in python how do I convert a single digit number into a double digits string?

In python 3.6, the fstring or "formatted string literal" mechanism was introduced.

f"{a:02}"

is the equivalent of the .format format below, but a little bit more terse.

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python 3 before 3.6 prefers a somewhat more verbose formatting system:

"{0:0=2d}".format(a)

You can take shortcuts here, the above is probably the most verbose variant. The full documentation is available here: http://docs.python.org/3/library/string.html#string-formatting

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print "%02d"%a is the python 2 variant

The relevant doc link for python2 is: http://docs.python.org/2/library/string.html#format-specification-mini-language

a = 5
print '%02d' % a
# output: 05

The '%' operator is called string formatting operator when used with a string on the left side. '%d' is the formatting code to print out an integer number (you will get a type error if the value isn't numeric). With '%2d you can specify the length, and '%02d' can be used to set the padding character to a 0 instead of the default space.