Where can I find the source code of os.urandom()?

I want to investigate and study the code of python 3.7's os.urandom() function. I had a look at the respective standard library's os.py, but neither is it defined there, nor is it imported there. I also tried to grep for the definition:

/usr/lib/python3.7 $ grep -rFl "def urandom" 2> /dev/null But nothing. I suspect that there is some magic happening regarding os.urandom(). Where can I find the function definition for study and cryptoanalysis?