PyCon Korea 2017 - Manoj Pandey (Attack of Pythons: Gotchas and Landmines in Python)

gotchas.ipynb

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   "source": [
    "<img src=\"https://s3.amazonaws.com/manojp-personal/intro.png\">"
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   "source": [
    "<img src=\"https://s3.amazonaws.com/manojp-personal/about.png\">"
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  {
   "cell_type": "markdown",
   "metadata": {
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   },
   "source": [
    "// * Python 2"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "# List repetition with nested lists\n"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {
    "slideshow": {
     "slide_type": "-"
    }
   },
   "outputs": [],
   "source": [
    "my_list = [[0]*5]*4\n",
    "for i in range(3):\n",
    "    my_list[2][i] = 1"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "[[1, 1, 1, 0, 0], [1, 1, 1, 0, 0], [1, 1, 1, 0, 0], [1, 1, 1, 0, 0]]\n"
     ]
    }
   ],
   "source": [
    "print my_list"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {
    "collapsed": true,
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [],
   "source": [
    "new_list = [[0]*5 for x in range(4)]"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "[[0, 0, 0, 0, 0], [0, 0, 0, 0, 0], [1, 1, 1, 0, 0], [0, 0, 0, 0, 0]]"
      ]
     },
     "execution_count": 4,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "for i in range(3):\n",
    "    new_list[2][i] = 1\n",
    "new_list"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "# Don't mix spaces and tabs\n",
    "\n",
    "> Just don't. You would cry.\n"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "where, Tab(→) = 4 spaces\n",
    "\n",
    "    →  Some code here(firstArgument,\n",
    "    →   →   →   →  ···secondOne,\n",
    "    →   →   →  ·······modified);"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "where, Tab(→)= 2 spaces\n",
    "\n",
    "    → Some code here(firstArgument,\n",
    "    → → → → ···secondOne,\n",
    "    → → → ·······modified);"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "# Explicit type cast of strings"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [],
   "source": [
    "float('i-am-a-string')"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [],
   "source": [
    "float('inf')"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [],
   "source": [
    "float('inf') > 6"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "# Same hash in a dict"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [],
   "source": [
    "a={}\n",
    "a[1.0]= 'quora'\n",
    "a[True] = 'twitter'\n",
    "a[1]= 'facebook'"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [],
   "source": [
    "print a[True]"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [],
   "source": [
    "hash(1) == hash(1.0)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "# Integers in Python"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": true,
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [],
   "source": [
    "a = 25\n",
    "b = 25"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [],
   "source": [
    "print id(a)\n",
    "print id(b)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "a = 300\n",
    "b = 300"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [],
   "source": [
    "print id(a)\n",
    "print id(b)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": []
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "# Local Variable Optimization\n",
    "\n",
    "> This flaw bites people fairly often. Consider the following function; what do you think happens when you run it?:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
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   },
   "outputs": [],
   "source": [
    "i=1\n",
    "def f():\n",
    "    #global i\n",
    "    print \"i=\",i\n",
    "    i = i + 1 \n",
    "\n",
    "f()"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "# Trailing spaces"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [],
   "source": [
    "for i in range(10): print \"*\","
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [],
   "source": [
    "import sys\n",
    "for i in range(10): sys.stdout.write(\"*\")"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "# Catching Multiple Exceptions"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
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   },
   "outputs": [],
   "source": [
    "try:\n",
    "    #something that raises an error...\n",
    "except IndexError, ValueError:\n",
    "    # expects to catch IndexError and ValueError\n",
    "    # wrong!"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
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   "outputs": [],
   "source": [
    "# correct\n",
    "try:\n",
    "    1/0\n",
    "except (ZeroDivisionError, IndexError, ValueError), e:\n",
    "    print e\n",
    "    # integer division or modulo by zero"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "# The += operator\n"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "```python\n",
    "x += 42\n",
    "\n",
    "x = x + 42\n",
    "```"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "```python\n",
    "a = 1\n",
    "a = a + 42\n",
    "# a is 43\n",
    "a = 1\n",
    "a += 42\n",
    "# a is 43\n",
    "```"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "outputs": [],
   "source": [
    "z = [1, 2, 3]"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [],
