Simply import R's source function like any other!

import rpy2.robjects as robjects
r_source = robjects.r['source']
r_source("path/to/file.R")

source is a r function, which runs a r source file. Therefore in rpy2, we have two ways to call it, either:

import rpy2.robjects as robjects
r = robjects.r
r'source'

or

import rpy2.robjects as robjects
r = robjects.r
r.source('script.R')

r[r.source("script.R")] is a wrong way to do it.

Same idea may apply to the next line.

The rpy2 object r is meant to represent "R" running embedded. You can pass it a string in a call (e.g., r("1+2")) and that string will be evaluated as R code and the output returned.

On the other hand, globalenv is the R "environment" .Globalenv. That's basically where objects go when you declare them in an R terminal. R code may make this clearer:

x <- 1
# get "x" back, the less-easy way
get("x", .GlobalEnv)

When doing pi in an R console, the symbol will be searched first in the .GlobalEnv, and if not found the rest of the "search path" (the loaded libraries) will be searched. pi is in the R package "base", IIRC.

pi # returns 3.14...
pi <- "abc"
pi # return "abc"