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Add section on hashing
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‎source-code/object-orientation/attr_intro.ipynb

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"source": [
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"alice1 == alice3"
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]
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},
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{
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"cell_type": "markdown",
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"id": "748fa453-3827-4eb8-a012-f0d8d15aa183",
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"metadata": {},
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"source": [
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"You can exclude attrubutes from being used to compare objects for equality by adding the argument `eq=False` to the `attr.ib` function call. Similarly, you can exclude an attribute from order comparisons by setting `order` to `False`."
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]
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},
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{
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"cell_type": "markdown",
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"id": "6e3b0777-f567-47cb-8823-1e462d370be5",
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"metadata": {},
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"source": [
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"## Hashing"
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]
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},
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{
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"cell_type": "markdown",
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"id": "03d02551-2b9f-4d28-86a6-64c7550b847b",
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"metadata": {},
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"source": [
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"Mutable objects are not hashable, which means they can not be stored in sets, or used as keys in dictionaries."
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]
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},
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{
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"cell_type": "code",
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"execution_count": 32,
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"id": "6d576453-68e9-4bae-b8b8-378565960668",
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"metadata": {},
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"outputs": [
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{
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"name": "stdout",
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"output_type": "stream",
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"text": [
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"unhashable type: 'Person'\n"
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]
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}
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],
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"source": [
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"try:\n",
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" people = {\n",
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" Person(firstname='Alice', lastname='Zosimo', age=43),\n",
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" Person(firstname='Aaron', lastname='Leibovitch', age=31),\n",
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" }\n",
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"except Exception as error:\n",
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" print(error)"
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]
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},
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{
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"cell_type": "markdown",
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"id": "46e907e6-3278-4cec-b595-fc15a79a18ad",
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"metadata": {},
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"source": [
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"One options is to make the objects unmutable."
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]
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},
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{
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"cell_type": "code",
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"execution_count": 33,
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"id": "8c34e55f-f798-417c-808a-a70cab41d9b0",
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"metadata": {},
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"outputs": [],
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"source": [
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"@attr.s(frozen=True)\n",
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"class Person:\n",
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" lastname: str = attr.ib()\n",
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" firstname: str = attr.ib()\n",
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" age: int = attr.ib()\n",
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" \n",
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" @age.validator\n",
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" def age_validator(self, attribute, value):\n",
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" if value < 0 or value >= 130:\n",
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" raise ValueError(f'age should be between 0 and 130')"
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]
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},
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{
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"cell_type": "code",
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"execution_count": 53,
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"id": "922f49dd-c4cf-4554-9376-5e0185158f31",
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"metadata": {},
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"outputs": [
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{
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"data": {
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"text/plain": [
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"{Person(lastname='Leibovitch', firstname='Aaron', age=31),\n",
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" Person(lastname='Zosimo', firstname='Alice', age=43)}"
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]
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},
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"execution_count": 53,
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"metadata": {},
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"output_type": "execute_result"
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}
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],
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"source": [
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"people = {\n",
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" Person(firstname='Alice', lastname='Zosimo', age=43),\n",
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" Person(firstname='Aaron', lastname='Leibovitch', age=31),\n",
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"}\n",
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"people"
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]
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},
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{
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"cell_type": "markdown",
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"id": "0133fe30-3973-4398-8c8d-e62bd504c519",
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"metadata": {},
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"source": [
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"However, this implies you can not modify the objects."
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]
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},
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{
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"cell_type": "code",
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"execution_count": 55,
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"id": "37c2e8a4-2977-42b9-9529-30dfe1089f44",
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"metadata": {},
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"outputs": [
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{
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"name": "stdout",
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"output_type": "stream",
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"text": [
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"Person(lastname='Leibovitch', firstname='Aaron', age=31)\n",
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"the objects is frozen\n",
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"Person(lastname='Leibovitch', firstname='Aaron', age=31)\n"
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]
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}
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],
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"source": [
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"person = people.pop()\n",
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"print(person)\n",
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"try:\n",
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" person.age = 12\n",
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"except AttributeError:\n",
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" print('the objects is frozen')\n",
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"print(person)"
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]
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},
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{
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"cell_type": "markdown",
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"id": "5554878d-ad20-4317-b58a-54c3c8958e2b",
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"metadata": {},
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"source": [
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"This is of course fine if you do not need mutable ojbects. The alternative is to hash based on object identity."
