I have failed to find documentation for the operator % as it is used on strings in Python. What does this operator do when it is used with a string on the left hand side?
It's the string formatting operator. Read up on string formatting in Python.
format % values
Creates a string where format specifies a format and values are the values to be filled in.
It applies printf-like formatting to a string, so that you can substitute certain parts of a string with values of variables. Example
# assuming numFiles is an int variable
print "Found %d files" % (numFiles, )
See the link provided by Konrad
The '%' operator is used for string interpolation. Since Python 2.6 the String method "format" is used insted. For details see http://www.python.org/dev/peps/pep-3101/
Note that starting from Python 2.6, it's recommended to use the new str.format() method:
>>> "The sum of 1 + 2 is {0}".format(1+2)
'The sum of 1 + 2 is 3'
If you are using 2.6, you may want to keep using % in order to remain compatible with older versions, but in Python 3 there's no reason not to use str.format().
来源：https://stackoverflow.com/questions/1238306/what-does-do-to-strings-in-python