/historical/filtertools.py

#
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"""filtertools - Filter utilities

Functions:
filterthru - Transform a sequence through a stack of filters

"""

__all__ = [
    'filterthru',
    ]


def _append_lists(a, b):
    return a + b
    
def filterthru(data, stack):
    """Return a sequence transformed through a stack of filters
    
    The data argument is a list of zero or more items.

    The stack argument is a list of zero or more filters.  Each filter is a
    function that should take a single data item, and return a list of zero
    or more data items. Filters may simply transform the data, returning a
    single valued list, may add values by returning multiple valued list, or
    eliminate an item from further processing by return an empty list.
    
    For each filter in the stack, this function takes each item in the data
    and passes it through the filter, concatenating the results. This new,
    transformed data list becomes the input for processing with the next filter 
    in the stack. The final transformed list is the result.
    
    Note: The order of the results is maintained when concatenating.
    Note: If at any stage, there resulting list is empty, the final result
    will be empty.
    
    """
    
    for filter in stack:
        data = reduce(_append_lists, map(filter, data), [])
    return data