wanted to install 3.6.15, but the source tarball installs 3.6.8

Where did you get the tarball?

From 
https://www.python.org/downloads/release/python-3615/

Downloaded the XZ one:
https://www.python.org/ftp/python/3.6.15/Python-3.6.15.tar.xz

I am currently building the Gzipped one, let's see what it turns out.

I've just confirmed that both tarballs (gzip and xz) on the official download page (https://www.python.org/downloads/release/python-3615/) contain 3.6.15.

Note that 3.6.8 was the last bugfix release that included Windows and macOS installers.  If you're on one of those platforms, you may need to go through some extra steps to replace an existing 3.6.8 install with 3.6.15, and at that point you would likely be better off upgrading to 3.10 (or at least 3.9, which is the oldest release that still has a current binary installer).

Also note that 3.6 is now EOL, so an upgrade is highly recommended anyway :)

I hear you, but I continue to need 3.6 for a bit longer.
I am on Linux, Centos 7; building from sources in a sandbox, no installation conflict with any else...

How do you confirm that the tarball indeed contains that it says? 

I build it and then I do "./python --version" and I get 3.6.8, instead of 3.6.15   

I also have 3.7, and 3.8 installs (I also need them) and working on installing 3.9 and 3.10

I confirmed by downloading a fresh copy of each, extracting, and looking at Include/patchlevel.h.  You could further check by downloading a 3.6.8 tarball and comparing it against the 3.6.15 tarball.

Is 3.6.8 the version you already have installed, and are you building with `./configure --enable-shared`?  If so, my best guess would be that your new `./python` binary is picking up the system `libpython3.6.so`.  Try `LD_LIBRARY_PATH=$(pwd) ./python --version`.

Pre-post-edit: you are right, just saw your answer as I was typing this one. 


Aaaahhhh....

./python --version

is not enough, I guess I need to properly setup PATH, LD_LIBRARY_PATH and possibly PKG_CONFIG_PATH.

Sorry for the bother, guys. All is good. My apologies and thank you very much for the super fast response, it prompted to question and double question what I was doing.

So, help me understand, please. 


if I use "--enable-shared":
the build produces a rather small python executable; but, 
the build produces a dynamic library, too
and the path to the dynamic library must be included into LD_LIBRARY_PATH for the python executable to work correctly.

if I do NOT use "--enable-shared"
the build produces a larger python executable; and,
it does not produce a dynamic library, but
it produces an static library.
Does this mean that I do not have to worry about setting up LD_LIBRARY_PATH?

In the non-shared, static case, I thought the static library would be inside the executable, but, funny enough, I noticed that the python executable is smaller than the static library, so...I am confused. 

I would like to have 3.6, next to 3.7 and 3.8 with minimal setup, thus the static route.

Please advise.

That's a question better suited to a forum such as the Users category of discuss.python.org, the python-list@python.org mailing list, or StackOverflow, not a bug tracker issue.