mypy.ipynb

{
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      "file_extension": ".py",
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  "cells": [
    {
      "cell_type": "markdown",
      "source": "# Brief example of type-checking Python code using `mypy`",
      "metadata": {}
    },
    {
      "cell_type": "markdown",
      "source": "## Install `mypy` and register a custom `%%typecheck` magic in this JupyterLite notebook",
      "metadata": {}
    },
    {
      "cell_type": "code",
      "source": "%pip install mypy\n%pip install mypy_extensions\n\n\"\"\"\nAdd mypy type-checking cell magic to jupyter/ipython.\n\nSave this script to your ipython profile's startup directory.\n\nIPython's directories can be found via `ipython locate [profile]` to find the current ipython directory and ipython profile directory, respectively.\n\nFor example, this file could exist on a path like this on mac:\n\n/Users/yourusername/.ipython/profile_default/startup/typecheck.py\n\nwhere /Users/yourusername/.ipython/profile_default/ is the ipython directory for\nthe default profile.\n\nThe line magic is called \"typecheck\" to avoid namespace conflict with the mypy\npackage.\n\nhttps://gist.github.com/knowsuchagency/f7b2203dd613756a45f816d6809f01a6\n\n\"\"\"\n\nimport typing_extensions\n\nfrom IPython.core.magic import register_cell_magic\n\n\n@register_cell_magic\ndef typecheck(line, cell):\n    \"\"\"\n    Run the following cell though mypy.\n\n    Any parameters that would normally be passed to the mypy cli\n    can be passed on the first line, with the exception of the\n    -c flag we use to pass the code from the cell we want to execute\n\n     i.e.\n\n    %%typecheck --ignore-missing-imports\n    ...\n    ...\n    ...\n\n    mypy stdout and stderr will print prior to output of cell. If there are no conflicts,\n    nothing will be printed by mypy.\n    \"\"\"\n\n    from IPython import get_ipython\n    from mypy import api\n\n    # inserting a newline at the beginning of the cell\n    # ensures mypy's output matches the the line\n    # numbers in jupyter\n\n    cell = '\\n' + cell\n\n    mypy_result = api.run(['-c', cell] + line.split())\n\n    if mypy_result[0]:  # print mypy stdout\n        print(mypy_result[0])\n\n    if mypy_result[1]:  # print mypy stderr\n        print(mypy_result[1])\n\n    shell = get_ipython()\n    shell.run_cell(cell)",
      "metadata": {
        "trusted": true,
        "jupyter": {
          "source_hidden": true
        }
      },
      "outputs": [],
      "execution_count": 1
    },
    {
      "cell_type": "markdown",
      "source": "## a) Define an untyped function and call it correctly",
      "metadata": {}
    },
    {
      "cell_type": "code",
      "source": "def takes(x):\n    return x + \"b\"\n\ntakes(\"a\")",
      "metadata": {
        "trusted": true
      },
      "outputs": [
        {
          "execution_count": 2,
          "output_type": "execute_result",
          "data": {
            "text/plain": "'ab'"
          },
          "metadata": {}
        }
      ],
      "execution_count": 2
    },
    {
      "cell_type": "markdown",
      "source": "## b) Define an untyped function and call it with the wrong type - runtime `TypeError`",
      "metadata": {}
    },
    {
      "cell_type": "code",
      "source": "def takes(x):\n    return x + \"b\"\n\ntakes(3)",
      "metadata": {
        "trusted": true
      },
      "outputs": [
        {
          "ename": "<class 'TypeError'>",
          "evalue": "unsupported operand type(s) for +: 'int' and 'str'",
          "traceback": [
            "\u001b[0;31m---------------------------------------------------------------------------\u001b[0m",
