Sergey Zhuk has written up a post to his site that wonders if the factory method violates the open/closed principle, a part of the SOLID set of principles for software development.

Consider an application that provides some statistics reports. Reports are present in different formats: JSON for API, HTML for viewing in a browser and pdf for printing on the paper. It has StatisticsController that receives a required format from the request and returns a formatted report. The logic for choosing a formatting strategy is hidden behind the factory.

He works through a code example of using the factory pattern to create this functionality, generating the fomatter from behind the factory. He then talks about adding a new formatter for CSVs and the update to the factory that would come with it. It's this last change he's wondering about as the Open/Closed principle states that objects should be open for extension but not modification. While the answer is technically "yes" he explains that the purpose of the factory is to abstract the logic away so you only have to deal with one type of thing rather than making it yourself every time.

According to Open-Closed Principle the “correct” solution would be to create a new factory with the same interface. That said, adherence to this principle should always be weighed against other design principles like KISS and YAGNI.