default-parameter-python-gotcha.ipynb

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   "source": [
    "## Unexpected behavior when using mutable default parameter values"
   ]
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      "f:\n",
      "  first call: 1\n",
      "  second call: 1\n",
      "g:\n",
      "  first call: [1]\n",
      "  first call: [1, 1]\n"
     ]
    }
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   "source": [
    "def f(a=0):\n",
    "    a += 1\n",
    "    return a\n",
    "\n",
    "def g(a=[]):\n",
    "    a += [1]\n",
    "    return a\n",
    "\n",
    "print('f:')\n",
    "print(f'  first call: {f()}')\n",
    "print(f'  second call: {f()}')\n",
    "\n",
    "print('g:')\n",
    "print(f'  first call: {g()}')\n",
    "print(f'  first call: {g()}')"
   ]
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    "## Explaining the behavior\n",
    "A function definition creates a function object. One of the attributes of such an object is the function's default parameter. The default parameter of f is immutable, but the default parameter of g is mutable. Therefore, every change that is made to g's default parameter in it's body in every call changes an attribute of the global function object. \n",
    "\n",
    "Essentially the following is occurring: "
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      "a points to a different object than \"attribute\" because \"0\" is immutable:\n",
      "  attribute: 0, a: 1\n",
      "  attribute: 0, a: 1\n",
      "a points to same object as \"attribute\" because \"[]\" is mutable:\n",
      "  attribute: [1], a: [1]\n",
      "  attribute: [1, 1], a: [1, 1]\n"
     ]
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   "source": [
    "attribute = 0\n",
    "a = attribute\n",
    "a += 1\n",
    "print(f'a points to a different object than \"attribute\" because \"0\" is immutable:')\n",
    "print(f'  attribute: {attribute}, a: {a}')\n",
    "a = attribute\n",
    "a += 1\n",
    "print(f'  attribute: {attribute}, a: {a}')\n",
    "\n",
    "attribute = []\n",
    "a = attribute\n",
    "a += [1]\n",
    "print(f'a points to same object as \"attribute\" because \"[]\" is mutable:')\n",
    "print(f'  attribute: {attribute}, a: {a}')\n",
    "a = attribute\n",
    "a += [1]\n",
    "print(f'  attribute: {attribute}, a: {a}')\n"
   ]
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   "source": [
    "## Best practice\n",
    "\n"
   ]
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     "text": [
      "good_g:\n",
      "  first call: [1]\n",
      "  first call: [1]\n"
     ]
    }
   ],
   "source": [
    "def good_g(a=None):\n",
    "    if a is None:\n",
    "        a = []\n",
    "    a += [1]\n",
    "    return a\n",
    "\n",
    "print('good_g:')\n",
    "print(f'  first call: {good_g()}')\n",
    "print(f'  first call: {good_g()}')"
   ]
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   "cell_type": "markdown",
   "id": "a1200c55",
   "metadata": {},
   "source": [
    "## Resources\n",
    "\n",
    "http://www.omahapython.org/IdiomaticPython.html#default-parameter-values"
   ]
  },
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   "execution_count": null,
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