How to remove the oldest element from a dictionary?

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

Python 3.1 has an ordered dict. use the class collections.OrderedDict to keep the elements in their order of insertions. beware that if you are overwriting an element, it keeps its place in the order, you need to delete and re-insert an element to make it last.

if you are using an older release, a patch may be available to get OrderedDict.

anyway, if it is not available, you may simply use a list of tuples: it can be easily converted to and from a dictionary, keeps its ordering, can be used like a queue with append and pop, ...

Answer 2

Python dictionaries are ordered now (from 3.6 and above). more details
next(iter(dict)) would give the oldest (or first) key. stackoverflow answer

So to delete oldest (or first) key, we can use the following...

dict.pop(next(iter(dict)))