06-Debugging.ipynb

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   "source": [
    "**Course Announcements**\n",
    "\n",
    "- A1 due Monday (11:59 PM)\n",
    "- CL3 & A2 will be released Friday afternoon"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
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   "source": [
    "# Debugging\n",
    "\n",
    "- Errors\n",
    "    - Syntax\n",
    "    - Exceptions\n",
    "- `try`/`except`"
   ]
  },
  {
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   "metadata": {
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   "source": [
    "<div class=\"alert alert-success\">\n",
    "Debugging is the process of finding and fixing errors in a computer program.\n",
    "</div>"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
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   "source": [
    "## Errors"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
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   },
   "source": [
    "<div class=\"alert alert-success\">\n",
    "Errors are problems with code definition or execution that interrupt running Python code.\n",
    "</div>"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
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   "source": [
    "### Syntax Errors\n",
    "\n",
    "- Syntax Errors\n",
    "- Indentation Errors"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
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   "source": [
    "<div class=\"alert alert-success\">\n",
    "Syntax & Indentation Errors reflect code that doesn't follow Python structure, and will necessarily fail. \n",
    "</div>"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
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   },
   "source": [
    "### Syntax Error Examples"
   ]
  },
  {
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   "source": [
    "# will produce a syntax error\n",
    "if True\n",
    "    print('Yep.')"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
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   },
   "source": [
    "Python does its best to tell you:\n",
    "- what type of error it is\n",
    "- and where it _thinks_ it occurred (`^`)"
   ]
  },
  {
   "cell_type": "code",
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   "outputs": [],
   "source": [
    "# will produce a syntax error\n",
    "# and specifically an indentation error\n",
    "if 3 < 5:\n",
    "print(value)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "Python gives you a readout about what it was expecting and where you appear to have gone wrong."
   ]
  },
  {
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   "source": [
    "## Exceptions\n",
    "\n",
    "<div class=\"alert alert-success\">\n",
    "Exceptions are errors that occur when a code is executed.\n",
    "</div>"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
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   },
   "source": [
    "For these, there's nothing wrong with the _syntax_ or _structure_ of the code, but in your specific case and how you're trying to use it, Python says 'no'."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
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   },
   "source": [
    "### ZeroDivisionError\n",
    "\n",
    "ZeroDivisionError occurs when you try to divide by zero. "
   ]
  },
  {
   "cell_type": "code",
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   "metadata": {
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   "source": [
    "# produces ZeroDivisionError\n",
    "1 / 0"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
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     "slide_type": "fragment"
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   },
   "source": [
    "Python specifies:\n",
    "- the Exception and specific type / error\n",
    "- points you to where the error occurred"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
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   },
   "source": [
    "### NameError\n",
    "\n",
    "NameError occurs when you try to access a name that Python does not know."
   ]
  },
  {
   "cell_type": "code",
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   "source": [
    "# Define a variable\n",
    "variable = 12"
   ]
  },
  {
   "cell_type": "code",
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   "metadata": {
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   "outputs": [],
   "source": [
    "# If you typo a name, you will get a NameError\n",
    "varaible"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
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     "slide_type": "fragment"
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   },
   "source": [
    "While it's annoying, it's helpful that Python doesn't just _guess_ that you _meant_ 'variable'....because sometimes Python would guess wrong. It's better for Python to just give us the error."
   ]
  },
  {
   "cell_type": "code",
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   "source": [
    "# You also get a name error if you try to use the wrong operator for assignment\n",
    "new_variable == 1"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
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   "source": [
    "### IndexError\n",
    "\n",
    "IndexError occurs when you try to access an index that doesn't exist."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
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   "outputs": [],
   "source": [
    "my_string = 'COGS18'\n",
    "my_string[6]"
   ]
  },
  {
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   "source": [
    "# Relatedly, 'KeyError' occurs if you ask for a dictionary key that doesn't exist\n",
    "# we'll talk about dictionaries soon\n",
    "my_dictionary = {'name1' : 1, 'name2' : 2}\n",
    "my_dictionary['name3']"
   ]
  },
  {
   "cell_type": "markdown",
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   "source": [
    "### ValueError\n",
    "\n",
    "ValueError occurs when you try to use an illegal value for something."
   ]
  },
  {
   "cell_type": "code",
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   "outputs": [],
   "source": [
    "int('cat')"
   ]
  },
  {
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   },
   "source": [
    "### TypeError"
   ]
  },
  {
   "cell_type": "code",
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   "outputs": [],
   "source": [
    "'a_string' + 12"
   ]
  },
  {
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   "source": [
    "## Error Recap\n",
    "\n",
    "- Syntax Errors\n",
    "    - `SyntaxError`\n",
    "    - `IndentationError`\n",
    "- Exceptions\n",
    "    - `ZeroDivisionError` \n",
    "    - `NameError`\n",
    "    - Index Errors\n",
    "        - `IndexError`\n",
    "        - `KeyError`\n",
    "    - `ValueError`\n",
    "    - `TypeError`\n"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "#### Clicker Question #1\n",
    "\n",
    "What type of error will the following code produce?"
