In python 2.7, the comma is to show that the string will be printed on the same line
For example:
for i in xrange(10):
print i,
This will print
1 2 3 4 5 6 7 8 9
To do this in python 3 you would do this:
for i in xrange(10):
print(i,end=" ")
You will probably find this answer helpful
Printing horizontally in python
—- Edit —
The documentation, http://docs.python.org/2/reference/simple_stmts.html#the-print-statement, says
A ‘\n’ character is written at the end, unless the print statement ends with a comma.