kek == None #bad
kek != None #bad
kek is None #good
kek is not None #good

from cStringIO import StringIO
class StringBuilder:
    def __init__(self): 
        self._file_str = StringIO()
    def append(self, str):
        self._file_str.write(str)
    def __str__(self): 
        return self._file_str.getvalue()

class StringBuilder:
    def __init__(self): 
        self._file_str = []
    def append(self, str):
        self._file_str.append(str)
    def __str__(self): 
        self._file_str = [''.join(self._file_str),]
        return self._file_str[0]

l = [True, True, True, False, False, False, True, False, False]
c = len(l) - sum(l)

# вариант быстрый
l[:] = map(c.__le__, range(len(l)))

#вариант красивый
l[:] = (c <= i for i in range(len(l)))