Binary search to find a character in a str

def isIn(char, aStr):
    '''
    char: a single character
    aStr: an alphabetized string
   
    returns: True if char is in aStr; False otherwise
    '''
    # Your code here
    if aStr == '':
        return False
    if len (aStr) == 1:
        return char == aStr
    midindex = len(aStr)//2
    midchar = aStr [midindex]
    if char == midchar:
        return True
    elif char < midchar:
        return isIn (char, aStr[:midindex])
    else:
        return isIn (char, aStr[midindex+1:])