Error I do not understand [closed] - python-3.x

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I generated this error in Python 3.5:
Traceback (most recent call last):
File "C:\Users\Owner\AppData\Local\Programs\Python\Python35\lib\shelve.py", line 111, in __getitem__
value = self.cache[key]
KeyError: 'P4_vegetables'
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "C:\Users\Owner\Documents\Python\Allotment\allotment.py", line 217, in
main_program()
File "C:\Users\Owner\Documents\Python\Allotment\allotment.py", line 195, in main_program
main_program()
File "C:\Users\Owner\Documents\Python\Allotment\allotment.py", line 49, in main_program
print("Plot 4 - ", s["P4_vegetables"])
File "C:\Users\Owner\AppData\Local\Programs\Python\Python35\lib\shelve.py", line 113, in __getitem__
f = BytesIO(self.dict[key.encode(self.keyencoding)])
File "C:\Users\Owner\AppData\Local\Programs\Python\Python35\lib\dbm\dumb.py", line 141, in __getitem__
pos, siz = self._index[key] # may raise KeyError
KeyError: b'P4_vegetables'

Answer 1

It has been a while, but in case somebody comes across this: The following error
Traceback (most recent call last):
File "filepath", line 111, in __getitem__
value = self.cache[key]
KeyError: 'item1'
can occur if one attempts to retrieve the item outside of the with block. The shelf is closed once we start executing code outside of the with block. Therefore, any operation performed on the shelf outside of the with block in which the shelf is opened will be considered an invalid operation. For example,
import shelve
with shelve.open('ShelfTest') as item:
item['item1'] = 'item 1'
item['item2'] = 'item 2'
item['item3'] = 'item 3'
item['item4'] = 'item 4'
print(item['item1']) # no error, since shelf file is still open
# If we try print after file is closed
# an error will be thrown
# This is quite common
print(item['item1']) #error. It has been closed, so you can't retrieve it.
Hope this helps anyone who comes across a similar issue as the original poster.

Answer 2

It means that the dictionary (or of whatever type it is) cache does not contain the key named key which is of value 'P4_vegetables'. Make sure you added the key before using it.