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Make the dtype object immutable and not coerce other types when compared (hashable requirements) (Trac #1127) #1725
Description
Original ticket http://projects.scipy.org/numpy/ticket/1127 on 2009-06-05 by @FrancescAlted, assigned to unknown.
The dtype object seems to be shared after a copy of a structured array is done. This is inconvenient when you change properties in one of arrays and these are propagated to the other.
The next snippet exposes the problem:
In [21]: dt = np.dtype('f4,f8')
In [22]: ra = np.empty(1, dt)
In [23]: ra.dtype
Out[23]: dtype([('f0', '<f4'), ('f1', '<f8')])
In [24]: rb = ra[:]
In [25]: ra.dtype.names = ('float', 'double')
In [26]: ra.dtype
Out[26]: dtype([('float', '<f4'), ('double', '<f8')])
In [27]: rb.dtype
Out[27]: dtype([('float', '<f4'), ('double', '<f8')]) # not the original!
Activity
thouis commentedon Oct 19, 2012
Milestone changed to
1.4.0by @cournape on 2009-11-25thouis commentedon Oct 19, 2012
@mwiebe wrote on 2011-03-24
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().
thouis commentedon Oct 19, 2012
Milestone changed to
NumPy 2.0by @mwiebe on 2011-03-24thouis commentedon Oct 19, 2012
Title changed from
dtype is shared across structured arrays copiestoMake the dtype object immutable and not coerce other types when compared (hashable requirements)by @mwiebe on 2011-03-24thouis commentedon Oct 19, 2012
@mwiebe wrote on 2011-05-23
The dtype pickling also needs to change, because you can't have a setstate function on an immutable object.
charris commentedon Feb 18, 2014
Still in progress.