dualfroz
518ab0a093
Defining __eq__ without __hash__ sets __hash__ to None in Python 3, so Delorean instances were unhashable and could not be used in sets or as dict keys (unlike datetime). Hash on self.epoch, the same value __eq__ compares, so equal instances hash equal.
66 lines
1.9 KiB
Markdown
66 lines
1.9 KiB
Markdown
# Make `Delorean` hashable again
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## Problem
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`Delorean` instances cannot be put in a `set` or used as `dict` keys:
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```python
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>>> from datetime import datetime
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>>> from delorean import Delorean
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>>> d = Delorean(datetime(2015, 1, 1), timezone="UTC")
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>>> hash(d)
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Traceback (most recent call last):
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...
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TypeError: unhashable type: 'Delorean'
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>>> {d}
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TypeError: unhashable type: 'Delorean'
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```
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This is surprising for a date/time value object: the standard library's
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`datetime.datetime` is hashable, and `Delorean` supports equality and ordering,
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so grouping instances in sets or keying dictionaries by them is a natural thing
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to want.
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## Root cause
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`delorean/dates.py` defines `__eq__` (and the rich-comparison operators) on
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`Delorean`, but does not define `__hash__`. In Python 3, defining `__eq__`
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without `__hash__` sets `__hash__` to `None`, which makes the class unhashable:
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```python
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>>> Delorean.__hash__ is None
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True
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```
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Under Python 2 (the version delorean originally targeted) defining `__eq__`
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left the inherited `__hash__` in place, so this is effectively a regression
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introduced by the move to Python 3.
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## Fix
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Add a `__hash__` consistent with the existing `__eq__`. `__eq__` compares
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`self.epoch == other.epoch`, so hashing on `self.epoch` guarantees the hash
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invariant (equal objects hash equal), including two instances describing the
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same instant in different timezones:
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```python
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def __hash__(self):
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return hash(self.epoch)
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```
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## Test
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Added two tests to `tests/delorean_tests.py`:
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- `test_hashable`: `hash()` works and an instance can be stored in a set and
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used as a dict key.
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- `test_hash_consistent_with_equality`: two equal instances (same instant,
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different timezone) hash equal and collapse to one element in a set.
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## Gate
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- `python -m pytest tests/` -> 232 passed, 44 subtests passed.
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Counterfactual (fix reverted, tests kept): both new tests fail with
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`TypeError: unhashable type: 'Delorean'`.
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