Python 3 virtual environment is executing python 2 - python-3.x

Question

This issue has had me scratching my head for a while now.
I've encountered this problem twice now, on windows 7 and windows 10. As its a closed network I've no security concerns with W7.
I have an existing Python 2.7 install - densely populated, with some fairly old library versions.
Whilst I would like to create a virtual environment for Python 2.7 I'm reluctant to do so because wheels are not available for many of the library versions I need to use, and revision histories do not appear to be available for some of these older libraries either, so without trying them I can't tell if the later library versions are backwards compatible or not. As the Python 2.7 code is from a production environment I can't break/replace it.
The python 2.7 libraries are all installed from executable installers as many of them predate
the introduction of the python wheel standard. Some also require to be built with older versions of MSVC, which is another reason for not wanting to attempt to rebuild.
It will be much easier to leave the python 2.7 install as is and start running Python 3 in virtual environments, although my original plan was to create a separate 2.7 virtual environment.
Example Python 3 virtual environment created as "py37".
In my virtual environment.
include-system-site-packages = false
When I activate the virtual environment all appears to be good.
Python -V returns 3.7.3, which is what I expect
I modified my script to output the python version (so I can track what is happening)
(py37) C:\my_folder\my_script.py
3.7.3 (v3.7.3:ef4c6ed12, Mar 25, 2019, 21:26:53) [MSC v.1916 32 bit (Intel)]
Traceback (most recent call last):
file "my_script.py", line 24, in (module)
from tk import ttk
ModuleNotFoundError: No module named 'tk'. 'tk' (python 3 lib) is not being found.
I then created two simple scripts, one using the python 2 print format, the other python 3.
Even though the virtual environment says python 3, and the python version says 3.7.3 the python 3 script fails to run.
The python 2 version of print prints out correctly, so clearly even though all indications are that Python 3 is running, it's actually Python 2.
PythonPath is null/not defined on the PC and my path variable points to my python 3 virtual environment, not my Python 2 install.
All the searching and examples I have come up with stop when version() or Python -V returns 3.x, so if it is a duplicate issue I've not managed to find the original request.

Answer 1

The solution to this was simple, but unpleasant.
In order for everything to work as it should I needed to explicitly identify the executable I wanted to run, every time I wanted to run it i.e. full path to the python executable in my python3 virtual environment.
I ended up creating a batch file wrapper to facilitate this
e.g. c:\apps\my_venv3\scripts\python.exe %*
Clunky, but it works, reliably, and does not break any of the existing environments.