Coding Mechanics

[buran]

I don't pass anything into that method to tell it to make that connection, but somehow it knows what Y is. Prior to the method call, I set the column set to a variable called Y. But I don't pass that variable value to the ols method. So how does the code inside the method know what Y is?

But you bind name Y to column fat['body_fat_percent'] on line 7!
It uses patsy as language to describe the model. It parse the string 'Y ~ triceps_skinfold_thickness_mm + midarm_circumference_cm + thigh_circumference_cm' and uses the name Y you defined as well as the other columns from fat DataFrame.

If you prefer you can drop line 7 and just use

model = sms.ols('body_fat_percent ~ triceps_skinfold_thickness_mm + midarm_circumference_cm + thigh_circumference_cm', data = fat).fit()

Here's again the link to patsy package docs: https://patsy.readthedocs.io/en/latest/