dictionary - Why do python dictionaries change order?

Question

The order of objects stored in dictionaries in python3.5 changes over different executions of the interpreter, but it seems to stay the same for the same interpreter instance.

$ python3 <(printf 'print({"a": 1, "b": 2})
print({"a": 1, "b": 2})
print({"a": 1, "b": 2})
print({"a": 1, "b": 2})')
{'b': 2, 'a': 1}
{'b': 2, 'a': 1}
{'b': 2, 'a': 1}
{'b': 2, 'a': 1}
$ python3 <(printf 'print({"a": 1, "b": 2})
print({"a": 1, "b": 2})
print({"a": 1, "b": 2})
print({"a": 1, "b": 2})')
{'a': 1, 'b': 2}
{'a': 1, 'b': 2}
{'a': 1, 'b': 2}
{'a': 1, 'b': 2}

I always thought the order was based off of the hash of the key. Why is the order different between different executions of python?

Answer 1

Dictionaries use hash function, and the order is based on the hash of the key all right.

But, as stated somewhere in this Q&A, starting from python 3.3, the seed of the hash is randomly chosen at execution time (not to mention that it depends on the python versions) .

Note that as of Python 3.3, a random hash seed is used as well, making hash collisions unpredictable to prevent certain types of denial of service (where an attacker renders a Python server unresponsive by causing mass hash collisions). This means that the order of a given dictionary is then also dependent on the random hash seed for the current Python invocation.

So each time you execute your program, you may get a different order.

Since order of dictionaries are not guaranteed (not before python 3.6 anyway), this is an implementation detail that you shouldn't consider.