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.
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
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