Django Exceptions

What is an Exception in Django?

Exception is an unusual exceptional event(describing an error), which occurs only during the runtime of a program.

During the execution of a program, when an exception occurs, it halts the normal execution of the program and it ends abruptly. That's why we should handle an exception when it is thrown by the program. The process of handling an exception is called Exception Handling.

Some situations due to which an Exception may arise :

When an integer number is divided by zero, it raises an exception.
When we try to access an array with an out-of-range value, an exception is thrown.
When we are trying to read a file which doesn't exist.
When we are trying to access a value in a database, which doesn't exist.

What happens when an Exception is thrown?

Exceptions are thrown at the runtime of a program. If an exception, which is thrown during the runtime of a program, is not handled, the program terminates right away without executing the remaining statements. Let us understand this by an example here.

Looking at the diagram above, it is easy to understand that if an exception is thrown when statement2 is executed, the program ends abruptly without executing any statement after it. Hence, statement3 and statement4 will not be executed as a result of an exception at statement2.

Some important types of Exceptions -:

Django raises and supports its own exceptions classes and also supports all the core python exceptions classes. First, let us see the Django core exception classes which are defined in django.core.exceptions, as shown in the table below.

Django Core Exception Classes	Description
AppRegistryNotReady	This exception is raised when attempting to use models before the app loading process.
ObjectDoesNotExist	The base class for DoesNotExist exceptions and it will catch all kinds of DoesNotExist exceptions.
EmptyResultSet	This exception may be raised during the query generation i.e.when a query doesn't result any result.
FieldDoesNotExist	This exception is raised by the model when the requested field does not exist on the model or on the model's parents.
MultipleObjectsReturned	This exception is raised by a query, when one object was expected but multiple objects were returned.
Suspicious Operation	This exception is raised when a user has performed an suspicious operation from a security point of view, such as tampering with a session cookie.
PermissionDenied	This exception is raised when a user doesn not have a permission to perform a certain action.
ViewDoesNotExist	This type of exception is raised by django.urls when a requested view does not exist.
MiddlewareNotUsed	This type of exception is raised when a middleware is not used in a server configuration.
ImproperlyConfigured	This type of exception is raised when Django is improperly configured, for example - when a value in settings.py is incorrect.
FieldError	This type of exception is raised when there is a problem with a model's field, which could happen due to one of the following reasons: A field name is invalid. A field cannot be determined from a keyword in the query. A field in the model is clashing with the same field name in its abstract base class.
ValidationError	This type of exception is when data fails form or model field validation.

Django URL Resolver Exceptions

Let us see the Django URL resolver exception classes which are defined in django.urls, as shown in the table below.

Django URL Resolver Exception Classes	Description
Resolver404	This exception is raised if the path passed to resolve() doesn’t map to a view.
NoReverseMatch	This exception is raised by django.urls when a matching URL defined in URLconf cannot be identified based on the supplied parameters.

Django Database Exceptions

Let us see the Django Database exception classes which are defined in django.db, as shown in the table below.

Django Database Classes	Description
Error	This is the base class of all other error exceptions. You can use this to catch all errors with one single except statement.
Error	This is the base class of all other error exceptions. You can use this to catch all errors with one single except statement.
InterfaceError	This exception is raised for the errors that are related to the database interface rather than the database itself.
DatabaseError	This exception is raised for the errors that are related to the database.
DataError	This exception is raised for the errors which are raised while processing data like, division by zero, numeric value out of range.
OperationalError	This exception is raised for the errors that are related to the database's operation and not necessarily under the control of the programmer, for example - an unexpected disconnect occurs, a transaction could not be processed, a memory allocation error occurred during processing, etc
IntegrityError	This exception is raised when the relational integrity of the database is affected, for example - when a foreign key check has failed.
InternalError	This exception raised when the database encounters an internal error, for example - the cursor is not valid anymore.
ProgrammingError	This exception is raised for the programming errors, for example - table not found or already exists, syntax error in the SQL statements .
NotSupportedError	This exception is raised in a situation when a method or database API used, is not supported by the database..

Django Http Exceptions

Let us see the Django Http exception classes which are defined in django.http, as shown in the table below.

Django Http Exception Class	Description
UnreadablePostError	This type of exception is raised when a user cancels an upload.

Django Transaction Exceptions

Let us see the Django Transaction exceptions are defined in django.db.transaction, as shown in the table below.

Django Transaction Exception Class	Description
TransactionManagementError	This type of exception is raisedfor all problems related to database transactions.

Django Testing Exceptions

Let us see the Django Testing exceptions are defined in django.test, as shown in the table below.

Django Testing Exception Class	Description
RedirectCycleError	This type of exception is raised when the test client detects a loop or an overly long chain of redirects.

Apart from the above described Django specific exceptions, Django also raises built-in Python exceptions.

In the next few articles, we will show you a exception situation in a Django project and will also show how to handle the exceptions by using the try, except finally and raise keywords.

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