m \u001b[0mphone\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m\u001b[1;32m 17\u001b[0m \u001b[0mself\u001b[0m\u001b[0;34m.\u001b[0m\u001b[0m_phone\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0mphone\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m 18\u001b[0m \u001b[0;34m\u001b[0m\u001b[0m\n",
"\u001b[0;31mValueError\u001b[0m: Bad phone number: u'bad number'"
]
},
{
"output_type": "stream",
"stream": "stdout",
"text": [
"Person(Joe, 12345567)\n",
"set_phone bad number\n"
]
}
],
"prompt_number": 112
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"**Problem** Create a `slug` proprty for this following class.\n",
"\n",
" class Post(object):\n",
" def __init__(self, date, title):\n",
" self.date = date\n",
" self.title = title\n",
" \n",
"The date will be an instance of `datetime.date` class and title will be a string. The URL will be in the form of `\"/2013/09/11/title-comes-here\"`\n",
"\n",
"\n",
" from datetime import date\n",
" p = Post(date(2013, 9, 11), \"Hello world\")\n",
" print p.slug"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"**old-style and new-style classes**"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"class A:\n",
" pass\n",
"a = A()\n",
"print type(a)"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"<type 'instance'>\n"
]
}
],
"prompt_number": 113
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"type(A)"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 114,
"text": [
"classobj"
]
}
],
"prompt_number": 114
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"class B(object):\n",
" pass\n",
"b = B()\n",
"print type(b)\n",
"print type(B)"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"<class '__main__.B'>\n",
"<type 'type'>\n"
]
}
],
"prompt_number": 115
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"len(a)"
],
"language": "python",
"metadata": {},
"outputs": [
{
"ename": "AttributeError",
"evalue": "A instance has no attribute '__len__'",
"output_type": "pyerr",
"traceback": [
"\u001b[0;31m---------------------------------------------------------------------------\u001b[0m\n\u001b[0;31mAttributeError\u001b[0m Traceback (most recent call last)",
"\u001b[0;32m<ipython-input-116-af8c77e09569>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m()\u001b[0m\n\u001b[0;32m----> 1\u001b[0;31m \u001b[0mlen\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0ma\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m",
"\u001b[0;31mAttributeError\u001b[0m: A instance has no attribute '__len__'"
]
}
],
"prompt_number": 116
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"a.__len__ = lambda: 0\n",
"b.__len__ = lambda: 0"
],
"language": "python",
"metadata": {},
"outputs": [],
"prompt_number": 117
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"len(a)"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 118,
"text": [
"0"
]
}
],
"prompt_number": 118
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"len(b)"
],
"language": "python",
"metadata": {},
"outputs": [
{
"ename": "TypeError",
"evalue": "object of type 'B' has no len()",
"output_type": "pyerr",
"traceback": [
"\u001b[0;31m---------------------------------------------------------------------------\u001b[0m\n\u001b[0;31mTypeError\u001b[0m Traceback (most recent call last)",
"\u001b[0;32m<ipython-input-119-f311c9fb5505>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m()\u001b[0m\n\u001b[0;32m----> 1\u001b[0;31m \u001b[0mlen\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0mb\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m",
"\u001b[0;31mTypeError\u001b[0m: object of type 'B' has no len()"
]
}
],
"prompt_number": 119
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"new-style class looks for `__xxx__` methods in the class where as old-style lcass looks for it in the object."
