### 21.6. `YearCollection.__findPrev()`: Find predecessor month

For the algorithm, see Section 21.5, “`YearCollection.neighbors()`: Find previous and next month”.

noteweb
```# - - -   Y e a r C o l l e c t i o n . _ _ f i n d P r e v

def __findPrev ( self, yyyy_mm ):
'''Find the month preceding yyyy_mm.

[ yyyy_mm is a month in self with key string 'yyyy-mm' ->
if self has any months before yyyy_mm ->
return the month key of the immediate predecessor
else -> return None ]
'''
```

First we find the `YearRow` containing `yyyy_mm`, and ask that instance if the month has a predecessor in the same year.

noteweb
```        #-- 1 --
# [ yyyy  :=  year part of yyyy_mm
#   mm    :=  month part of yyyy_mm ]
yyyy, mm  =  yyyy_mm.split('-')

#-- 2 --
# [ yyyy is a year in self ->
#     yearRow  :=  YearRow instance for year yyyy ]
yearRow  =  self[yyyy]

#-- 3 --
# [ if yearRow has a predecessor to month mm ->
#     return that month's key as a string 'mm'
#   else -> I ]
try:
prev  =  yearRow.predecessor ( mm )
return '%s-%s' % (yyyy, prev)
except KeyError:
pass
```

If there is a predecessor, it must be in a previous year. First we form a list of the year numbers in descending order. Then we find the position of `yyyy` in that list, and step through the remaining elements (if any) until we find a year that has at least one month in it, and return the last month.

noteweb
```        #-- 4 --
# [ yyyyList  :=  all year key strings in self, sorted
#                 in descending order ]
yyyyList  =  self.__yearMap.keys()
yyyyList.sort()
yyyyList.reverse()

#-- 5 --
# [ yyyy is an element of yyyyList ->
#     pos  :=  yyyy's position in yyyyList ]
pos  =  yyyyList.index ( yyyy )
```

For the method that looks for the last month of a year, see Section 22.5, “`YearRow.lastMonth()`: Return the last month”.

noteweb
```        #-- 6 --
# [ if any year with a key in yyyyList[pos+1:] has at
#   least one month in it ->
#     return the yyyy-mm key of the last month in the
#     first such year
#   else -> I ]
for prevYear in yyyyList[pos+1:]:
prevRow  =  self[prevYear]
if len(prevRow) > 0:
lastMM  =  prevRow.lastMonth()
return '%s-%s' % (prevYear, lastMM)

#-- 7 --
return None
```