2024-10-10 03:26:21 +00:00
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2024-10-10 03:36:38 +00:00
"# Python Objects & References\n",
2024-10-10 03:26:21 +00:00
"\n",
"Everything in Python is an `object`. An object is a piece of memory with values & associated operations.\n",
"\n",
"We've seen plenty of methods on `str`, `list`, `dict`, etc. \n",
"\n",
"`a_string.lower()`, `dict.pop(val)`, etc.\n",
"\n",
"As we'll see in time, every type in Python works this way. \n",
"\n",
"Operators like `+`, `-`, `and` and `or` are \"associated operations\" when we're using scalars like `int` or `bool`."
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},
"source": [
"Variables in python are referred to as *names* or *identifiers*.\n",
"\n",
"\n",
"\n",
" -- Learning Python 2013\n",
" \n",
"A name does not uniquely identify an object!"
]
},
{
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"source": [
2024-10-10 03:36:38 +00:00
"## objects are typed, not variables\n",
2024-10-10 03:26:21 +00:00
"\n",
"\n",
" -- Learning Python 2013"
]
},
{
"cell_type": "markdown",
"id": "9f499594-559b-447e-b0a1-40b81b583913",
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"source": [
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"## Shared references\n",
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"\n",
"Setting a variable to a new value does not alter the original.\n",
"\n",
"It causes the variable to reference a brand new object."
]
},
{
"cell_type": "code",
"execution_count": null,
"id": "ed9dd65a-5792-4340-a354-0bb10b2828b3",
"metadata": {
"slideshow": {
"slide_type": "subslide"
}
},
"outputs": [],
"source": [
"x = 10\n",
"y = x\n",
"x = 20\n",
"print(x, y)"
]
},
{
"cell_type": "code",
"execution_count": null,
"id": "ab5bbf92-9b1a-4159-a9ce-4a9f1bea7785",
"metadata": {
"slideshow": {
"slide_type": "subslide"
}
},
"outputs": [],
"source": [
"# what does this mean for mutable objects?\n",
"x = [1, 2, 3]\n",
"y = x\n",
"y.append(4)\n",
"print(x)\n",
"print(y)"
]
},
{
"cell_type": "code",
"execution_count": null,
"id": "b8410e5b-f231-48df-8cef-7c947f6945fc",
"metadata": {},
"outputs": [],
"source": [
"a = 3\n",
"b = a\n",
"a *= 2\n",
"print(a, b)"
]
},
{
"cell_type": "code",
"execution_count": null,
"id": "bc471c6f-e4ac-4c15-84b1-e613644f066e",
"metadata": {
"tags": []
},
"outputs": [],
"source": [
"a = 3113\n",
"id(a)"
]
},
{
"cell_type": "code",
"execution_count": null,
"id": "75f6025a-4295-4a17-b08b-abc519bd24c4",
"metadata": {
"tags": []
},
"outputs": [],
"source": [
"b = 39209328\n",
"id(b)"
]
},
{
"cell_type": "code",
"execution_count": null,
"id": "927231d8-4f4c-47e5-8016-dbc6c436325a",
"metadata": {
"tags": []
},
"outputs": [],
"source": [
"id(1)"
]
},
{
"cell_type": "code",
"execution_count": null,
"id": "ed396d6b-fad3-4c43-9412-dbd9db181410",
"metadata": {
"tags": []
},
"outputs": [],
"source": [
"id(1)"
]
},
{
"cell_type": "code",
"execution_count": null,
"id": "3cb6f668-817f-4a37-aa08-253139f17ebb",
"metadata": {
"tags": []
},
"outputs": [],
"source": [
"c = b\n",
"id(c)"
]
},
{
"cell_type": "code",
"execution_count": null,
"id": "02a94a49-a376-4879-9004-527140dd7059",
"metadata": {
"tags": []
},
"outputs": [],
"source": [
"id(\"hello\")"
]
},
{
"cell_type": "code",
"execution_count": null,
"id": "8ce3f035-dd03-49e9-9f4e-a14947eb377c",
"metadata": {
"tags": []
},
"outputs": [],
"source": [
"id(\"hello\")"
]
},
{
"cell_type": "code",
"execution_count": null,
"id": "72893a54-b91d-43a6-bb78-3b19719e53d8",
"metadata": {
"tags": []
},
"outputs": [],
"source": [
"x = []"
]
},
{
"cell_type": "code",
"execution_count": null,
"id": "b11c23b9-b511-45ef-a1c8-1bf2eb3e48e1",
"metadata": {
"tags": []
},
"outputs": [],
"source": [
"y = []"
]
},
{
"cell_type": "code",
"execution_count": null,
"id": "ccf72016-cac0-469e-a1ff-7a8065b78e44",
"metadata": {
"tags": []
},
"outputs": [],
"source": [
"id(x)"
]
},
{
"cell_type": "code",
"execution_count": null,
"id": "c61cc294-14f9-4513-b160-3246efe0d501",
"metadata": {
"tags": []
},
"outputs": [],
"source": [
"id(y)"
]
},
{
"cell_type": "markdown",
"id": "99744fac-c1d9-4532-a7d4-0dcbc1a0d2a3",
"metadata": {},
"source": [
2024-10-10 03:36:38 +00:00
"## Garbage Collection\n",
2024-10-10 03:26:21 +00:00
"\n",
"Python is a garbage collected language. \n",
"\n",
"We don't free our own memory, Python does instead.\n",
"\n",
"Behind the scenes, Python stores a reference counter on each `object`. How many names/objects reference the object.\n",
"\n",
"When reference count drops to zero, Python can reclaim the memory."
