BUG: Fix concatenation when the output is "S" or "U" #18063
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Previously, the dtype was used, this now assumes that we want to cast to a string of (unknown) length. This is a simplified version of what happens in `np.array()` or `arr.astype()` (it does never inspect the values, e.g. for object casts). This is more complex as I would like, and with the refactor of ResultType and similar can be cleaned up a bit more hopefully. Note that currently, object to "S" or "U" casts simply return length 64 strings, but with the new version, this will be an error (although the error message probably needs improvement). This is a behaviour inherited from other places however.
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Backport of #18052.
Previously, the dtype was used, this now assumes that we want to
cast to a string of (unknown) length. This is a simplified version
of what happens in
np.array()orarr.astype()(it does neverinspect the values, e.g. for object casts).
This is more complex as I would like, and with the refactor of
ResultType and similar can be cleaned up a bit more hopefully.
Note that currently, object to "S" or "U" casts simply return
length 64 strings, but with the new version, this will be an error
(although the error message probably needs improvement).
This is a behaviour inherited from other places however.
The issue here is that NumPy 1.20 is just broken if you pass
dtype="U"in concatenate. Unfortunately, this is about as "minimal" as I could think of (some things around it should be cleaned up also in the new code paths). Things are simply fairly complicated if you have "flexible dtypes" or DType (type/class) in my way of thinking about it...There is of course more inanity here, e.g.
concatenateusesresult_typewhich uses value-based casting for 0-D objects, but there is not much to do about it right at this instance.