Basic Usage


Install TidyExc using pip:

$ pip install tidyexc

Defining exceptions

TidyExc exceptions inherit from tidyexc.Error and are typically empty:

>>> from tidyexc import Error
>>> class MyError(Error):
...     pass

While it’s possible to use the Error class directly, it’s good practice to define a custom exception type for your library. This makes it possible to distinguish between exceptions from different libraries.

How many exception classes to define for your library is a matter of taste. I prefer to define one for each kind of user that might be expected to fix the problem causing the exception. For example, this might include:

  • UsageError: For faulty input that the end-user would need to fix.

  • ApiError: For programming mistakes that a developer would need to fix.

Raising exceptions

There are three steps to raising a TidyExc exception:

  • Provide any parameters relevant to diagnosing and fixing the problem.

  • Provide message templates that format and describe the above parameters.

  • Raise the exception.

It’s important that the parameters and the message templates are provided separately. This allows downstream exception handlers to access every parameter relevant to the exception when attempting to formulate a response. For example, one common way to handle an exception is to raise another exception with a more context-specific message (e.g. “missing configuration” instead of “file not found”). This is much easier if the parameters of the original exception (e.g. the file name) can be accessed directly, rather than needing to be parsed from its error message.

Parameters are typically provided as keyword arguments to the exception constructor, although they can also be added to the attribute after the fact:

>>> err = MyError(a=1)
>>> = 2
(1, 2)

There are four different kinds of message templates that can be provided, all optional:

  • brief: For a brief statement of the problem.

  • info: For descriptions of the context in which the error occurred.

  • blame: For descriptions of the specific cause of the error.

  • hints: For suggestions on how to fix the error.

Each of these message templates can either be a string or a callable. If a string, the str.format method will be used to do parameter substitution. If a callable, the parameters will be provided as an argument and the return value will be used as the ultimate message. See the API documentation for more detail on how these templates are interpreted.

The brief message can be specified as an argument to the constructor, but all the other messages must be specified after the exception object has been created. Note that brief is specified using the assignment operator while info, blame, and hints are specified using the in-place addition operator. This is because there can only be one brief message, but any number of the others:

>>> err.brief = "a problem occured"
>>> += "some relevant context: {a}"
>>> err.blame += "something unexpected: {b}"
>>> err.hints += "try this instead"

Once the exception object contains all of the relevant information, it can be raised as usual:

>>> raise err
Traceback (most recent call last):
MyError: a problem occured
• some relevant context: 1
✖ something unexpected: 2
• try this instead