How to downcase the first character of a string?

There is a function to capitalize a string, I would like to be able to change the first character of a string to be sure it will be lowercase.

How can I do that in Python?

One-liner which handles empty strings and None:

func = lambda s: s[:1].lower() + s[1:] if s else ''

>>> func(None)
>>> ''
>>> func('')
>>> ''
>>> func('MARTINEAU')
>>> 'mARTINEAU'

s = "Bobby tables"
s = s[0].lower() + s[1:]

def first_lower(s):
   if len(s) == 0:
      return s
   else:
      return s[0].lower() + s[1:]

print first_lower("HELLO")  # Prints "hELLO"
print first_lower("")       # Doesn't crash  :-)