Python's Built-in Exceptions: A Walkthrough With Examples

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May 15, 2024
Python's Built-in Exceptions: A Walkthrough With Examples
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Python has a complete set of built-in exceptions that provide a quick and efficient way to handle errors and exceptional situations that may happen in your code. Knowing the most commonly used built-in exceptions is key for you as a Python developer. This knowledge will help you debug code because each exception has a specific meaning that can shed light on your debugging process.

You’ll also be able to handle and raise most of the built-in exceptions in your Python code, which is a great way to deal with errors and exceptional situations without having to create your own custom exceptions.

In this tutorial, you’ll:

  • Learn what errors and exceptions are in Python
  • Understand how Python organizes the built-in exceptions in a class hierarchy
  • Explore the most commonly used built-in exceptions
  • Learn how to handle and raise built-in exceptions in your code

To smoothly walk through this tutorial, you should be familiar with some core concepts in Python. These concepts include Python classes, class hierarchies, exceptions, try … except blocks, and the raise statement.

Errors and Exceptions in Python

Errors and exceptions are important concepts in programming, and you’ll probably spend a considerable amount of time dealing with them in your programming career. Errors are concrete conditions, such as syntax and logical errors, that make your code work incorrectly or even crash.

Often, you can fix errors by updating or modifying the code, installing a new version of a dependency, checking the code’s logic, and so on.

For example, say you need to make sure that a given string has a certain number of characters. In this case, you can use the built-in len() function:

Python 
>>> len("Pythonista") = 10
  File "<input>", line 1
    ...
SyntaxError: cannot assign to function call here.
    Maybe you meant '==' instead of '='?

In this example, you use the wrong operator. Instead of using the equality comparison operator, you use the assignment operator. This code raises a SyntaxError, which represents a syntax error as its name describes.

To fix the error, you need to localize the affected code and correct the syntax. This action will remove the error:

Python 
>>> len("Pythonista") == 10
True

Now the code works correctly, and the SyntaxError is gone. So, your code won’t break, and your program will continue its normal execution.

There’s something to learn from the above example. You can fix errors, but you can’t handle them. In other words, if you have a syntax error like the one in the example, then you won’t be able to handle that error and make the code run. You need to correct the syntax.

On the other hand, exceptions are events that interrupt the execution of a program. As their name suggests, exceptions occur in exceptional situations that should or shouldn’t happen. So, to prevent your program from crashing after an exception, you must handle the exception with the appropriate exception-handling mechanism.

To better understand exceptions, say that you have a Python expression like a + b. This expression will work if a and b are both strings or numbers:

Python 
>>> a = 4
>>> b = 3

>>> a + b
7

In this example, the code works correctly because a and b are both numbers. However, the expression raises an exception if a and b are of types that can’t be added together:

Python 
>>> a = "4"
>>> b = 3

>>> a + b
Traceback (most recent call last):
  File "<input>", line 1, in <module>
    a + b
    ~~^~~
TypeError: can only concatenate str (not "int") to str

Because a is a string and b is a number, your code fails with a TypeError exception. Since there is no way to add text and numbers, your code has faced an exceptional situation.

Python uses classes to represent exceptions and errors. These classes are generically known as exceptions, regardless of what a concrete class represents, an exception or an error. Exception classes give us information about an exceptional situation and also errors detected during the program’s execution.

The first example in this section shows a syntax error in action. The SyntaxError class represents an error but it’s implemented as a Python exception. This could be confusing, but Python uses exception classes for both errors and exceptions.

Read the full article at https://realpython.com/python-built-in-exceptions/ »

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May 15, 2024 at 07:30PM
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