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    "# 常量\n",
    "\n",
    "## numpy.nan\n",
    "\n",
    "- 表示空值。\n",
    "\n",
    "```\n",
    "nan = NaN = NAN\n",
    "```\n",
    "\n",
    "\n",
    "【例】两个`numpy.nan`是不相等的。\n",
    "\n",
    "```python\n",
    "import numpy as np\n",
    "\n",
    "print(np.nan == np.nan)  # False\n",
    "print(np.nan != np.nan)  # True\n",
    "```\n",
    "\n",
    "- `numpy.isnan(x, *args, **kwargs)` Test element-wise for NaN and return result as a boolean array.\n",
    "\n",
    "【例】\n",
    "```python\n",
    "import numpy as np\n",
    "\n",
    "x = np.array([1, 1, 8, np.nan, 10])\n",
    "print(x)\n",
    "# [ 1.  1.  8. nan 10.]\n",
    "\n",
    "y = np.isnan(x)\n",
    "print(y)\n",
    "# [False False False  True False]\n",
    "\n",
    "z = np.count_nonzero(y)\n",
    "print(z)  # 1\n",
    "```\n",
    "\n",
    "\n",
    "\n",
    "## numpy.inf\n",
    "- 表示正无穷大。\n",
    "\n",
    "```\n",
    "Inf = inf = infty = Infinity = PINF\n",
    "```\n",
    "\n",
    "【例】\n",
    "```python\n",
    "\n",
    "```\n",
    "\n",
    "## numpy.pi\n",
    "\n",
    "- 表示圆周率\n",
    "\n",
    "```python\n",
    "pi = 3.1415926535897932384626433...\n",
    "```\n",
    "\n",
    "## numpy.e\n",
    "\n",
    "- 表示自然常数\n",
    "\n",
    "```python\n",
    "e = 2.71828182845904523536028747135266249775724709369995...\n",
    "```\n",
    "\n"
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