[NumPy-Tickets] [NumPy] #1127: Make the dtype object immutable and not coerce other types when compared (hashable requirements) (was: dtype is shared across structured arrays copies)

[NumPy Trac]

#1127: Make the dtype object immutable and not coerce other types when compared
(hashable requirements)
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 Reporter:  faltet      |       Owner:  somebody      
     Type:  defect      |      Status:  needs_decision
 Priority:  normal      |   Milestone:  2.0.0         
Component:  numpy.core  |     Version:  devel         
 Keywords:              |  
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Changes (by mwiebe):

  * version:  => devel
  * milestone:  1.5.1 => 2.0.0


Comment:

 This is the PyArray_Descr object, which implements the __hash__ method.
 From http://docs.python.org/glossary.html, below is the definition of
 hashable. To be correct, the dtype object should be made immutable and
 should not coerce other types to dtype during comparisons.


 ==hashable==
 An object is hashable if it has a hash value which never changes during
 its lifetime (it needs a __hash__() method), and can be compared to other
 objects (it needs an __eq__() or __cmp__() method). Hashable objects which
 compare equal must have the same hash value.

 Hashability makes an object usable as a dictionary key and a set member,
 because these data structures use the hash value internally.

 All of Python’s immutable built-in objects are hashable, while no mutable
 containers (such as lists or dictionaries) are. Objects which are
 instances of user-defined classes are hashable by default; they all
 compare unequal, and their hash value is their id().

-- 
Ticket URL: <http://projects.scipy.org/numpy/ticket/1127#comment:4>
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