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"source": "# Chapter 139: List destructuring (aka packing and unpacking)"
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"cell_type": "markdown",
"source": "## Section 139.1: Destructuring assignment"
},
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"cell_type": "code",
"source": "a, b = (1, 2)\nprint(a, b)",
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"outputs": [
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"output_type": "stream",
"text": "1 2\n",
"name": "stdout"
}
]
},
{
"metadata": {
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"trusted": true
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"cell_type": "code",
"source": "a, b, c = [1]",
"execution_count": 3,
"outputs": [
{
"output_type": "error",
"ename": "ValueError",
"evalue": "not enough values to unpack (expected 3, got 1)",
"traceback": [
"\u001b[0;31m---------------------------------------------------------------------------\u001b[0m",
"\u001b[0;31mValueError\u001b[0m Traceback (most recent call last)",
"\u001b[0;32m<ipython-input-3-ce8ca3f6da0c>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m()\u001b[0m\n\u001b[0;32m----> 1\u001b[0;31m \u001b[0ma\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0mb\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0mc\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0;34m[\u001b[0m\u001b[0;36m1\u001b[0m\u001b[0;34m]\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m",
"\u001b[0;31mValueError\u001b[0m: not enough values to unpack (expected 3, got 1)"
]
}
]
},
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"cell_type": "markdown",
"source": "### Destructuring as a list"
},
{
"metadata": {
"hidden": true,
"trusted": true
},
"cell_type": "code",
"source": "head, *tail = [1, 2, 3, 4, 5]\nprint(head)\nprint(tail)",
"execution_count": 4,
"outputs": [
{
"output_type": "stream",
"text": "1\n[2, 3, 4, 5]\n",
"name": "stdout"
}
]
},
{
"metadata": {
"hidden": true
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"cell_type": "markdown",
"source": "等於下面這個："
},
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"trusted": true
},
"cell_type": "code",
"source": "l = [1, 2, 3, 4, 5]\nhead = l[0]\ntail = l[1:]\nprint(head)\nprint(tail)",
"execution_count": 6,
"outputs": [
{
"output_type": "stream",
"text": "1\n[2, 3, 4, 5]\n",
"name": "stdout"
}
]
},
{
"metadata": {
"hidden": true
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"cell_type": "markdown",
"source": "想怎麼取就怎麼取"
},
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"hidden": true,
"trusted": true
},
"cell_type": "code",
"source": "a, b, *other, z = [1, 2, 3, 4, 5] \nprint(a, b, z, other)",
"execution_count": 9,
"outputs": [
{
"output_type": "stream",
"text": "1 2 5 [3, 4]\n",
"name": "stdout"
}
]
},
{
"metadata": {
"hidden": true
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"cell_type": "markdown",
"source": "`*_` 適合用在事前不知道 list 長度但又想要取得特定位置的 elements. 一行搞定"
},
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"hidden": true,
"trusted": true
},
"cell_type": "code",
"source": "a, *_, b = [1, 2, 3, 4, 5] \nprint(a, b)",
"execution_count": 10,
"outputs": [
{
"output_type": "stream",
"text": "1 5\n",
"name": "stdout"
}
]
},
{
"metadata": {
"hidden": true,
"trusted": true
},
"cell_type": "code",
"source": "l = (x for x in range(10000))\nhead, *_, tail = l\nprint(head, tail)",
"execution_count": 11,
"outputs": [
{
"output_type": "stream",
"text": "0 9999\n",
"name": "stdout"
}
]
},
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"metadata": {},
"cell_type": "markdown",
"source": "## Section 139.2: Packing function arguments"
},
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"trusted": true
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"cell_type": "code",
"source": "def func1(arg1, arg2, arg3): \n return (arg1,arg2,arg3)",
"execution_count": 16,
"outputs": []
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"trusted": true
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"cell_type": "code",
"source": "func1(1, 2, 3)",
"execution_count": 17,
"outputs": [
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"output_type": "execute_result",
"execution_count": 17,
"data": {
"text/plain": "(1, 2, 3)"
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"metadata": {}
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"cell_type": "code",
"source": "func1(1, 2)",
"execution_count": 18,
"outputs": [
{
"output_type": "error",
"ename": "TypeError",
"evalue": "func1() missing 1 required positional argument: 'arg3'",
"traceback": [