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## Descriptors"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"class A(object):\n",
" @property\n",
" def x(self):\n",
" return 0\n",
" \n",
"a = A()\n",
"A.__dict__"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 125,
"text": [
"<dictproxy {'__dict__': <attribute '__dict__' of 'A' objects>,\n",
" '__doc__': None,\n",
" '__module__': '__main__',\n",
" '__weakref__': <attribute '__weakref__' of 'A' objects>,\n",
" 'x': <property at 0x1019c5158>}>"
]
}
],
"prompt_number": 125
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"A.x"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 126,
"text": [
"<property at 0x1019c5158>"
]
}
],
"prompt_number": 126
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"dir(A.x)"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 127,
"text": [
"['__class__',\n",
" '__delattr__',\n",
" '__delete__',\n",
" '__doc__',\n",
" '__format__',\n",
" '__get__',\n",
" '__getattribute__',\n",
" '__hash__',\n",
" '__init__',\n",
" '__new__',\n",
" '__reduce__',\n",
" '__reduce_ex__',\n",
" '__repr__',\n",
" '__set__',\n",
" '__setattr__',\n",
" '__sizeof__',\n",
" '__str__',\n",
" '__subclasshook__',\n",
" 'deleter',\n",
" 'fdel',\n",
" 'fget',\n",
" 'fset',\n",
" 'getter',\n",
" 'setter']"
]
}
],
"prompt_number": 127
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"A.x.__get__(a)"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 130,
"text": [
"0"
]
}
],
"prompt_number": 130
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"class Zero(object):\n",
" def __get__(self, obj, type=None):\n",
" print \"Zero.__get__\", obj, type\n",
" return 0\n",
" \n",
" def __set__(self, obj, value):\n",
" print \"__set__\"\n",
" raise AttributeError()\n",
" \n",
" def __repr__(self):\n",
" return \"<Zero>\"\n",
"\n",
"class A(object):\n",
" x = Zero()\n",
" \n",
"a = A()\n",
"print a.x"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"Zero.__get__ <__main__.A object at 0x101996e10> <class '__main__.A'>\n",
"0\n"
]
}
],
"prompt_number": 151
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"Lets try to implement property decorator ourselves."
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"class my_property(object):\n",
" def __init__(self, getter, setter=None):\n",
" print \"my_property.__init__\", getter\n",
" self._getter = getter\n",
" self._setter = setter\n",
" \n",
" def setter(self, func):\n",
" self._setter = func\n",
" return self\n",
" \n",
" def __get__(self, obj, type=None):\n",
" print \"my_property.__get__\", obj\n",
" return self._getter(obj)\n",
" \n",
" def __set__(self, obj, value):\n",
" print \"my_property.__set__\", obj, value\n",
" if self._setter is None:\n",
" raise AttributeError\n",
" else:\n",
" return self._setter(obj, value)\n",
"\n",
"class A(object):\n",
" def __init__(self):\n",
" self._x = 0\n",
" \n",
" @my_property\n",
" def x(self):\n",
" print \"A.x\"\n",
" return self._x\n",
" \n",
" @x.setter\n",
" def x(self, value):\n",
" print \"A.x setter\"\n",
" self._x = value\n",
" \n",
"print \"end of class\" \n",
"a = A()\n",
"print a.x\n",
"a.x = 1"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"my_property.__init__ <function x at 0x10198fc08>\n",
"end of class\n",
"my_property.__get__ <__main__.A object at 0x101b22950>\n",
"A.x\n",
"0\n",
"my_property.__set__ <__main__.A object at 0x101b22950> 1\n",
"A.x setter\n"
]
}
],
"prompt_number": 168
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"**Problem** Implement a descriptor Constant, which doesn't allow changing the value.\n",
"\n",
" class Circle:\n",
" pi = Constant(3.14)\n",
" \n",
" c = Circle()\n",
" print c.pi\n",
" \n",
" c.pi = 3.0 # error"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"We can use descriptors for doing various validations."
