Is there a better way to create a dictionary key in python than converting to string?

Question

I have a function that returns an energy associated with a vector in 3D space, F(i,j,k).

F() is a slow function, so I have a dictionary that checks if F() has been called for a specific i,j,k, and if it has then I just take that value from the dictionary.

I'm currently doing this as:

key = "_".join( [ str(i), str(j), str(k)  ])

if key not in dic: 
    dic[key] = F(i,j,k)

Energy = dic[key]

Is there a better way to create a key than by joining the vectors like this?

For future reference, that first line can be simplified using map, to key = "_".join(map(str, (i, j, k))) — jonrsharpe
– jonrsharpe, Commented Jun 23, 2014 at 9:44

Martijn Pieters · Accepted Answer · 2014-06-23 09:52:48Z

8

Dictionary keys can be anything hashable; a tuple of the 3 values does fine:

key = i, j, k  # a comma makes this a tuple
try:
    Energy = dic[key]
except KeyError:
    Energy = dic[key] = F(i, j, k)

I'm using exception handling here (asking for forgiveness) rather than explicitly testing if the key is present (asking for permission); the former is faster when the absence of the key is the key is the norm.

edited Jun 23, 2014 at 9:52

answered Jun 23, 2014 at 9:41

Martijn Pieters

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6 Comments

egoburnswell Over a year ago

I was using shelve to store my dictionaries. Apparently this won't work with tuples as keys. Is pickle the next best alternative?

Martijn Pieters Over a year ago

@egoburnswell: shelve uses pickle as the implementation. Tuple keys should work just fine with shelve; what is the error you see?

egoburnswell Over a year ago

'TypeError: String or Integer object expected for key, tuple found'

Martijn Pieters Over a year ago

@egoburnswell: My apologies, I indeed see that shelve is documented as only accepting strings as keys. That's because the values are pickled, but these are then stored in a DBM-style file, and it is that format that limits your keys.

Martijn Pieters Over a year ago

@egoburnswell: Either store your dictionary nested in shelve (shelveobj['data'] = yourdict, make sure to reassign to that key on changes) or use only Pickle, in that case.

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jbaiter · Accepted Answer · 2014-06-23 09:42:25Z

1

Since Python dicts can take any hashable object as keys[1], you can just use the (i, j, k) tuple:

if (i, j, k) not in dic:
    dic[(i, j, k)] = F(i,j,k)
Energy = dic[(i, j, k)]

[1] https://docs.python.org/2/library/stdtypes.html#mapping-types-dict

answered Jun 23, 2014 at 9:42

jbaiter

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