Use of openpyxl.utils.cell._get_column_letter assumes <= 18278 columns

[poulson]

I am unable to unflatten some UK government procurement data because flatten-tool's unflatten routine is internally making use of openpyxl.utils.cell's _get_column_letter routine to convert column indices into strings of the form [A-Z]+.

The issue is that _get_column_letter explicitly enforces the constraint that at most three letters are generated, which corresponds to a constraint of at most 26 + 26 * 26 + 26 * 26 * 26 = 18,278 columns.

Thus, for spreadsheets with more than 18,278 columns, the unflatten routine is failing with a dubious error message about an invalid column index.

Thankfully, the fix is quite simple: the _get_column_letter routine is self-contained and the limitation to three characters is self-imposed and easily removed.

The implementation in its entirety is:

def _get_column_letter(col_idx):
    """Convert a column number into a column letter (3 -> 'C')

    Right shift the column col_idx by 26 to find column letters in reverse
    order.  These numbers are 1-based, and can be converted to ASCII
    ordinals by adding 64.
    """
    # these indicies corrospond to A -> ZZZ and include all allowed
    # columns
    if not 1 <= col_idx <= 18278:
        raise ValueError("Invalid column index {0}".format(col_idx))
    letters = []
    while col_idx > 0:
        col_idx, remainder = divmod(col_idx, 26)
        # check for exact division and borrow if needed
        if remainder == 0:
            remainder = 26
            col_idx -= 1
        letters.append(chr(remainder+64))
    return ''.join(reversed(letters))

The routine functions for arbitrarily large column indices if you simply remove these four lines.

    # these indicies corrospond to A -> ZZZ and include all allowed
    # columns
    if not 1 <= col_idx <= 18278:
        raise ValueError("Invalid column index {0}".format(col_idx))

I recommend avoiding the external dependency on _get_column_letter via an internally implemented function of the form:

def get_column_string(col_idx):
    """Convert a column number into a string of characters between A and Z."""
    letters = []
    while col_idx > 0:
        col_idx, remainder = divmod(col_idx, 26)
        # check for exact division and borrow if needed
        if remainder == 0:
            remainder = 26
            col_idx -= 1
        letters.append(chr(remainder+64))
    return ''.join(reversed(letters))