Question

I would like to know the best way to remove the oldest element in a dictionary in order to control the maximum dictionary size.

example:

MAXSIZE = 4
dict = {}
def add(key,value):
  if len(dict) == MAXSIZE:
    old = get_oldest_key() # returns the key to the oldest item
    del dict[old]
  dict[key] = value

add('a','1') # {'a': '1'}
add('b','2') # {'a': '1', 'b': '2'}
add('c','3') # {'a': '1', 'c': '3', 'b': '2'}
add('d','4') # {'a': '1', 'c': '3', 'b': '2', 'd': '4'}
add('e','5') # {'c': '3', 'b': '2', 'e': '5', 'd': '4'}

Was this clear?

Edit: Forgot that len(dict) lags one item behind.

Answer 1

Dictionaries don’t preserve order, so you can’t tell which element had been added first. You could combine the dictionary with a list of it’s keys to preserve order.

Here’s an activestate recipe for an ordered dict that does just this.

There’s also PEP-0372 with this patch for an odict class.