Fix Python – How to comment out a block of Python code in Vim

I was wondering if there was any key mapping in Vim to allow me to indent certain lines of code (whether those lines have been selected in visual mode, or n lines above/below current cursor position).
So basically something that converts the following
def my_fun(x, y):
return x + y

to
#def my_fun(x, y):
# return x + y

I am okay with usin….