"Portfolio Optimazation"

Hi,

I’m trying to understand a function in the file called “rf.py” for “Portfolio Optimization” in "Analyze Financial Data with Python.

https://www.codecademy.com/paths/finance-python/tracks/visualizing-finance-data/modules/portfolio-optimization/lessons/mv-portfolio-optimization/exercises/review

Could anyone teach me what the following code does.
Why is expected_returns.axes in parentheses?

selected = (expected_returns.axes)[0]

Thanks.

Satoru