python字典分配不正确

Question

I wanted to use a dictionary both with the keys and also like lists with indexing. but i found that assignment to an dictionary is not in order.

my python script is:

left=['E','Z','T','Y','F']

for lhs in left:
  first[lhs]=set()
  follow[lhs]=set()
  print first

and i get the output as:

{'E': set([])}
{'Z': set([]), 'E': set([])}
{'Z': set([]), 'E': set([]), 'T': set([])}
{'Y': set([]), 'Z': set([]), 'E': set([]), 'T': set([])}
{'Y': set([]), 'Z': set([]), 'E': set([]), 'T': set([]), 'F': set([])}

one time it's inserted at end and another time it's at the beginning. this made me to think that i dont know the dictionary at all. where can i know the dictionary in depth. and how can iterate through a dictionary with keys as well as with indexes. for this i'm using now:

for lhs in left:
  print first[lhs]

this some how helps with indexing. but is there any other method?

Answer 1

Use collections.OrderedDict . It requires Python 2.7, but it remembers the order in which keys are added, rather than storing them in an arbitrary order based on the underlying hash algorithm.

Update: to be precise, the storage of the dict is unchanged, but the iteration is implemented with an additional data structure to provide a fixed order based on the original insertion order of the keys.