   ]
  },
  {
   "cell_type": "code",
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   "metadata": {
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   "source": [
    "if num > 0\n",
    "    print(\"Greater than 0\")"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
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     "slide_type": "fragment"
    }
   },
   "source": [
    "- A) Syntax \n",
    "- B) Name\n",
    "- C) Type\n",
    "- D) Index\n",
    "- E) Value"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "#### Clicker Question #2\n",
    "\n",
    "What type of error will the following code produce?"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
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   },
   "outputs": [],
   "source": [
    "if num > 0:\n",
    "    print(\"Greater than 0\")"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "- A) Syntax \n",
    "- B) Name\n",
    "- C) Type\n",
    "- D) Index\n",
    "- E) Value"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "## Stack Trace\n",
    "\n",
    "The trace (log) of what Python did as it went through your code. Gets printed out if Python runs into an error."
   ]
  },
  {
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   "source": [
    "running_sum = 0\n",
    "my_list = [1, 2, 3, 4, 5]\n",
    "\n",
    "for val in my_list:\n",
    "    \n",
    "    if val % 2 == 0:\n",
    "        temp = val / (val - 4)\n",
    "        #+= allows you to add the value on the right to the variable on the left \n",
    "        # and assign it to the variable on the left\n",
    "        running_sum += temp \n",
    "        # equivalent to:\n",
    "        # running_sum = running_sum + temp"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
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   },
   "source": [
    "Sometimes these get really complex. We're here to get better at interpreting these traces.\n",
    "\n",
    "Note that if external functions are being used, these will get longer."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
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   },
   "source": [
    "## Try / Except\n",
    "\n",
    "<div class=\"alert alert-success\">\n",
    "Exceptions do not necessarily have to lead to breaking the program - they can be programmatically dealt with, using <code>try</code> and <code>except</code>. \n",
    "</div>"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
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   },
   "source": [
    "### Try / Except Block"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
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   "outputs": [],
   "source": [
    "# Try / Except Block\n",
    "try:\n",
    "    # Tries to do this code\n",
    "    pass # pass just says is not an operation; carry on\n",
    "except:\n",
    "    # If there is an error (an exception), keep going and do this instead\n",
    "    pass"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "### Try / Except Example "
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
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   },
   "outputs": [],
   "source": [
    "# Example: we want to get an input number from the user\n",
    "\n",
    "my_num = input(\"Please type a number: \")\n",
    "\n",
    "print('\\nmy_num is: ', my_num)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "### Example with Try / Except"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
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   "outputs": [],
   "source": [
    "try:\n",
    "    int(input('Number'))\n",
    "except:\n",
    "    print(\"nahhh\")"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "## Raising Errors\n",
    "\n",
    "<div class=\"alert alert-success\">\n",
    "You can also write code to raise an Exception if something unexpected happens.\n",
    "</div>"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "### Raise Exception Examples\n",
    "\n",
    "`raise` is a keyword that tells Python you want to create your own error."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "scrolled": false,
    "slideshow": {
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   "outputs": [],
   "source": [
    "my_int = input('An integer please: ')\n",
    "if not my_int.isnumeric():\n",
    "    raise ValueError('I wanted a number! :(')\n",
    "    \n",
    "print('My integer is: ', my_int) "
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
    }
   },
   "source": [
    "#### Clicker Question #3\n",
    "\n",
    "Edit the code below (replacing `---` with either values or variable names) so that when executed, this cell returns `None`."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
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   "source": [
    "num1 = ---\n",
    "num2 = ---\n",
    "\n",
    "try:\n",
    "    output = num1 / num2\n",
    "except ZeroDivisionError:\n",
    "    output = None\n",
    "    \n",
    "print(output)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "- A) I did it!\n",
    "- B) I _think_ I did it...\n",
    "- C) I'm totally lost."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
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   },
   "source": [
    "## Helping yourself when debugging\n",
    "\n",
    "- You aren't sure how to approach a problem: add 'in python'\n",
    "- Start with some pseudocode; then add in real code\n",
    "- You don't understand an error message: \n",
    "    - Use `print()` statements\n",
    "    - Google the error message...but then you need to understand what you're reading"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "### Unsure how to approach a problem\n",
    "\n",
    "\"Write code that will check whether the value stored in `my_val` is both positive and even. If it is, store the integer `1` in the variable `output`\"\n",
    "\n",
    "...How do I check whether a variable stores an even value?\n",
    "\n",
    "Google: \"How to check whether a variable stores an even value _in python_\""
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "slideshow": {
     "slide_type": "-"
    }
   },
   "outputs": [],
   "source": []
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "### Don't understand the error message\n",
    "\n",
    "So you try to accomplish the task...below is your first attempt"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "slideshow": {
     "slide_type": "-"
    }
   },
   "outputs": [],
   "source": [
    "# this code has errors\n",
    "# we're going to debug together in class\n",
    "my_val = 6\n",
    "\n",
    "if my_val > 0 and my_val % 2 = 0:\n",
    "    output == 1"
   ]
  }
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