Inside a bash function, how should I pipe output to a python program without invoking the python program, at definition time?

Question

I apologize in advance, I'm not good at writing shell scripts.

I have a bash function that looks like this:

tagDownload() {
    mp3 "$@" | /home/username/bin/tag.py
}

where mp3 is another bash function that produces some output that is used by the python script. This function is stored in a dotfile that is sourced whenever a terminal is opened.

What I expect (and want) to happen is for tagDownload and its constituent components to only be invoked when called. What is actually happening is that the python script is invoked when the dotfile that houses this definition is sourced when a new terminal is opened.

I have two questions:

1. How should I structure tagDownload so that when its parent file is sourced, the python program isn't invoked?
2. What rules does bash use to determine if something is an invocation vs a statement? I've had problems with this before and want to avoid such headaches in the future.

If I'm thinking about this incorrectly, please, correct me.

Answer 1

As discussed in the comments to the question, when defining a function in the shell, the act of defining it does not cause it to be called.

As the user realised later, they were in fact calling the function from another place, which caused the Python script to run.