Why are they some new releases of Python 3 on older Python 3 versions?

Question

Right now, on the "All releases" page of the Python Software Foundation website, the "Download latest release" links to the Python 3.6.4 version. However, you can find on the page that the release is from the 2017-12-19 and there has been two others release since, for Python 3.5.5 and Python 3.4.8 .

I understand why there is two parallel version of Python with 3 and 2.7, but I do not understand why they are multiple versions of Python 3, as it should be backward compatible with Python 3 code.

Answer 1

x.y.Z point releases are usually bug fix releases.
x.Y releases are usually feature releases, but might contain minor backwards incompatibilities.
X releases are large changes possibly breaking a lot of existing code.

In practice you cannot always upgrade your x.Y version immediately; reasons range from actual code incompatibilities which cannot be fixed quickly to internal deployment limitations to scheduling reasons. Linux distributions also often distribute one specific x.Y version and will pick up the next version only a year later in their annual release cycle or so. However, x.y.Z versions are often picked up soon and can typically be installed without breaking existing code.

The reason several x.y.Z versions are maintained in parallel is that users want to reap the benefits of bug fixes without being forced to upgrade to a new major version.