Python Errors and Built-in Exceptions

Python Errors

While writing code, we can make some mistakes that lead to errors when running the code.

A Python program is terminated as soon as it encounters an unhandled error. These errors can be classified into two types:

• Syntax Errors
• Logical Errors (Exceptions)

Python Syntax Errors

Syntax error(also called parsing error) is caused by not following the Python language's proper structure (syntax).

Let us see the following example:

if x > 0

Output

File "<interactive input>", line 1
       if x > 0  
                ^ 
SyntaxError: invalid syntax

As we can see above, an arrow indicates where the parser ran into the syntax error.

The error was caused by the missing of the colon : in the if statement.

Python Logical Errors (Exceptions)

Errors that happen at runtime (after passing the syntax check) are called exceptions or logical errors.

For example, they occur when we try to divide a number by zero (ZeroDivisionError), try to open a file(for reading) that does not exist (FileNotFoundError), or try to import a module that does not exist (ImportError).

Whenever a runtime error happens, Python creates an exception object. If not handled correctly, it prints a traceback to that error and some details about why it occurs.

Let us in the following example, how Python treats these errors:

>>> open("non_existing.txt")
FileNotFoundError Traceback (most recent call last)
----> 1 open("non_existing.txt")
FileNotFoundError: [Errno 2] No such file or directory: 'non_existing.txt'


>>> 5/0
ZeroDivisionError Traceback (most recent call last)
----> 1 5/0
ZeroDivisionError: division by zero

Python Built-in Exceptions

There are many built-in exceptions in Python that are raised when corresponding errors occur.

To view all the built-in exceptions, we can use the built-in function locals() as foolows:

print(dir(locals()['__builtins__']))

Output

['ArithmeticError', 'AssertionError', 'AttributeError', 'BaseException', 'BlockingIOError', 'BrokenPipeError', 'BufferError', 'BytesWarning', 'ChildProcessError', 'ConnectionAbortedError', 'ConnectionError', 'ConnectionRefusedError', 'ConnectionResetError', 'DeprecationWarning', 'EOFError', 'Ellipsis', 'EnvironmentError', 'Exception', 'False', 'FileExistsError', 'FileNotFoundError', 'FloatingPointError', 'FutureWarning', 'GeneratorExit', 'IOError', 'ImportError', 'ImportWarning', 'IndentationError', 'IndexError', 'InterruptedError', 'IsADirectoryError', 'KeyError', 'KeyboardInterrupt', 'LookupError', 'MemoryError', 'ModuleNotFoundError', 'NameError', 'None', 'NotADirectoryError', 'NotImplemented', 'NotImplementedError', 'OSError', 'OverflowError', 'PendingDeprecationWarning', 'PermissionError', 'ProcessLookupError', 'RecursionError', 'ReferenceError', 'ResourceWarning', 'RuntimeError', 'RuntimeWarning', 'StopAsyncIteration', 'StopIteration', 'SyntaxError', 'SyntaxWarning', 'SystemError', 'SystemExit', 'TabError', 'TimeoutError', 'True', 'TypeError', 'UnboundLocalError', 'UnicodeDecodeError', 'UnicodeEncodeError', 'UnicodeError', 'UnicodeTranslateError', 'UnicodeWarning', 'UserWarning', 'ValueError', 'Warning', 'ZeroDivisionError', '__IPYTHON__', '__build_class__', '__debug__', '__doc__', '__import__', '__loader__', '__name__', '__package__', '__spec__', 'abs', 'all', 'any', 'ascii', 'bin', 'bool', 'breakpoint', 'bytearray', 'bytes', 'callable', 'chr', 'classmethod', 'compile', 'complex', 'copyright', 'credits', 'delattr', 'dict', 'dir', 'display', 'divmod', 'dreload', 'enumerate', 'eval', 'exec', 'execfile', 'filter', 'float', 'format', 'frozenset', 'get_ipython', 'getattr', 'globals', 'hasattr', 'hash', 'help', 'hex', 'id', 'input', 'int', 'isinstance', 'issubclass', 'iter', 'len', 'license', 'list', 'locals', 'map', 'max', 'memoryview', 'min', 'next', 'object', 'oct', 'open', 'ord', 'pow', 'print', 'property', 'range', 'repr', 'reversed', 'round', 'runfile', 'set', 'setattr', 'slice', 'sorted', 'staticmethod', 'str', 'sum', 'super', 'tuple', 'type', 'vars', 'zip']

The locals()['__builtins__'] returns a module of built-in exceptions, attributes, and functions. The dir() function allows us to list these attributes as strings.

In the following table, some of the standard built-in exceptions in Python with the error that cause them.

We can also define our own exceptions in Python. You can visit Python Custom Exceptions to learn more about them.

To handle these built-in and user-defined exceptions in Python, we can use try, except, and finally statements. You can visit Python try, except and finally statements to learn more about them.