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]
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},
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{
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"cell_type": "code",
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"execution_count": 56,
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"id": "f6269b27-ff89-40e8-bb85-5602cb0d6c72",
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"metadata": {},
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"outputs": [],
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"source": [
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"@attr.s(eq=False)\n",
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"class Person:\n",
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" lastname: str = attr.ib()\n",
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" firstname: str = attr.ib()\n",
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" age: int = attr.ib()\n",
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" \n",
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" @age.validator\n",
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" def age_validator(self, attribute, value):\n",
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" if value < 0 or value >= 130:\n",
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" raise ValueError(f'age should be between 0 and 130')"
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]
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},
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{
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"cell_type": "markdown",
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"id": "1e82961c-5ffb-43b4-b7e3-8e607beed8c7",
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"metadata": {},
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"source": [
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"However, this implies that objects will be distinct, regardless of the values of their attributes."
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]
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},
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{
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"cell_type": "code",
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"execution_count": 58,
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"id": "4ca729cc-2ce4-470e-befd-bab544ae1f3c",
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"metadata": {},
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"outputs": [
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{
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"data": {
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"text/plain": [
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"{Person(lastname='Leibovitch', firstname='Aaron', age=31),\n",
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" Person(lastname='Leibovitch', firstname='Robert', age=49),\n",
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" Person(lastname='Zosimo', firstname='Alice', age=43),\n",
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" Person(lastname='Zosimo', firstname='Alice', age=43)}"
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]
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},
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"execution_count": 58,
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"metadata": {},
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"output_type": "execute_result"
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}
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],
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"source": [
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"people = {\n",
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" Person(firstname='Alice', lastname='Zosimo', age=43),\n",
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" Person(firstname='Aaron', lastname='Leibovitch', age=31),\n",
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" Person(firstname='Robert', lastname='Leibovitch', age=49),\n",
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" Person(firstname='Alice', lastname='Zosimo', age=43),\n",
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"}\n",
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"people"
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]
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},
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{
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"cell_type": "code",
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"execution_count": 59,
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"id": "086f2880-5772-4cd1-ba58-44576678d0b6",
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"metadata": {},
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"outputs": [],
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"source": [
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"alice1 = Person(firstname='Alice', lastname='Zosimo', age=43)\n",
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"alice2 = Person(firstname='Alice', lastname='Zosimo', age=43)\n",
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"alice3 = Person(firstname='alice', lastname='zosimo', age=43)"
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]
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},
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{
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"cell_type": "markdown",
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"id": "ca98822f-691f-40a3-bb67-bbf989d87698",
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"metadata": {},
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"source": [
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"`alice1` has the same attributes as `alice2`, but they are distinct objects nevertheless."
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]
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},
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{
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"cell_type": "code",
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"execution_count": 60,
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"id": "d5cb600d-f440-4919-9397-1e9b75af0d0f",
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"metadata": {},
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"outputs": [
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{
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"data": {
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"text/plain": [
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"False"
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]
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},
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"execution_count": 60,
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"metadata": {},
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"output_type": "execute_result"
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}
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],
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"source": [
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"alice1 == alice2"
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]
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},
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{
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"cell_type": "code",
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"execution_count": 61,
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"id": "07a70926-54a2-425f-acb2-6aa22260f7c3",
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"metadata": {},
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"outputs": [
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{
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"data": {
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"text/plain": [
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"False"
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]
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},
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"execution_count": 61,
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"metadata": {},
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"output_type": "execute_result"
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}
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],
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"source": [
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"alice1 is alice2"
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]
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},
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{
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"cell_type": "markdown",
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"id": "1600b5e8-e10c-4eff-a369-87bc3920a8d3",
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"metadata": {},
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"source": [
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"Of course, you can still impliment your own hash method and comparison methods if required."
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]
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}
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],
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"metadata": {

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