Generate range of characters
What if you would operate with character code ranges and use the ord() and chr() built-in functions instead:

def char_range(mini, maxi, step=1):
    for code in range(ord(mini), ord(maxi) + 1, step):
        yield chr(code)


Or, with the yield from (Python 3.3+):

def char_range(mini, maxi, step=1):
    yield from range(ord(mini), ord(maxi) + 1, step)


Demo:

$ ipython3 -i test.py
In [1]: for char in char_range('a', 'h'):
   ...:     print(char, end=' ')
   ...:     
a b c d e f g h 


Couple notes about the code style of your solution:


according to PEP8, there should be two blank lines after the imports
use the "lower_case_with_underscores" style to name the function - charRange() vs char_range() (PEP8 reference)