Python as a language has evolved a lot over the years, but despite this fact, a program written in Python 1.0 will still be able to run in Python 2.7, which is a version that was released 19 years after Python 1.0.

Though a great benefit for the developers of Python applications, this backward compatibility of the language is also a major hurdle in the growth and development of major improvements in the language specification, since a great amount of the older code base will break if major changes are made to the language specification.

With the release of Python 3, this chain of backward compatibility was broken. The language in version 3 dropped the support for programs that were written in earlier versions in favor of allowing a larger set of long-overdue improvements to the language. However, this decision disappointed quite a lot of developers in the community.