CODEWITHSUNDEEP
pythonarraysnumpydictionary
JZ_42
JZ_42

Reputation: 615

Python - Alternative for using numpy array as key in dictionary

I'm pretty new to Python numpy. I attempted to use the numpy array as the key in the dictionary in one of my functions and then was told by the Python interpreter that the numpy array is not hashable. I've just found out that one way to work this issue around is to use repr() function to convert a numpy array to a string but it seems very expensive. Is there any better way to achieve the same effect?

Update: I could create a new class to contain the numpy array, which seems to be the right way to achieve what I want. Just wondering if there is any better method.

update 2: Using a class to contain data in the array and then override __hash__ function is acceptable, however, I'd prefer the solution provided by @hpaulj. Converting the array/list to a tuple fits my need in a better way as it does not require an additional class.

Upvotes: 36

Views: 26543

Answers (3)

Lurosset
Lurosset

Reputation: 37

I just ran into that issue and there's a very simple solution to it using list comprehension:

import numpy as np

dict = {'key1':1, 'key2':2}
my_array = np.array(['key1', 'key2'])

result = np.array( [dict[element] for element in my_array] )
print(result)

The result should be:

[1 2]

I don't know how efficient this is but seems like a very practical and straight-forward solution, no conversions or new classes needed :)

Upvotes: -3

Peter Olson
Peter Olson

Reputation: 906

If you want to quickly store a numpy.ndarray as a key in a dictionary, a fast option is to use ndarray.tobytes() which will return a raw python bytes string which is immutable

my_array = numpy.arange(4).reshape((2,2))
my_dict = {}
my_dict[my_array.tobytes()] = None

Upvotes: 37

JZ_42
JZ_42

Reputation: 615

After done some researches and reading through all comments. I think I've known the answer to my own question so I'd just write them down.

  1. Write a class to contain the data in the array and then override __hash__ function to amend the way how it is hashed as mentioned by ZdaR
  2. Convert this array to a tuple, which makes the list hashable instantaneously.Thanks to hpaulj

I'd prefer method No.2 because it fits my need better, as well as simpler. However, using a class might bring some additional benefits so it could also be useful.

Upvotes: 13

Related Questions