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Explore exceptions - a special type of error in Python!

Exceptions

Congratulations, you have now mastered many techniques for interacting with exceptions in Python! We learned:
* How exceptions differ from syntax errors
* How to read tracebacks
* How `try`/`except`/`else`/`finally` provides us with a powerful control flow for handling exceptions
* How to create and raise custom exceptions to provide more helpful errors to users of our code

These tools will get you very far as a Python developer! 

Review

The exception handlers from the previous exercise handled any exception hit during the `try` clause. However, in most cases, we will have an idea of the types of exceptions that might occur within our code. It is generally considered best practice to be as specific as possible with the exceptions we want to raise unless there is a specific reason for catching any type of exception. 

We can catch a specific exception by listing it after the `except` keyword, as in the example below:
```py
try:
    print(undefined_var)
except NameError:
    print('We hit a NameError')
```

In this case, the `except` block is only executed if a `NameError` is encountered (in the `try` block) rather than any exception. If any other exception occurs, it is unhandled, and the program terminates. 

When we specify exception types, Python also allows us to capture the exception object using the `as` keyword. The exception object hosts information about the specific error that occurred. Examine our previous function but now capturing the exception object as `errorObject`: 

```py
try:
    print(undefined_var)
except NameError as errorObject:
    print('We hit a NameError')
    print(errorObject)
```

Would output: 
```bash
We hit a NameError
name 'undefined_var' is not defined
```

Its worth noting `errorObject` is an arbitrary name and can be replaced with any name we see fit. The following code would work exactly the same:
```py
try:
    print(undefined_var)
except NameError as e:
    print('We hit a NameError')
    print(e)
```

Let’s get some practice capturing specific exceptions.

Catching Specific Exceptions

So far, the exceptions we’ve encountered have caused our programs to stop executing. However, it is possible for programs to continue executing even after encountering an exception. This process is known as _exception handling_ and is accomplished using the Python `try`/`except` clauses. 

The following flow chart demonstrates the mechanics of `try`/`except`:

![Try/Except](https://static-assets.codecademy.com/Courses/Intermediate-Python/Flow%20diagram%202.svg)

Let's break it down: 
* Python will first attempt to execute code inside the `try` clause code block. 
* If no exception is encountered in the code, the `except` clause is skipped and the program continues normally. 
* If an exception does occur inside of the `try` code block, Python will immediately stop executing the code and begin executing the code inside the `except` code block (sometimes called a _handler_). 

Let's see this in action in a small program that prints colors:

```py
colors = {
    'red': '#FF0000',
    'blue': '#0000FF',
    'yellow': '#FFFF00',
}

for color in ('red', 'green', 'yellow'):
  try:
    print('The hex value of ' + color + ' is ' + colors[color])
  except:
    print('An exception occurred! Color does not exist.')
  print('Loop continues...')
```

We get the following output: 
```
The hex value of red is #FF0000
Loop continues...
An exception occurred! Color does not exist.
Loop continues...
The hex value of yellow is #FFFF00
Loop continues...
```

In the above code, the `try` block runs until it hit an exception. The hex value of the color red was successfully printed before it tried to access the hex value of green, which caused a `KeyError` since green is not in our `colors` dictionary and ran the code in the `except` block. However, the exception was handled so Python continued executing our code and went onto print the hex value of yellow. 

Exception handling is a powerful tool that lets us gain more flexibility in dealing with errors in our applications. We can use it to perform an action multiple times until it succeeds, or perhaps simply print a message when a non-critical part of our program doesn’t work properly. 

Let's try performing some exception handling!

Try / Except

While handling a single exception is useful, Python also gives us the ability to handle multiple exceptions at once. We can list more than one exception type in a tuple with a single `except` clause. Here is what the syntax would look like: 
```py
try:
    # Some code to try!
except (NameError, ZeroDivisionError) as e:
    print('We hit an Exception!')
    print(e)
```

In the above example, we expect to encounter either a `NameError` or a `ZeroDivisionError`. We can list any number of exceptions in this tuple format as long as it makes sense for the code in our `try` block. This is where we can see the benefit of capturing our exception object (via the `as` clause) since it enables us to print (or operate on) the specific exception that is caught.

In addition to catching multiple exceptions, we can also pair multiple `except` clauses with a single `try` clause, enabling specific exceptions to be handled differently. For example: 

```py
try:
    # Some code to try!
except NameError:
    print('We hit a NameError Exception!')
except KeyError:
    print('We hit a TypeError Exception!')
except Exception:
    print('We hit an exception that is not a NameError or TypeError!')
```

In the above program, a `NameError` or `KeyError` will trigger one of the first two exception handlers. Any other exception will trigger the third handler. Note that the order of handlers is important here - if an exception is encountered, Python will execute the first one that matches its type. In this case, and a valid strategy for exception handling, we use the last `except` clause as a generic `Exception` as a backup if no other specific exception gets caught. 

