How is the runtime of a function being determined?

[csong16]

In one of my functions, I have set spec.runtime to python:3.8 in the config file. However, the base image that I provided for this function contains Python version 3.10.x. To test which Python interpreter is being used, I created a simple function that returns the Python version when invoked. Surprisingly, the function responds with 3.10.x, which is the Python interpreter inside the base image.

My question is: How is the runtime exactly determined?

My hypothesis is that when spec.runtime is set to python:x.x, the version specified (x.x) may not matter. Instead, the function might use the Python interpreter invoked by the python command.

Could someone clarify this behavior for me?

[TomerShor]

Hi @csong16
When you set spec.runtime to a specific python version, we parse the value and take the appropriate version's base image - e.g. python:3.8.

I ran a test just like you described, with the spec.runtime="python:3.8" and the following handler:

import platform


def handler(context, event):
    return platform.python_version()

And I receive the following response:

3.8.13

Please let me know if this still happens, and perhaps provide more details to reproduce.

[TomerShor]

@csong16
I have managed to reproduce with a different bas image.

When setting a custom base image, the runtime version is determined by the runtime that's pre-installed in the given image.
Meaning - when a custom image is given, the value in spec.runtime is essentially ignored.
This is done because we can't (and won't) install the required runtime version on the given base image.

I agree that this is a bit confusing, and we will add a log that communicates this information when deployed a function with a custom image.

Bottom line - if you want to use a custom base image and a specific runtime version, make sure your given image has this version already installed.

[csong16]

Thanks for your clarification. Actually, I encountered this issue because I have to use an old version of nuclio which does not allow me to specify python version above 3.8. However, I aim to develop a custom function based on an docker image with Python 3.10. That raises my question.