"\u001b[0;31m---------------------------------------------------------------------------\u001b[0m",
"\u001b[0;31mTypeError\u001b[0m Traceback (most recent call last)",
"\u001b[0;32m<ipython-input-18-315c5e9d954e>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m()\u001b[0m\n\u001b[0;32m----> 1\u001b[0;31m \u001b[0mfunc1\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0;36m1\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0;36m2\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m",
"\u001b[0;31mTypeError\u001b[0m: func1() missing 1 required positional argument: 'arg3'"
]
}
]
},
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"metadata": {},
"cell_type": "markdown",
"source": "### Packing a list of arguments"
},
{
"metadata": {
"trusted": true
},
"cell_type": "code",
"source": "l = ['w', 't', 'f']\ns = 'wtf'\nprint(func1(*l))\nprint(func1(*s))",
"execution_count": 27,
"outputs": [
{
"output_type": "stream",
"text": "('w', 't', 'f')\n('w', 't', 'f')\n",
"name": "stdout"
}
]
},
{
"metadata": {
"trusted": true
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"cell_type": "code",
"source": "func1(*l[::-1])",
"execution_count": 23,
"outputs": [
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"output_type": "execute_result",
"execution_count": 23,
"data": {
"text/plain": "('f', 't', 'w')"
},
"metadata": {}
}
]
},
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"metadata": {
"trusted": true
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"cell_type": "code",
"source": "func1(*'wtf!')",
"execution_count": 26,
"outputs": [
{
"output_type": "error",
"ename": "TypeError",
"evalue": "func1() takes 3 positional arguments but 4 were given",
"traceback": [
"\u001b[0;31m---------------------------------------------------------------------------\u001b[0m",
"\u001b[0;31mTypeError\u001b[0m Traceback (most recent call last)",
"\u001b[0;32m<ipython-input-26-bbe37ddfb9cd>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m()\u001b[0m\n\u001b[0;32m----> 1\u001b[0;31m \u001b[0mfunc1\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0;34m*\u001b[0m\u001b[0;34m'wtf!'\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m",
"\u001b[0;31mTypeError\u001b[0m: func1() takes 3 positional arguments but 4 were given"
]
}
]
},
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"metadata": {},
"cell_type": "markdown",
"source": "### Packing keyword arguments"
},
{
"metadata": {
"trusted": true
},
"cell_type": "code",
"source": "d = { 'arg1': 1, 'arg2': 2, 'arg3': 3}\nfun1(**d)",
"execution_count": 29,
"outputs": [
{
"output_type": "execute_result",
"execution_count": 29,
"data": {
"text/plain": "(1, 2, 3)"
},
"metadata": {}
}
]
},
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"metadata": {
"trusted": true
},
"cell_type": "code",
"source": "def func3(arg1, arg2='b', arg3='c'):\n return (arg1, arg2, arg3)",
"execution_count": 33,
"outputs": []
},
{
"metadata": {
"trusted": true
},
"cell_type": "code",
"source": "func3(*['arg1'], **{\"arg3\": \"YOOOO\"})",
"execution_count": 34,
"outputs": [
{
"output_type": "execute_result",
"execution_count": 34,
"data": {
"text/plain": "('arg1', 'b', 'YOOOO')"
},
"metadata": {}
}
]
},
{
"metadata": {},
"cell_type": "markdown",
"source": "## Section 139.3: Unpacking function arguments"
},
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"metadata": {
"trusted": true
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"cell_type": "code",
"source": "def func(*args, **kwargs):\n print(args, kwargs)",
"execution_count": 35,
"outputs": []
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"trusted": true
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"cell_type": "code",
"source": "func()",
"execution_count": 36,
"outputs": [
{
"output_type": "stream",
"text": "() {}\n",
"name": "stdout"
}
]
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"metadata": {
"trusted": true
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"cell_type": "code",
"source": "func(**{'kwarg1': 'YO', 'kwarg2': \"HI\"})",
"execution_count": 38,
"outputs": [
{
"output_type": "stream",
"text": "() {'kwarg1': 'YO', 'kwarg2': 'HI'}\n",
"name": "stdout"
}
]
},
{
"metadata": {},
"cell_type": "markdown",
"source": "Unpack `dict` as keyword arguments"
},
{
"metadata": {
"trusted": true
},
"cell_type": "code",
"source": "func(*['arg1', 'arg2'], {'kwarg1': \"HI\"})\nfunc(*['arg1', 'arg2'], **{'kwarg1': 'HI'})",
"execution_count": 41,
"outputs": [
{
"output_type": "stream",
"text": "('arg1', 'arg2', {'kwarg1': 'HI'}) {}\n('arg1', 'arg2') {'kwarg1': 'HI'}\n",
"name": "stdout"
}
]
}
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