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Added Exception stuff
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‎week6/w6.ipynb‎

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@@ -18,6 +18,190 @@
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"---"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"## Expect Everything\n",
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"Throughout the course, you may have run into an error message or two (or three or...). In Python, we typically refer to these as **Exceptions**.\n",
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"\n",
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"It would be really inconvenient if all programs, ever, crashed the first time they ran into a problem. Luckily, we can write code to deal with these exceptions, should they arise - this is **Exception Handling**. For example, look at this short script that takes an input, converts it to an `int` and doubles it:"
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]
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},
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{
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"cell_type": "code",
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"execution_count": null,
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"metadata": {},
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"outputs": [],
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"source": [
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"num = int(input(\"Enter a number: \"))\n",
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"num *= 2\n",
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"print(num)"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"Seems fine? But what if the user decides they *don't* want to enter a number, what if, e.g., they enter \"Hello\"? Python doesn't know how to convert a string to an integer, so it will throw a **ValueError** and exit the program.\n",
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"\n",
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"In reality, entering the wrong type of data isn't the end of the world. To stop Python from exiting immeadiately, we can surround any potentially problem-causing code in a `try-expect` block like this:"
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]
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},
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{
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"cell_type": "code",
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"execution_count": null,
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"metadata": {},
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"outputs": [],
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"source": [
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"try:\n",
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" num = int(input(\"Enter a number\"))\n",
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" num *= 2\n",
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" print(num)\n",
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"except ValueError:\n",
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" print(\"uh oh - that wasn't a number\")"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"The `try` statement tells the Python interpreter that something could go wrong in the following statements, but we know and have ways to deal with it. \n",
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"\n",
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"Note: we need `num *= 2` and `print(num)` in the `try` block because if the user inputs something wrong, `num` won't be defined. If the statements were *after* the try statement, this could cause an error as `num` wouldn't be defined.\n",
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"\n",
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"We can also catch multiple types of exceptions!"
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]
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},
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{
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"cell_type": "code",
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"execution_count": null,
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"metadata": {},
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"outputs": [],
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"source": [
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"try:\n",
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" num = int(input(\"Enter a number\"))\n",
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" result = 2 / num\n",
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" print(result)\n",
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"except ValueError:\n",
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" print(\"uh oh - that wasn't a number\")\n",
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"except ZeroDivisionError:\n",
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" print(\"Friends don't let friends divide by zero\")"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"All exceptions are a type of class, all inheriting from the **Exception** class. Also, certain exceptions are also subclasses of other exceptions, e.g. `ZeroDivisionError` is a subclass of `ArithmeticError`, here's a full hierarchy: \n",
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"\n",
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"***Image of Exception hierarchy goes here***"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"So what if we were to do this:"
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]
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},
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{
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"cell_type": "code",
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"execution_count": null,
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"metadata": {},
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"outputs": [],
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"source": [
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"try:\n",
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" num = int(input(\"Enter a number\"))\n",
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" result = 2 / num\n",
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" print(result)\n",
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"except Exception:\n",
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" print(\"Something went wrong :(\")\n",
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"except ValueError:\n",
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" print(\"uh oh - that wasn't a number\")\n",
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"except ZeroDivisionError:\n",
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" print(\"Friends don't let friends divide by zero\")"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"Try running the above block and input a string. Now input 0. We've caused two different errors, but the same statement is being executed. Why?\n",
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"\n",
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"Since all exceptions are a subclass of **Exception**, Python considers them to be of type *Exception*. Why does this matter? Python executes the *first relevant* catch statement - in this case it will *always* be an Exception, so the first statement will execute *regardless* of what type of exception we're throwing. You need to make sure that you put any catch statements for subclasses *above* those of their parent classes, otherwise the subclasses will never execute:"
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]
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},
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{
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"cell_type": "code",
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"execution_count": null,
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"metadata": {},
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"outputs": [],
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"source": [
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"try:\n",
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" num = int(input(\"Enter a number\"))\n",
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" result = 2 / num\n",
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" print(result)\n",
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"except ValueError:\n",
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" print(\"uh oh - that wasn't a number\")\n",
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"except ZeroDivisionError:\n",
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" print(\"Friends don't let friends divide by zero\")\n",
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"except Exception:\n",
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" print(\"Something went wrong :(\")"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"Sometimes we may not want to continue the program if we encounter an error, if we want to throw an exception, we use the `raise` keyword. For example:"
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]
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},
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{
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"cell_type": "code",
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"execution_count": null,
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"metadata": {},
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"outputs": [],
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"source": [
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"def division(num1: int, num2: int) -> float:\n",
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" if (num2 == 0):\n",
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" raise ZeroDivisionError(\"We've been through this - you can't divide by zero\")\n",
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" else:\n",
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" return num1 / num2\n",
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"\n",
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"inp_1 = input(\"Enter first number: \")\n",
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"inp_2 = input(\"Enter second number: \")\n",
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"try:\n",
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" inp_1 = int(inp_1)\n",
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" inp_2 = int(inp_2)\n",
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"except ValueError:\n",
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" print(\"uh oh - one of those numbers wasn't a number\")\n",
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"\n",
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"print(division(inp_1, inp_2))"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"We can also write our own custom exceptions. We define them the same way we define a class, making sure to say it inherits from **Exception**. Then, we can call it anywhere in our code by using `raise`:"
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]
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},
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{
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"cell_type": "code",
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"execution_count": null,
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"metadata": {},
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"outputs": [],
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"source": [
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"class MyException(Exception):\n",
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" def __init__(self, msg):\n",
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" super().__init__(msg)\n",
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"\n",
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"thing = input(\"Enter your favourite society: \")\n",
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"if thing != \"UWCS\":\n",
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" raise MyException(\"Ding dong, your opinion is wrong\")"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
@@ -1005,7 +1189,7 @@
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],
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"metadata": {
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"kernelspec": {
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"display_name": "Python 3.10.4 64-bit",
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"display_name": "Python 3.10.8 64-bit",
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"language": "python",
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"name": "python3"
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},
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"name": "python",
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"nbconvert_exporter": "python",
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"pygments_lexer": "ipython3",
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"version": "3.10.7"
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"version": "3.10.8"
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},
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"orig_nbformat": 4,
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"vscode": {
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"interpreter": {
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"hash": "268a003517095b3e00ec49a0832a87899bac50083c754616205934d33472dfcd"
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"hash": "bb33f6d328f18c07440802b8c66874c52744b86bff5cfe8eb1d71afeb55a2150"
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}
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}
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},

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