BUG: preserve trailing NULs for StringDType in append/pad/isin/select - #2
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np.append, np.pad (constant mode), np.isin, and np.select are pure-Python wrappers that convert scalar/sequence arguments with np.asarray/np.ravel/np.broadcast_arrays before the target array's dtype is known. When the argument is a bare Python str (or np.str_, or a sequence of these) and the other operand is a StringDType array, this premature conversion infers fixed-width unicode (U dtype), which silently strips trailing NUL characters before the value ever reaches StringDType-aware code. Fix each function by detecting when the result would be StringDType and, in that case, converting the raw (non-ndarray) operand directly against that dtype instead of through the lossy default inference, following the same pattern already used for np.strings.replace/ partition/rpartition (numpygh-32040). isin() is fixed symmetrically (element vs. test_elements), which also transparently fixes setdiff1d for the same reason. select() keeps its existing NEP-50 result_type detection unchanged, since passing a raw str directly into np.result_type raises rather than promoting it like int/float/complex do; the fix re-derives raw choices/default after the StringDType result is detected. Closes numpy#32431
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np.append, np.pad (constant mode), np.isin, and np.select are pure-Python wrappers that convert scalar/sequence arguments with np.asarray/np.ravel/np.broadcast_arrays before the target array's dtype is known. When the argument is a bare Python str (or np.str_, or a sequence of these) and the other operand is a StringDType array, this premature conversion infers fixed-width unicode (U dtype), which silently strips trailing NUL characters before the value ever reaches StringDType-aware code.
Fix each function by detecting when the result would be StringDType and, in that case, converting the raw (non-ndarray) operand directly against that dtype instead of through the lossy default inference, following the same pattern already used for np.strings.replace/ partition/rpartition (numpygh-32040).
isin() is fixed symmetrically (element vs. test_elements), which also transparently fixes setdiff1d for the same reason. select() keeps its existing NEP-50 result_type detection unchanged, since passing a raw str directly into np.result_type raises rather than promoting it like int/float/complex do; the fix re-derives raw choices/default after the StringDType result is detected.
Closes numpy#32431
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