The standard way is to use format string modifiers. These format string methods are available in most programming languages (via the sprintf function in c for example) and are a handy tool to know about.

To output a string of length 5:

... in Python 3.5 and above: f-strings.

i = random.randint(0, 99999)
print(f'{i:05d}')

Search for f-strings here for more details.

... Python 2.6 and above:

print '{0:05d}'.format(i)

... before Python 2.6:

print "%05d" % i

See: https://docs.python.org/3/library/string.html

You can use str.zfill:

print(str(1).zfill(2))
print(str(10).zfill(2))
print(str(100).zfill(2))

prints:

01
10
100