from typing import List, Set

def number_to_digits(n: int, base: int = 10) -> List[int]:

"""Convert a number to a list of digits.

>>> number_to_digits(0)

[0]

>>> number_to_digits(1)

[1]

>>> number_to_digits(10)

[1, 0]

>>> number_to_digits(100)

[1, 0, 0]

>>> number_to_digits(257, 16)

[1, 0, 1]

"""

digits = [0] if n == 0 else []

while n > 0:

digits.append(n % base)

n //= base

return digits[::-1]

def variable_name_generator(used: Set[str] = []):

"""Generate variable name not currently used in scope.

>>> generator = variable_name_generator()

>>> next(generator)

'a'

>>> for i in range(25):

... _ = next(generator)

...

>>> next(generator)

'A'

>>> for i in range(25):

... _ = next(generator)

...

>>> next(generator)

'aa'

"""

cur = 0

while True:

name = ''

for i, digit in enumerate(number_to_digits(cur, base=52)[::-1]):

base = 'a' if digit < 26 else 'A'

name = chr(ord(base) + ((digit % 26) - (i > 0))) + name # for 1st digit, a = 0. for subsequent, a = 1

if name not in used:

yield name

cur += 1