Python Exceptions : How to handle failures

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Python exceptions : unexpected event that occurs during program execution. For example,

divide_by_zero = 7 / 0

The above code causes an exception as it is not possible to divide a number by 0.

Let’s learn about Python Exceptions in detail.

Python Exceptions : Logical Errors

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

Whenever these types of runtime errors occur, Python creates an exception object.

If not handled properly, it prints a traceback to that error along with some details about why that error occurred.

Let’s look at how Python treats these errors:

divide_numbers = 7 / 0
prit(divide_numbers)

Output

Traceback (most recent call last):
 File "<string>", line 1, in <module>
ZeroDivisionError: division by zero

Here, while trying to divide 7 / 0, the program throws a system exception ZeroDivisionError

Python Built-in Exceptions

Illegal operations can raise exceptions. In Python, there are plenty of built-in exceptions thrown when corresponding errors occur.

We can view all the built-in exceptions using the built-in local() function as follows:

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

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

Some of the common built-in exceptions in Python programming along with the error that cause them:

Exception	Cause of Error
AssertionError	Raised when an assert statement fails.
AttributeError	Raised when attribute assignment or reference fails.
EOFError	Raised when the input() function hits end-of-file condition.
FloatingPointError	Raised when a floating point operation fails.
GeneratorExit	Raise when a generator’s close() method is called.
ImportError	Raised when the imported module is not found.
IndexError	Raised when the index of a sequence is out of range.
KeyError	Raised when a key is not found in a dictionary.
KeyboardInterrupt	Raised when the user hits the interrupt key (Ctrl+C or Delete).
MemoryError	Raised when an operation runs out of memory.
NameError	Raised when a variable is not found in local or global scope.
NotImplementedError	Raised by abstract methods.
OSError	Raised when system operation causes system related error.
OverflowError	Raised when the result of an arithmetic operation is too large to be represented.
ReferenceError	Raised when a weak reference proxy is used to access a garbage collected referent.
RuntimeError	Raised when an error does not fall under any other category.
StopIteration	Raised by next() function to indicate that there is no further item to be returned by iterator.
SyntaxError	Raised by parser when syntax error is encountered.
IndentationError	Raised when there is incorrect indentation.
TabError	Raised when indentation consists of inconsistent tabs and spaces.
SystemError	Raised when interpreter detects internal error.
SystemExit	Raised by sys.exit() function.
TypeError	Raised when a function or operation is applied to an object of incorrect type.
UnboundLocalError	Raised when a reference is made to a local variable in a function or method, but no value has been bound to that variable.
UnicodeError	Raised when a Unicode-related encoding or decoding error occurs.
UnicodeEncodeError	Raised when a Unicode-related error occurs during encoding.
UnicodeDecodeError	Raised when a Unicode-related error occurs during decoding.
UnicodeTranslateError	Raised when a Unicode-related error occurs during translating.
ValueError	Raised when a function gets an argument of correct type but improper value.
ZeroDivisionError	Raised when the second operand of division or modulo operation is zero.

If required, we can also define our own exceptions in Python. To learn more about them, visit Python User-defined Exceptions.

We can handle these built-in and user-defined exceptions in Python using try, except and finally statements. To learn more about them, visit Python try, except and finally statements.

Python Error and Exception

Errors represent conditions such as compilation error, syntax error, error in the logical part of the code, library incompatibility, infinite recursion, etc.

Errors are usually beyond the control of the programmer and we should not try to handle errors.

Exceptions can be caught and handled by the program.

Now we know about exceptions, we will learn about handling exceptions in the next tutorial.