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"class BaseField(object):\n",
" def __init__(self):\n",
" self.value = None\n",
" \n",
" def __get__(self, obj, type):\n",
" return self.value\n",
" \n",
" def __set__(self, obj, value):\n",
" self.value = self.parse(obj, value)\n",
" \n",
" def parse(self, obj, value):\n",
" return value \n",
" \n",
"class IntegerField(BaseField):\n",
" def parse(self, obj, value):\n",
" return int(value)\n",
" \n",
"class Point(object):\n",
" x = IntegerField()\n",
" y = IntegerField()\n",
" \n",
"p = Point()\n",
"p.x = 1\n",
"p.y = 34\n",
"print p.x, p.y\n",
"\n",
"p.x = \"foo\""
],
"language": "python",
"metadata": {},
"outputs": [
{
"ename": "ValueError",
"evalue": "invalid literal for int() with base 10: 'foo'",
"output_type": "pyerr",
"traceback": [
"\u001b[0;31m---------------------------------------------------------------------------\u001b[0m\n\u001b[0;31mValueError\u001b[0m Traceback (most recent call last)",
"\u001b[0;32m<ipython-input-172-57a9c35436e1>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m()\u001b[0m\n\u001b[1;32m 25\u001b[0m \u001b[0;32mprint\u001b[0m \u001b[0mp\u001b[0m\u001b[0;34m.\u001b[0m\u001b[0mx\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0mp\u001b[0m\u001b[0;34m.\u001b[0m\u001b[0my\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m 26\u001b[0m \u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0;32m---> 27\u001b[0;31m \u001b[0mp\u001b[0m\u001b[0;34m.\u001b[0m\u001b[0mx\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0;34m\"foo\"\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m",
"\u001b[0;32m<ipython-input-172-57a9c35436e1>\u001b[0m in \u001b[0;36m__set__\u001b[0;34m(self, obj, value)\u001b[0m\n\u001b[1;32m 7\u001b[0m \u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m 8\u001b[0m \u001b[0;32mdef\u001b[0m \u001b[0m__set__\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0mself\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0mobj\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0mvalue\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m:\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0;32m----> 9\u001b[0;31m \u001b[0mself\u001b[0m\u001b[0;34m.\u001b[0m\u001b[0mvalue\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0mself\u001b[0m\u001b[0;34m.\u001b[0m\u001b[0mparse\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0mobj\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0mvalue\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m\u001b[1;32m 10\u001b[0m \u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m 11\u001b[0m \u001b[0;32mdef\u001b[0m \u001b[0mparse\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0mself\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0mobj\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0mvalue\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m:\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n",
"\u001b[0;32m<ipython-input-172-57a9c35436e1>\u001b[0m in \u001b[0;36mparse\u001b[0;34m(self, obj, value)\u001b[0m\n\u001b[1;32m 14\u001b[0m \u001b[0;32mclass\u001b[0m \u001b[0mIntegerField\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0mBaseField\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m:\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m 15\u001b[0m \u001b[0;32mdef\u001b[0m \u001b[0mparse\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0mself\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0mobj\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0mvalue\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m:\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0;32m---> 16\u001b[0;31m \u001b[0;32mreturn\u001b[0m \u001b[0mint\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0mvalue\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m\u001b[1;32m 17\u001b[0m \u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m 18\u001b[0m \u001b[0;32mclass\u001b[0m \u001b[0mPoint\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0mobject\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m:\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n",
"\u001b[0;31mValueError\u001b[0m: invalid literal for int() with base 10: 'foo'"
]
},
{
"output_type": "stream",
"stream": "stdout",
"text": [
"1 34\n"
]
}
],
"prompt_number": 172
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## Context Managers"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"with open(\"numbers.txt\") as f:\n",
" f.read()"
],
"language": "python",
"metadata": {},
"outputs": [],
"prompt_number": 174
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"The with statement takes care of closing the file."
