Result Design Pattern

Result Pattern: (Bonus pattern not in the book) Defines an object representing both the status and the output of an operation.

A result object is a container for the result of an operation, which can be either a success or a failure, together with the output of the operation. The result pattern is a fundamental part of the Rust programming language in the form of the Result enum.

Class Diagram

classDiagram
    class Result {
        status: bool
        output: Any
    }

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Example

The filereader example uses the Result pattern to handle common errors reading and parsing json files. It puts all file reading and parsing logic into a single function, which returns a Result object. The calling code can check the status without needing to understand the details of the error.

In this case we have correct files, a missing file and a bad file. All can be handled with the same approach, in this case using the LBYL philosophy.

Running the code

  python filereader.py

Discussion

People often say that in Python you should use exceptions - "Easier to Ask Forgiveness than Permission" (EAFP) over guard clauses - "Look Before You Leap" (LBYL), but the Result design pattern - returning an object which explicitly states whether the operation succeeded is a useful alternative where error handling is required and can be compatible with both approaches.

Use in Python

The asyncio Future object can be seen as a result object - it represents the result of an asynchronous operation but rather than a success or failure it is either done or not done. Confusingly it contains a .result attribute which is the output of the operation, but is not the same as the Result object in the Result pattern.

The requests library is often cited as a quintessentially pythonic library - its response object (representing the HTTP response) is essentially a Result object. The raise_for_status() method lets you easily use it with the EAFP philosophy.