When Did Dictionary Ordering in Python Become Non-Deterministic?

Non-Deterministic Dictionary Ordering in Python: An Explanation

Dictionary ordering in Python has witnessed a significant change between versions 2.7 and 3.3. In Python 2.7, the order of dictionary keys remained consistent and arbitrary, while in Python 3.3, the ordering has become unpredictable. This non-determinism has raised questions about the underlying reason and how it affects the behavior of Python dictionaries.

The Source of Non-Determinism

The non-deterministic nature of dictionary ordering in Python 3.3 stems from a security fix implemented in 2012, activated by default in Python 3.3. This security measure introduced "hash randomization," a process that makes the iteration order of dictionaries unpredictable to prevent security vulnerabilities.

Explanation of Hash Randomization

Hash randomization involves altering the hash function used to determine the position of elements within a dictionary. By altering the hash function, the order in which keys are stored and retrieved becomes unpredictable and differs across Python runs. This added layer of unpredictability enhances the security of Python applications by making it more difficult for malicious actors to exploit predictable hash functions.

Implications for Dictionary Ordering

The introduction of hash randomization has significant implications for the ordering of dictionary keys. Applications that rely on the ordering of dictionary keys may encounter unexpected behavior in Python 3.3 and subsequent versions. For instance, the order in which variables are displayed using vars() can vary between runs.

Consistent Ordering in Select Cases

Despite the non-deterministic ordering in Python 3.3, certain cases still exhibit consistent ordering. For example, the ordering of keys in a dictionary constructed using a list comprehension remains predictable because the order of keys in the original list is preserved.

Disable Hash Randomization (Optional)

In older versions of Python affected by hash randomization, it is possible to disable it by setting the PYTHONHASHSEED environment variable to 0. However, this is not advised as it reduces the security benefits of hash randomization.

Future Updates

In Python 3.6, a new implementation of dict was introduced that preserves insertion order. Additionally, in Python 3.7, this order-preserving behavior is guaranteed, ensuring consistent dictionary ordering in those versions and beyond.

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