]
},
{
"cell_type": "markdown",
"id": "4fe201ab-1d9e-441a-b90c-27ca22da2e53",
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"source": [
"## Identity\n",
"\n",
"The built-in `id(...)` function returns the identity of an object, which is an integer value guaranteed to be unique and constant for lifetime of object\n",
"\n",
"In the ofificial (\"CPython\") Interpeter we are using in this class, it is the address of the memory location storing the object."
]
},
{
"cell_type": "code",
"execution_count": 59,
"id": "ec197a99-5b09-480e-b7aa-8129474e03d2",
"metadata": {},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"4576794160\n"
]
}
],
"source": [
"x = \"MPCS\"\n",
"print(id(x)) # Unique integer-value for the object pointed by x"
]
},
{
"cell_type": "code",
"execution_count": null,
"id": "68e3ade8-d9a1-4d9c-b2f6-382fb90e745a",
"metadata": {},
"outputs": [],
"source": []
},
{
"cell_type": "code",
"execution_count": 60,
"id": "93b42c37-41d5-41ff-a92e-863bcf2ee78e",
"metadata": {},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"Fruit1 id = 4576869760 \n",
" Fruit2 id = 4576746816\n",
"Fruit3 id= 4576746816\n"
]
}
],
"source": [
"fruit1 = (\"Apples\", 4)\n",
"fruit2 = (\"Apples\", 4)\n",
"fruit3 = fruit2\n",
"print(f\"Fruit1 id = {id(fruit1)} \\n Fruit2 id = {id(fruit2)}\")\n",
"print(f\"Fruit3 id= {id(fruit3)}\")"
]
},
{
"cell_type": "markdown",
"id": "f36d4ec8-eefa-4474-84b1-b8be365329bc",
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"source": [
"#### Equality vs. Identity\n",
"\n",
"Two different ways of testing if objects are the \"same\":\n",
"\n",
"- Equality operator (`==`): Returns true if two objects are equal (i.e., have the same value)\n",
"- Identity operator (`is`): Returns true if two objects identities are the same.\n",
"\n",
"`a is b` means `id(a) == id(b)`"
]
},
{
"cell_type": "code",
"execution_count": 61,
"id": "67ee08d9-96b4-426f-bc05-4dc7bfecdf96",
"metadata": {
"slideshow": {
"slide_type": "subslide"
}
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"a == b True\n",
"4576799552\n",
"4576826368\n",
"a is b False\n"
]
}
],
"source": [
"a = [1, 2, 3]\n",
"b = [1, 2, 3]\n",
"print(\"a == b\", a == b)\n",
"\n",
"print(id(a))\n",
"print(id(b))\n",
"print(\"a is b\", a is b) # The id values are different"
]
},
{
"cell_type": "markdown",
"id": "37ad8b3a-2b9b-4879-afb2-a05cc3ab659e",
"metadata": {},
"source": [
"#### `is None`\n",
"\n",
"If you ever need to check if a value is `None`, you'd use `is None` or `is not None`"
]
},
{
"cell_type": "markdown",
"id": "9a48460e-a342-4d18-8383-b84483a3a010",
"metadata": {},
"source": [
2024-10-10 03:36:38 +00:00
"### list / string mutability revisited"
2024-10-10 03:26:21 +00:00
]
},
{
"cell_type": "code",
"execution_count": null,
"id": "3f65348f-9118-4840-ab30-6ef05e7dbf93",
"metadata": {},
"outputs": [],
"source": [
"# list d\n",
"d = [1, 2, 3]\n",
"print(id(d))\n",
"d.append(4)\n",
"print(d)\n",
"print(id(d))"
]
},
{
"cell_type": "code",
"execution_count": null,
"id": "7d1b2659-239c-4333-ac62-8533406bb025",
"metadata": {},
"outputs": [],
"source": [
"# str D\n",
"s = \"Hello\"\n",
"print(id(s))\n",
"s += \" World\"\n",
"print(s)\n",
"\n",
"# did s change?\n",