   "source": [
    "z += [4]\n",
    "id(z)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [],
   "source": [
    "z = z + [5]\n",
    "id(z)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "# Class attributes vs instance attributes"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "```python\n",
    ">>> class Foo:\n",
    "...     a = 42\n",
    "...     def __init__(self):\n",
    "...         self.a = 43\n",
    "...     \n",
    ">>> f = Foo()\n",
    ">>> f.a\n",
    "43\n",
    "```"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "```python\n",
    ">>> class Foo:\n",
    "...     a = 42\n",
    "...     \n",
    ">>> f = Foo()\n",
    ">>> f.a\n",
    "42\n",
    "```"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "outputs": [],
   "source": [
    "class Foo:\n",
    "    bar = []\n",
    "    def __init__(self, x):\n",
    "        self.bar += [x]\n",
    "#         self.bar = self.bar + [x]"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 6,
   "metadata": {
    "collapsed": true,
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [],
   "source": [
    "f = Foo(42)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 7,
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "[42]"
      ]
     },
     "execution_count": 7,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "f.bar"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 8,
   "metadata": {
    "collapsed": true,
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [],
   "source": [
    "g = Foo(100)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 9,
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "[42, 100]"
      ]
     },
     "execution_count": 9,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "g.bar"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "# List slicing\n",
    "\n",
    "```python\n",
    ">>> x = [10, 20, 30, 40, 50]\n",
    "```"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 11,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "x = [10, 20, 30, 40, 50]"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 12,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "[]"
      ]
     },
     "execution_count": 12,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "x[6:]"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "# Mutable default arguments"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "```python\n",
    ">>> def myfunc(x=[]):\n",
    "...     x.append(222)\n",
    "...     print x\n",
    "\n",
    ">>> myfunc([1, 2, 3])\n",
    "[1, 2, 3, 222]\n",
    "\n",
    ">>> x = [1, 2]\n",
    "\n",
    ">>> myfunc(x)\n",
    "[1, 2, 222]\n",
    "\n",
    ">>> x\n",
    "[1, 2, 222]\n",
    "```"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "### This was expected. But now .. "
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "```python\n",
    ">>> myfunc()\n",
    "[222]\n",
    "\n",
    ">>> myfunc()\n",
    "[222, 222]\n",
    "\n",
    ">>> myfunc()\n",
    "[222, 222, 222]\n",
    "```"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "# String concatenation"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "To illustrate this, a simple benchmark. (timeit is a simple function that runs another function and returns how long it took to complete, in seconds.)\n",
    "\n",
    "```python\n",
    ">>> def f():\n",
    "...     s = \"\"\n",
    "...     for i in range(100000):\n",
    "...         s = s + \"abcdefg\"[i % 7]\n",
    "...     \n",
    ">>> timeit(f)\n",
    "23.7819999456\n",
    "\n",
    ">>> def g():\n",
    "...     z = []\n",
    "...     for i in range(100000):\n",
    "...         z.append(\"abcdefg\"[i % 7])\n",
    "...     return ''.join(z)\n",
    "...     \n",
    ">>> timeit(g)\n",
    "0.343000054359\n",
    "```"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "# QUICK QUESTION\n",
    "\n",
    "```python\n",
    ">>> t = (1,2,[1,2,3],)\n",
    ">>> t[2] += [4, 5]\n",
    "```"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "t = (1,2,[1,2,3],)\n",
    "t[2] += [4,5]"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "t"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "# [Tip] Ask for forgiveness than permission"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "Don't :\n",
    "```python\n",
    "if os.path.isfile(file_path) :\n",
    "    file = open(file_path)\n",
    "else :\n",
    "    # do something\n",
    "    ```"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "Do :\n",
    "```python\n",
    "try :\n",
    "    file =  open(file_path)\n",
    "except OSError as e:\n",
    "    # do something\n",
    "```"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "## Resources\n",
    "- Reasons for the gotchas above: https://gist.github.com/manojpandey/41b90cba1fd62095e247d1b2448ef85b\n",
    "- 'A \"Best of the Best Practices\" (BOBP) guide to developing in Python.:'https://gist.github.com/sloria/7001839\n",
    "- Brief explanation for the 'same hash in a dict' gotcha: https://dbader.org/blog/python-mystery-dict-expression\n",
    "- Tuple mystery: https://stackoverflow.com/a/29747287/2124480\n",
    "- https://docs.python.org/2/whatsnew/2.6.html#other-language-changes\n",
    "- https://waymoot.org/home/python_string/"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "# QUESTIONS ? O.o"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "collapsed": true
   },
   "source": [
    "## Twitter: @onlyrealmvp"
   ]
  }
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