Learn

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 - 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:

print(NameError.__bases__)

Will output:

<class 'Exception'>

We can even call __bases__ on the Exception class to see its origins:

print(Exception.__bases__)

Will output:

<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 is 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.

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.

Instructions

1.

Instrument World has a program that prints some of the most popular instruments it has on sale.

Take some time to look over the program, and then run the code in instruments.py. We’ll encounter another exception that we might not have seen before. What might this exception be telling us?

2.

Take a look over traceback - it ends with the exception type and a brief message. Above that is the exact line that caused the exception.

Fix this line so that the exception no longer occurs and then re-run the code.

3.

Looks like the exception we saw in the previous step was a TypeError. Let’s confirm which base class it is derived from.

Use print() to output a TypeError.__bases__.

4.

There is another exception that gets hit - once again, read the traceback, fix the exception, and re-run the code.

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