Let's now practice handling multiple exceptions!

Handling Multiple Exceptions

In the previous exercise (and probably many times before), we saw one type of exception called the `NameError`. The `NameError` is just one of the many _built-in exceptions_ &mdash; exceptions that are built into the Python language. Other built-in exceptions cover fields ranging from mathematical errors all the way to operating system errors. We don’t need to memorize them all, but it’s helpful to be familiar with some common ones and, more importantly, understand where they come from inside Python.

Exceptions are objects just like anything else. Most exceptions inherit directly from a class called `Exception`; however, they all are derived directly or indirectly from the `BaseException` class. We can examine the base classes by using the `__bases__` attribute on any specific exception: 

```py
print(NameError.__bases__)
```

Will output:
```bash
<class 'Exception'>
```

We can even call `__bases__` on the `Exception` class to see its origins: 
```py
print(Exception.__bases__)
```

Will output:

```bash
<class 'BaseException'>
```

The full hierarchy of built-in exceptions is the following: 
```
BaseException
 +-- Exception
      +-- StopIteration
      +-- StopAsyncIteration
      +-- ArithmeticError
      |    +-- FloatingPointError
      |    +-- OverflowError
      |    +-- ZeroDivisionError
      +-- AssertionError
      +-- AttributeError
      +-- BufferError
      +-- EOFError
      +-- ImportError
      |    +-- ModuleNotFoundError
      +-- LookupError
      |    +-- IndexError
      |    +-- KeyError
      +-- MemoryError
      +-- NameError
      |    +-- UnboundLocalError
      +-- OSError
      |    +-- BlockingIOError
      |    +-- ChildProcessError
      |    +-- ConnectionError
      |    |    +-- BrokenPipeError
      |    |    +-- ConnectionAbortedError
      |    |    +-- ConnectionRefusedError
      |    |    +-- ConnectionResetError
      |    +-- FileExistsError
      |    +-- FileNotFoundError
      |    +-- InterruptedError
      |    +-- IsADirectoryError
      |    +-- NotADirectoryError
      |    +-- PermissionError
      |    +-- ProcessLookupError
      |    +-- TimeoutError
      +-- ReferenceError
      +-- RuntimeError
      |    +-- NotImplementedError
      |    +-- RecursionError
      +-- SyntaxError
      |    +-- IndentationError
      |         +-- TabError
      +-- SystemError
      +-- TypeError
      +-- ValueError
      |    +-- UnicodeError
      |         +-- UnicodeDecodeError
      |         +-- UnicodeEncodeError
      |         +-- UnicodeTranslateError
```

Note that there are ***a lot*** of exceptions built into the language of Python. Again, we don't need to memorize all of them, but at some point, we may see them pop up in our programs. We can find details on each of the exceptions listed above in the [Python documentation](https://docs.python.org/3/library/exceptions.html#built-in-exceptions).

Later in this lesson, we’ll be using the `Exception` base class to create custom exceptions. For now, let’s get some practice encountering built-in exceptions and reading their tracebacks.

Built-in Exceptions

Encountering exceptions isn’t always an accident. We can throw an exception at any time by using the `raise` keyword, even when Python would not normally throw it.

We might want to raise an exception anytime we think a mistake has or will occur in our program. This lets us stop program execution immediately and provide a useful error message instead of allowing mistakes to occur that may be difficult to diagnose at a later point.  

One way to use the `raise` keyword is by pairing it with a specific exception class name. We can either call the class by itself or call a constructor and provide a specific error message. So for example we could do: 
```py
raise NameError
# or 
raise NameError('Custom Message')
```

When only the class name is provided (as in the first example), Python calls the constructor method for us without any arguments (and thus no custom message will come up).

For a more concrete example, let's examine raising a `TypeError` for a function that checks if an employee tries to open the cash register but does not have the correct access:

```py
def open_register(employee_status):
  if employee_status == 'Authorized':
    print('Successfully opened cash register')
  else:
    # Alternatives: raise TypeError() or TypeError('Message')
    raise TypeError
```

When an employee_status is not `'Authorized'`, the function `open_register()` will throw a `TypeError` and stop program execution.

Alternatively, when no built-in exception makes sense for the type of error our program might experience, it might be better to use a generic exception with a specific message. This is where we can use the base `Exception` class and provide a single argument that serves as the error message. Let’s modify the previous example, this time raising an `Exception` object with a message:

```py
def open_register(employee_status):
  if employee_status == 'Authorized':
    print('Successfully opened cash register')
  else:
    raise Exception('Employee does not have access!')
```

As a general rule of thumb, use an exception that provides the best explanation for the expected error for both the user and anyone that will read the code. 

