What does `python3 ()` do? - zsh

Question

While trying to execute a timeit command on the command line using the python command line interface I accidentally put .function() on the outside of the command like so:
$ python3 -m timeit '<code>'.function()
Rather than the timeit command being executed, I was prompted as such:
function>
Thinking I had entered the python repl I tried to quit with q. Yes, I'm aware quit() is the correct way to do this. Having returned to the command line, I noticed the error and corrected it like so:
$ python3 -m timeit `<code>.function()`
I expected this code to execute correctly, but instead I received the following error:
python3:7: command not found: q
After discussing it with some colleagues, it was suggested that I check which python was being used:
$ which python3
python3 () {
q
}
This was not what I was expecting! Normally the result would be /usr/local/bin/python3. Through some trial and error I was able to determine that the minimal case to reproduce this is:
$ python3 ()
function> q
$
Now that the context is out of the way, I have two questions about the behaviour I witnessed:
1. What exactly does python3 () do?
2. How do I return execution to its original state in the same terminal window? I'm aware I can open a new terminal window and the original state exists in that window.

Answer 1

The syntax foo () is used in POSIX-compliant shells (such as bash, dash, and zsh) to define a function. Your entire snippet defines a function called python3 and executes the command q when it's ran. You can bypass shell functions and aliases using the command command: command -p python3 myfile.py
To remove the function from the current shell process, you can use unset -f python3. If it keeps coming back after starting new shells, then it's likely defined in one of you shell initialization files.