"print(id(s))"
]
},
{
"cell_type": "markdown",
"id": "c056ac96-58bc-41da-8014-443dc10485f6",
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"source": [
2024-10-10 03:36:38 +00:00
"### Aside: Object Creation Quirk\n",
2024-10-10 03:26:21 +00:00
"\n",
" Each time you generate a new value in your script by running an expression, Python creates a new object (i.e., a chunk of memory) to represent that value.\n",
" \n",
"-- Learning Python 2013\n",
"\n",
"Not quite! CPython does not guarantee this, and in fact sometimes caches & reuses immutable objects for efficiency.\n",
"\n"
]
},
{
"cell_type": "code",
"execution_count": null,
"id": "153d1f65-9d24-4f09-bb97-173c119e3528",
"metadata": {
"slideshow": {
"slide_type": "subslide"
}
},
"outputs": [],
"source": [
"a = 1000\n",
"b = 1000\n",
"\n",
"# Two different objects, two different ids.\n",
"print(a is b)\n",
"\n",
"# a = 100\n",
"# b = 100\n",
"\n",
"# However, for small integer objects, CPython caches them\n",
"# this means that a and b point to the same object\n",
"# print(a is b)\n",
"\n",
"for i in range(200, 300):\n",
" print(i, i is i)"
]
},
{
"cell_type": "code",
"execution_count": null,
"id": "d4e1b9d3-a5ee-475a-b187-af0a3bb1f257",
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [],
"source": [
"# CPython does the same for short strings\n",
"str1 = \"MPCS\" * 100\n",
"str2 = \"MPCS\" * 100\n",
"print(id(str1), id(str2))\n",
"str1 is str2"
]
},
{
"cell_type": "markdown",
"id": "12713d9c-f6da-44f3-9b12-db663cd3e6da",
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"source": [
2024-10-10 03:36:38 +00:00
"## copy & deepcopy\n",
2024-10-10 03:26:21 +00:00
"\n",
"If `y = x` does not make a copy, how can we get one?\n",
"\n",
"We've seen the `.copy()` method on a few of our types. Which ones?\n",
"\n",
"We can also use the `copy` module:"
]
},
{
"cell_type": "code",
"execution_count": null,
"id": "c7e81136-78ee-4503-bc66-443dfcbfbccf",
"metadata": {},
"outputs": [],
"source": [
"x = [1, 2, 3]\n",
"y = x.copy()\n",
"\n",
"print(id(x))\n",
"print(id(y))\n",
"\n",
"x.append(4)\n",
"print(x, y)"
]
},
{
"cell_type": "code",
"execution_count": 2,
"id": "10cafa1b-b1fe-4954-b005-4f6f953f36b5",
"metadata": {
"slideshow": {
"slide_type": "subslide"
}
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"x is y False\n",
"x[0] is y[0] True\n",
"[[1, 2, 5], [3, 4]] \n",
" [[1, 2, 5], [3, 4]]\n"
]
}
],
"source": [
"# shallow copy example (nested mutables are not copied)\n",
"\n",
"x = [[1, 2], [3, 4]]\n",
"y = x.copy() # or copy.copy(x)\n",
"\n",
"print(\"x is y\", x is y)\n",
"print(\"x[0] is y[0]\", x[0] is y[0])\n",
"\n",
"# print(x, y)\n",
"x[0].append(5)\n",
"print(x, \"\\n\", y)"
]
},
{
"cell_type": "code",
"execution_count": 3,
"id": "21855e2a-435c-4ecf-a224-586b55ad03b7",
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"x[0] is z[0] False\n"
]
}
],
"source": [
"# deep copy (nested mutables are copied)\n",
"import copy\n",
"\n",
"# copy.copy(obj) --> same as obj.copy()\n",
"z = copy.deepcopy(x)\n",
"print(\"x[0] is z[0]\", x[0] is z[0])"
]
}
],
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"display_name": "Python 3 (ipykernel)",
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