Modern software architecture tends to split an application into several components. Whether you embrace the service-oriented architecture paradigm, or you push it even further into the microservices realm, these components will have to exchange data. But even if you are coding a monolithic application, whose code base is contained in one project, chances are that you have to still exchange data with APIs, other programs, or simply handle the data flow between the frontend and the backend part of your website, which very likely won't speak the same language.
Choosing the right format in which to exchange information is crucial. A language-specific format has the advantage that the language itself is very likely to provide you with all the tools to make serialization and deserialization a breeze. However, you will lose the ability to talk to other components that have been written in different versions of the same language, or in different languages altogether. Regardless of what the future looks like, going with a language-specific format should only be done if it is the only possible choice for the given situation.
A much better approach is to choose a format that is language agnostic, and can be spoken by all (or at least most) languages. In the team I lead, we have people from England, Poland, South Africa, Spain, Greece, India, Italy, to mention just a few. We all speak English, so regardless of our native tongue, we can all understand each other (well... mostly!).
In the software world, some popular formats have become the de facto standard over recent years. The most famous ones probably are XML, YAML, and JSON. The Python standard library features the xml and json modules, and, on PyPI (https://docs.python.org/3.7/library/archiving.html), you can find a few different packages to work with YAML.
In the Python environment, JSON is probably the most commonly used one. It wins over the other two because of being part of the standard library, and for its simplicity. If you have ever worked with XML, you know what a nightmare it can be.