Later in this lesson, we’ll explore how to customize our exceptions further by creating user-defined exceptions. For now, let's practice using the `raise` keyword. 

Raising Exceptions

We’ve seen how exception handlers get executed when we encounter exceptions during a `try` clause - but what if we want to run some code only if we do not encounter an exception? Python provides us a way to do this as well - the `else` clause. 

Our updated flow chart shows what happens when an `else` clause is added to the mix:

![Try/Except/Else](https://static-assets.codecademy.com/Courses/Intermediate-Python/Flow%20diagram%203.svg)

Python will only execute the `else` clause if no exception was encountered in the `try` clause. 

Let's examine a hypothetical program that authenticates a user. For now, we will use two imaginary functions `check_password()` and `login_user()`. Here is what the program looks like: 

```py
try:
  check_password()
except ValueError:
  print('Wrong Password! Try again!')
else:
  login_user()
  # 20 other lines of imaginary code
```

In this program, we can assume if our function `check_password()` fails, it will return a `ValueError`. Thankfully, our exception handler takes care of this scenario. However, if our function doesn't fail, the `else` clause allows us to log the user in!

Now, one could argue, we could have written our program a different way to achieve a similar outcome: 

```py
try:
  check_password()
  login_user()
  # 20 other lines of imaginary code
except ValueError:
  print('Wrong Password! Try again!')
```

Here, if our `check_password() ` ever fails, we will be able to catch the exception just like before. Python does offer a bit of insight on this scenario in the official documentation: 

> The use of the else clause is better than adding additional code to the try clause because it avoids accidentally catching an exception that wasn’t raised by the code being protected by the try … except statement.

This suggestion is valid in this case since in the alternative style, the `ValueError` could occur in any of the other lines of code other than `check_password()`, and it would be challenging to tell where it came from.

Let's give the `else` clause a try! 

The else Clause

With `try`/`except`/`else`, we’ve seen how to run certain code when an exception occurs and other code when it does not. There is also a way to execute code regardless of whether an exception occurs - the `finally` clause. 

Here is our final flow chart demonstrating `try`/`except`/`else`/`finally`:

![Try/Except/Else/Finally](https://static-assets.codecademy.com/Courses/Intermediate-Python/Flow%20diagram.svg)

Let’s return to our fictional login program from earlier and examine a use case for the `finally` clause: 
```py
try:
  check_password()
except ValueError:
  print('Wrong Password! Try again!')
else:
  login_user()
  # 20 other lines of imaginary code
finally:
  load_footer()
```

In the above program, most of our code stayed the same. The one change we made was we added the `finally` clause to execute no matter if the user fails to login or not. In either case, we use an imaginary function called `load_footer()` to load the page's footer. Since the footer area of our imaginary application stays the same for both states, we always want to load it, and thus call it inside of the `finally` clause. 

Note that the `finally` clause can be used independently (without an `except` or `else` clause). This is a convenient way to guarantee that a behavior will occur, regardless of whether an exception occurs:

```py
try:
    check_password()
finally:
    load_footer()
    # Other code we always want to run 
```

Let’s put the `finally` clause into practice for our Instrument World application!

The finally Clause

We’ve just seen how defining a simple exception class can provide a more specific and useful error to users. Defining a simple class is just the first step to creating better exceptions in our programs. Python does not stop us from customizing our custom exception classes even further. 

Let's say we wanted to expand our `LocationTooFarError` exception from earlier to also provide a custom error message. Here is what the custom class might look like:

```py
class LocationTooFarError(Exception):
   def __init__(self, distance):
       self.distance = distance
       
   def __str__(self):
        return 'Location is not within 10 km: ' + str(self.distance)
```

Let's break this down: 

- Our class definition doesn't look much different from before. We have a class named `LocationTooFarError` that still inherits from the built-in `Exception` class.
- We have added a constructor that is going to take in a distance argument when we instantiate our exception class. Here, we have overridden the constructor of the `Exception` class to accept our own custom argument of `distance`. The reason for taking in a distance is to use it in our `__str__` method that will return a custom error message when the exception is hit! 
- The `__str__` method provides our exception a custom message by returning a string with the distance property from the constructor. 