            "\u001b[0;31mTypeError\u001b[0m                                 Traceback (most recent call last)",
            "Cell \u001b[0;32mIn[3], line 4\u001b[0m\n\u001b[1;32m      1\u001b[0m \u001b[38;5;28;01mdef\u001b[39;00m \u001b[38;5;21mtakes\u001b[39m(x):\n\u001b[1;32m      2\u001b[0m     \u001b[38;5;28;01mreturn\u001b[39;00m x \u001b[38;5;241m+\u001b[39m \u001b[38;5;124m\"\u001b[39m\u001b[38;5;124mb\u001b[39m\u001b[38;5;124m\"\u001b[39m\n\u001b[0;32m----> 4\u001b[0m \u001b[43mtakes\u001b[49m\u001b[43m(\u001b[49m\u001b[38;5;241;43m3\u001b[39;49m\u001b[43m)\u001b[49m\n",
            "Cell \u001b[0;32mIn[3], line 2\u001b[0m, in \u001b[0;36mtakes\u001b[0;34m(x)\u001b[0m\n\u001b[1;32m      1\u001b[0m \u001b[38;5;28;01mdef\u001b[39;00m \u001b[38;5;21mtakes\u001b[39m(x):\n\u001b[0;32m----> 2\u001b[0m     \u001b[38;5;28;01mreturn\u001b[39;00m \u001b[43mx\u001b[49m\u001b[43m \u001b[49m\u001b[38;5;241;43m+\u001b[39;49m\u001b[43m \u001b[49m\u001b[38;5;124;43m\"\u001b[39;49m\u001b[38;5;124;43mb\u001b[39;49m\u001b[38;5;124;43m\"\u001b[39;49m\n",
            "\u001b[0;31mTypeError\u001b[0m: unsupported operand type(s) for +: 'int' and 'str'"
          ],
          "output_type": "error"
        }
      ],
      "execution_count": 3
    },
    {
      "cell_type": "markdown",
      "source": "## c) Type the function and use `mypy` to catch the type error before execution",
      "metadata": {}
    },
    {
      "cell_type": "code",
      "source": "%%typecheck\ndef takes(x: str):\n    return x + \"b\"\n\ntakes(3)",
      "metadata": {
        "trusted": true
      },
      "outputs": [
        {
          "name": "stdout",
          "text": "<string>:5: error: Argument 1 to \"takes\" has incompatible type \"int\"; expected \"str\"  [arg-type]\nFound 1 error in 1 file (checked 1 source file)\n\n",
          "output_type": "stream"
        },
        {
          "ename": "<class 'TypeError'>",
          "evalue": "unsupported operand type(s) for +: 'int' and 'str'",
          "traceback": [
            "\u001b[0;31m---------------------------------------------------------------------------\u001b[0m",
            "\u001b[0;31mTypeError\u001b[0m                                 Traceback (most recent call last)",
            "Cell \u001b[0;32mIn[4], line 4\u001b[0m\n\u001b[1;32m      1\u001b[0m \u001b[38;5;28;01mdef\u001b[39;00m \u001b[38;5;21mtakes\u001b[39m(x: \u001b[38;5;28mstr\u001b[39m):\n\u001b[1;32m      2\u001b[0m     \u001b[38;5;28;01mreturn\u001b[39;00m x \u001b[38;5;241m+\u001b[39m \u001b[38;5;124m\"\u001b[39m\u001b[38;5;124mb\u001b[39m\u001b[38;5;124m\"\u001b[39m\n\u001b[0;32m----> 4\u001b[0m \u001b[43mtakes\u001b[49m\u001b[43m(\u001b[49m\u001b[38;5;241;43m3\u001b[39;49m\u001b[43m)\u001b[49m\n",
            "Cell \u001b[0;32mIn[4], line 2\u001b[0m, in \u001b[0;36mtakes\u001b[0;34m(x)\u001b[0m\n\u001b[1;32m      1\u001b[0m \u001b[38;5;28;01mdef\u001b[39;00m \u001b[38;5;21mtakes\u001b[39m(x: \u001b[38;5;28mstr\u001b[39m):\n\u001b[0;32m----> 2\u001b[0m     \u001b[38;5;28;01mreturn\u001b[39;00m \u001b[43mx\u001b[49m\u001b[43m \u001b[49m\u001b[38;5;241;43m+\u001b[39;49m\u001b[43m \u001b[49m\u001b[38;5;124;43m\"\u001b[39;49m\u001b[38;5;124;43mb\u001b[39;49m\u001b[38;5;124;43m\"\u001b[39;49m\n",
            "\u001b[0;31mTypeError\u001b[0m: unsupported operand type(s) for +: 'int' and 'str'"
          ],
          "output_type": "error"
        }
      ],
      "execution_count": 4
    },
    {
      "cell_type": "code",
      "source": "",
      "metadata": {
        "trusted": true
      },
      "outputs": [],
      "execution_count": null
    }
  ]
}