diff options
| -rw-r--r-- | Doc/library/functions.rst | 13 | ||||
| -rw-r--r-- | Doc/reference/datamodel.rst | 19 | ||||
| -rw-r--r-- | Misc/NEWS | 3 |
3 files changed, 27 insertions, 8 deletions
diff --git a/Doc/library/functions.rst b/Doc/library/functions.rst index 3d239ec..a37d496 100644 --- a/Doc/library/functions.rst +++ b/Doc/library/functions.rst @@ -587,11 +587,16 @@ are always available. They are listed here in alphabetical order. .. function:: hash(object) - Return the hash value of the object (if it has one). Hash values are integers. - They are used to quickly compare dictionary keys during a dictionary lookup. - Numeric values that compare equal have the same hash value (even if they are of - different types, as is the case for 1 and 1.0). + Return the hash value of the object (if it has one). Hash values are + integers. They are used to quickly compare dictionary keys during a + dictionary lookup. Numeric values that compare equal have the same hash + value (even if they are of different types, as is the case for 1 and 1.0). + .. note:: + + For object's with custom :meth:`__hash__` methods, note that :func:`hash` + truncates the return value based on the bit width of the host machine. + See :meth:`__hash__` for details. .. function:: help([object]) diff --git a/Doc/reference/datamodel.rst b/Doc/reference/datamodel.rst index 4f9b8b0..1a48c1f 100644 --- a/Doc/reference/datamodel.rst +++ b/Doc/reference/datamodel.rst @@ -1264,10 +1264,21 @@ Basic customization Called by built-in function :func:`hash` and for operations on members of hashed collections including :class:`set`, :class:`frozenset`, and - :class:`dict`. :meth:`__hash__` should return an integer. The only required - property is that objects which compare equal have the same hash value; it is - advised to somehow mix together (e.g. using exclusive or) the hash values for - the components of the object that also play a part in comparison of objects. + :class:`dict`. :meth:`__hash__` should return an integer. The only + required property is that objects which compare equal have the same hash + value; it is advised to somehow mix together (e.g. using exclusive or) the + hash values for the components of the object that also play a part in + comparison of objects. + + .. note:: + + :func:`hash` truncates the value returned from an object's custom + :meth:`__hash__` method to the size of a :c:type:`Py_ssize_t`. This is + typically 8 bytes on 64-bit builds and 4 bytes on 32-bit builds. If an + object's :meth:`__hash__` must interoperate on builds of different bit + sizes, be sure to check the width on all supported builds. An easy way + to do this is with + ``python -c "import sys; print(sys.hash_info.width)"`` If a class does not define an :meth:`__eq__` method it should not define a :meth:`__hash__` operation either; if it defines :meth:`__eq__` but not @@ -581,6 +581,9 @@ Tests Documentation ------------- +- Issue #18440: Clarify that `hash()` can truncate the value returned from an + object's custom `__hash__()` method. + - Issue #17844: Add links to encoders and decoders for bytes-to-bytes codecs. - Issue #14097: improve the "introduction" page of the tutorial. |