If we now ran it using our script from earlier: 
```py
def schedule_delivery(distance_from_store):
    if distance_from_store > 10:
        raise LocationTooFarError(distance_from_store)
    else:
        print('Scheduling the delivery...')
```

We would see our expanded custom exception in action: 

```error
Traceback (most recent call last):
  File "inventory.py", line 14, in <module>
    schedule_delivery(20)
  File "inventory.py", line 11, in schedule_delivery
    raise LocationTooFarError(distance_from_store)
__main__.LocationTooFarError: Location is not within 10 km: 20
```

Let's practice customizing our exceptions even further! 

Customizing User-defined Exceptions

It is truly an amazing feeling when our code works exactly the way we want it to. On the other hand, it can be equally frustrating when our code runs into errors. Since errors are such an integral part of working with Python, it's important to know how to control errors and use them to our advantage effectively. In this lesson, we will explore a specific type of error, an _exception_. 

At this point, we are probably very familiar with the most common type of error:  a _syntax error_. Syntax errors are mistakes in the structure of Python code. They are caught during a special parsing stage before a program is executed. They always prevent the entire program from running. For example, here is the error output of a syntax error:

```error
  File "script.py", line 1
    def print_five
                 ^
SyntaxError: invalid syntax
```

As opposed to a syntax error, an exception is a different kind of error that can occur with syntactically correct code. Exceptions are _runtime errors_ because they occur during program execution, only when the offending code (the code causing the error) is reached. An example of an exception, and one we have probably seen before, is a `NameError`: 

```error
Traceback (most recent call last):
  File "script.py", line 1, in <module>
    print(five)
NameError: name 'five' is not defined
```

Although the `NameError` has a similar output to a `SyntaxError` (both end with `Error`), it falls under the category of exceptions. Exceptions and syntax errors make up the two core categories for any error we will run into. 

![Python Error Categories](https://static-assets.codecademy.com/Courses/Intermediate-Python/Types%20of%20Errors.svg)


We’ll encounter many different kinds of exceptions, some of which will be unfamiliar. Luckily, as we saw in the example above, Python gives us a tool for gaining insight into exceptions - the _traceback_. A traceback is a summary that includes the exception type, a message, and the series of function calls preceding the exception, along with file names and line numbers. Here is another example of a traceback for a small program: 

```py
# Imaginary file script.py
print(1/0)
```

Would output:
```error
Traceback (most recent call last):
  File "script.py", line 1, in <module>
    print(1/0)
ZeroDivisionError: division by zero
```

In the traceback above, reading from the bottom line, we see the exception type (`ZeroDivisionError`) followed by a message (`division by zero`). Going up, we see that the exception originated on line 1 of a file called `script.py` while calling `print(1/0)`. We'll be using tracebacks throughout the rest of the lesson to track and identify why and where our exceptions are occurring. 

Let’s get some practice debugging syntax errors and exceptions. For this lesson, let's imagine we are hired by Instrument World, a musical instrument company with retail and online stores.

Introduction to Exceptions

So far we have seen how to raise and manage built-in exceptions. In most programs, using built-in exceptions won't always be the most detailed way to describe an error occurring. What if we could create custom exceptions that are more specific to a program or module? Well, Python gives us the ability to create _user-defined exceptions_. 

User-defined exceptions are exceptions that we create to allow for better readability in our program's errors. The core syntax looks like this: 

```py
class CustomError(Exception):
    pass
```

All we have to do to create a custom exception is to derive a subclass from the built-in `Exception` class. Although not required, most custom exceptions end in "Error" similar to the naming of the built-in exceptions. We'll learn how to customize these exceptions in the next exercise, but for now, let's see how a simple custom exception helps us better document our errors. 

Let’s imagine that Instrument World has an optional delivery service for instruments. If someone tries to schedule a delivery but their address is too far, we want to raise a custom `LocationTooFarError` exception. This isn't a type of exception that is built into Python, but rather one that is specific to our program and use case. Here is what our program might look like utilizing this custom exception: 

```py
class LocationTooFarError(Exception):
   pass

def schedule_delivery(distance_from_store):
    if distance_from_store > 10:
        raise LocationTooFarError
    else:
        print('Scheduling the delivery...')
```

Here, we have a class called `LocationTooFarError` that inherits from the `Exception` class. By doing so, we are telling Python that we would like to be able to use the class as our own custom exception. 

Now, if we call `schedule_delivery(20)`, we get the following output:
```error
Traceback (most recent call last):
  File "inventory.py", line 10, in <module>
    schedule_delivery(20)
  File "inventory.py", line 6, in schedule_delivery
    raise LocationTooFarError
__main__.LocationTooFarError
```

Since our class name populates into the traceback, even this simple class proves to be more useful than a generic `Exception` object or any built-in types! Users and developers alike will appreciate having specific exception details to work with.

Let's practice creating our own simple custom exceptions! 