def is_valid_index(idx, in_list, start_idx=0):
    """
    param: idx (str) - a string that is expected to
            contain an integer index to validate
    param: in_list - a list that the idx indexes
    param: start_idx (int) - an expected starting
            value for idx (default is 0); gets
            subtracted from idx for 0-based indexing

    The function checks if the input string contains
    only digits and verifies that the provided index
    idx is a valid positive index that can retrieve
    an element from in_list.

    returns:
    - True, if idx is a positive numeric index
    that can retrieve an element from in_list.
    - False if idx is not an integer value, is negative
    or exceeds the size of in_list.
    """
    # writing a function with the default parameter value
    # using Boolean value as a flag for whether printing needs to occur
    # working with dictionaries stored inside a list (a nested list)
    # indexing and printing dictionary entries (potentially using enumerate())
    # formatting the output according to the specifications
    if (idx - start_idx) >= 0 and (idx - start_idx) < len(in_list) and idx.isdigit():
        return True
    else:
        return False