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"f = open(\"numbers.txt\")"
],
"language": "python",
"metadata": {},
"outputs": [],
"prompt_number": 176
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"f.__enter__"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 177,
"text": [
"<function __enter__>"
]
}
],
"prompt_number": 177
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"f.__exit__"
],
"language": "python",
"metadata": {},
"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 178,
"text": [
"<function __exit__>"
]
}
],
"prompt_number": 178
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"**Example: chdir**"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"Lets try to implement a context manager to change directory for a block of code.\n",
"\n",
" print os.getcwd()\n",
" with chdir(\"/tmp\"):\n",
" print os.getcwd()\n",
" print os.getcwd() "
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"import os\n",
"\n",
"class chdir(object):\n",
" def __init__(self, path):\n",
" self.path = path\n",
" \n",
" def __enter__(self):\n",
" self.old_cwd = os.getcwd()\n",
" os.chdir(self.path)\n",
" return self\n",
" \n",
" def __exit__(self, *a):\n",
" os.chdir(self.old_cwd)\n",
" \n",
"\n",
"print os.getcwd()\n",
"with chdir(\"/tmp\"):\n",
" print os.getcwd()\n",
" print os.popen(\"pwd\").read()\n",
"print os.getcwd()"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"/Users/anand/Dropbox/Trainings/2013/advancedpython-nov2013/notes\n",
"/private/tmp\n",
"/private/tmp\n",
"\n",
"/Users/anand/Dropbox/Trainings/2013/advancedpython-nov2013/notes\n"
]
}
],
"prompt_number": 182
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"Lets see what happens on exceptions."
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"class foo(object):\n",
" def __enter__(self):\n",
" print \"__enter__\"\n",
" return 1\n",
" \n",
" def __exit__(self, type, exc, traceback):\n",
" print \"__exit__\", type, exc, traceback\n",
" # If __exit__ returns True, the exception is supressed\n",
" return True\n",
"\n",
"with foo() as x:\n",
" print x\n",
" raise Exception(\"with in with\")"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"__enter__\n",
"1\n",
"__exit__ <type 'exceptions.Exception'> with in with <traceback object at 0x101b37710>\n"
]
}
],
"prompt_number": 188
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"**Problem** Write a context manager `ignore_exception` to ignore any exception raised in the with block.\n",
"\n",
" with ignore_exception(IOError):\n",
" read_config_file()\n",
" \n",
"If ignore_exception is called with an exception type, all exceptions of that type are ignored. Otherwise all exceptions are ignored."
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"**Calling super class constructor**"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"class A0(object):\n",
" pass\n",
"\n",
"class A(A0):\n",
" def hello(self):\n",
" print \"A.Hello\"\n",
" \n",
"class B(object):\n",
" def hello(self):\n",
" print \"B.Hello\"\n",
"\n",
"class C(A, B):\n",
" pass\n",
"\n",
"c = C()\n",
"c.hello()\n",
"print C.__mro__"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"A.Hello\n",
"(<class '__main__.C'>, <class '__main__.A'>, <class '__main__.A0'>, <class '__main__.B'>, <type 'object'>)\n"
]
}
],
"prompt_number": 204
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"# Python 3"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"%%file python3.py\n",
"\n",
"# print is a function\n",
"print(\"hello\", \"world\", 2)\n",
"\n",
"# Python3 str type is what is unicode in python2\n",
"name = \"\\u0c06\"\n",
"\n",
"# It has bytes type, which is same as str in Python2\n",
"\n",
"x = b'hello'\n",
"print(x)\n",
"print(x[0])\n",
"\n",
"# some modules are moved around.\n",
"# python2 urlparse has become urllib.parse\n",
"import urllib.parse\n",
"\n",
"# some nice features!\n",
"\n",
"# keyword-only arguments\n",
"\n",
"def join(*args, sep):\n",
" return sep.join(args)\n",
"\n",
"print(join(\"hello\", \"world\", sep=\"-\"))\n",
"\n",
"def incr(x, *, amount=1):\n",
" return x+amount\n",
"\n",
"print(incr(2, amount=4))\n",
" \n",
"# new asyncio module\n",
"\n",
"# there is 2to3 utility to convert python 2 code to python 3."
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"Overwriting python3.py\n"
]
}
],
"prompt_number": 229
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"!python3 python3.py"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"hello world 2\r\n",
"b'hello'\r\n",
"104\r\n",
"hello-world\r\n",
"6\r\n"
]
}
],
"prompt_number": 230
},
{
"cell_type": "markdown",
"metadata": {},
"source": []
}
],
"metadata": {}
}
]
}
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