Complex 'compare'

Frank Millman frank at chagford.com
Tue Dec 18 05:50:28 EST 2018


Hi all

I want to compare two tuples. They each represent a row in a database, and 
each element represents a column, so I will use that terminology.

I need to know if one row is greater than or less than the other. The sort 
sequence can be complex - one or more columns, each of which can be sorted 
ascending or descending.

Below is the function I have come up with. Can anyone see any problem with 
it, or suggest a better way to do it?

I have taken one short cut. If all sort columns compare equal, it will 
return True, which is wrong. In practice I ensure that one of the sort 
columns will be the primary key, so the two rows should never be equal.

Thanks

Frank Millman

def compare(source_row, target_row, order, compare_type):

    # source_row - the row I want to compare - some sort columns could 
contain None
    # target_row - the row I want to compare it with - no sort columns will 
contain None
    # order - a list of 2-part tuples defining the sort sequence - the 
column number, and True if descending else False
    # compare_type - either 'gt' or 'lt'

    def compare_col(source_col, target_col, desc, compare_type):
        if compare_type == 'gt':
            if desc:
                return True if source_col is None else source_col < 
target_col
            else:
                return False if source_col is None else source_col > 
target_col
        elif compare_type == 'lt':
            if desc:
                return False if source_col is None else source_col > 
target_col
            else:
                return True if source_col is None else source_col < 
target_col

    for pos, desc in order:
        source_col = source_row[pos]
        target_col = target_row[pos]
        if compare_col(source_col, target_col, desc, compare_type):
            return True
        if source_col != target_col:
            return False
        # if we get here they are equal -
        #    compare the next column in the sort sequence
    return True




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