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For a given month key expressed as a ' string, this method finds the preceding and
following months, and returns a two-element tuple
containing the month keys of its immediate neighbors,
using yyyy-mm'None for the predecessor of the
first month in the set, or for the successor of the last
month.
# - - - Y e a r C o l l e c t i o n . n e i g h b o r s
def neighbors ( self, yyyy_mm ):
'''Find the previous/next months to yyyy_mm in sequence
'''
Because we use a two-level structure of years and months, and because the predecessor or successor of a month may be in a different year, we'll use this algorithm to find the predecessor.
If year yyyy has any months before
mm, the predecessor is the last
such month.
Work backwards through years yyyy-1,
yyyy-2, until we find a year that
has at least one month in it. When we do, the
predecessor is the last month in that year.
If we run out of years, the predecessor is None.
The algorithm to find the successor is symmetric. See
Section 21.6, “YearCollection.__findPrev(): Find
predecessor month” and Section 21.7, “YearCollection.findNext(): Find
successor month”.
#-- 1 --
# [ if yyyy_mm has a predecessor in self ->
# prev := that predecessor's 'yyyy-mm' month key
# else ->
# prev := None ]
prev = self.__findPrev ( yyyy_mm )
#-- 2 --
# [ if yyyy_mm has a successor in self ->
# next := that successor's 'yyyy-mm' month key
# else ->
# next := None ]
next = self.__findNext ( yyyy_mm )
#-- 3 --
return (prev, next )