What does .BY$ do in R? Apologies, I'm new to R and I am supposed to convert an R code to Python. I'm having a hard time understanding the syntax

Question

This is a sample of my dataframe(data.table in R) called "aligned_input_dt_in"

The line of code I am having trouble understanding is given below -->

i=3

output = aligned_input_dt_in[, .(Delta_denominator = aligned_input_dt_in[pt_period_life >= i + .BY$periods -1][.(.BY$product),uniqueN(patient_id, na.rm = T)]),
      by = .(periods, product)]

Can anyone please help me understand this line of code?

And also, if anyone can convert this to Python, it'll be really helpful..

Thanks!