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https://issues.apache.org/jira/browse/THRIFT-162?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12640778#action_12640778
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Esteve Fernandez commented on THRIFT-162:
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bq. From the Python docs: """If a class defines mutable objects and implements
a _cmp() or __eq() method, it should not implement __hash_(), since the
dictionary implementation requires that a key's hash value is immutable (if the
object's hash value changes, it will be in the wrong hash bucket)."""
Yes, I know, that's why I said that only immutable objects can have both
__eq__() and __hash__()
bq. Thrift structures are inherently mutable, so they should not define
_hash_(). Please use a map with simple types as keys if you are using Python.
No, using a map won't solve this problem. As I already said, dictionaries
(maps) are mutable and thus, can't be used as set elements. I'll either use a
tuple (i64, i64) or a tree-based set, such as ZODB's TreeSet
(http://www.zope.org/Wikis/ZODB/guide/node6.html#SECTION000630000000000000000),
it won't be O(1), though.
Anyway, would it be useful to add an immutable keyword to Thrift structures in
the IDL? This would make structures immutable (overriding __setattr__ in
Python, removing setters in Java, declaring attributes with attr_reader, but
not attr_writer in Ruby, etc.) I'm sure there are more implications, for
example, one would have to make struct members immutable as well, but would
like to know your opinions.
> Thrift structures are unhashable, preventing them from being used as set
> elements
> ---------------------------------------------------------------------------------
>
> Key: THRIFT-162
> URL: https://issues.apache.org/jira/browse/THRIFT-162
> Project: Thrift
> Issue Type: Bug
> Components: Compiler (Python), Library (Python)
> Reporter: Esteve Fernandez
> Priority: Minor
> Attachments: thrift_py_hash.patch
>
>
> Let Foo be a Thrift structure:
> struct Foo {
> 1: i32 bar
> }
> If you want to use it properly as a set element or a as a dictionary key, the
> autoegenerated Python code will complain about not being hashable:
> >>> f1 = Foo()
> >>> f1.bar = 1
> >>> f2 = Foo()
> >>> f2.bar = 1
> >>> f1 == f2
> True
> >>> set([f1]) & set([f2])
> set([])
> >>> d = {}
> >>> d[f1] = 2
> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> TypeError: unhashable instance
> Since Thrift structures already implement __eq__ and __ne__, they should
> implement __hash__ as well. The attached patch tries to mimic the behaviour
> of the Java compiler, including a HashCodeBuilder class written in Python.
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