Write scalars and NDArray values chunk by chunk; cast whole chunks in asarray/astype (part 3/4 of #744)

doc/reference/ndarray.rst

@@ -14,7 +14,7 @@ It is a direct alias for ``array.vlmeta`` and uses the same persistent storage.
 .. autoclass:: NDArray
     :members:
     :inherited-members:
-    :exclude-members: get_slice, set_slice, get_slice_numpy, get_oindex_numpy, set_oindex_numpy
+    :exclude-members: get_slice, set_slice, get_slice_numpy, get_basic_numpy, get_basic_start_stop, get_oindex_numpy, set_oindex_numpy
     :member-order: groupwise
 
     :Special Methods:

src/blosc2/__init__.py

@@ -503,6 +503,8 @@ def _raise(exc):
 cpu_info = get_cpu_info()
 nthreads = ncores = cpu_info.get("count", 1)
 """Number of threads to be used in compression/decompression.
+
+It starts from the ``BLOSC_NTHREADS`` environment variable if that is set.
 """
 # Protection against too many threads
 nthreads = min(nthreads, 64)
@@ -515,14 +517,22 @@ def _raise(exc):
     # Experiments say that, when using a large number of threads, it is better to not use them all
     if nthreads > 16:
         nthreads -= nthreads // 8
-    # Only call set_num_threads if within NUMEXPR_MAX_THREADS limit to avoid warning
+    # C-Blosc2 uses BLOSC_NTHREADS, when set to a valid number, for every compression and
+    # decompression, whatever the nthreads in their parameters; start from it (as C-Blosc2 reads
+    # it), so that nthreads and the CParams/DParams defaults say how many threads are used
+    from .blosc2_ext import context_nthreads
+
+    nthreads = context_nthreads(nthreads)
+    # Only call set_num_threads if within NUMEXPR_MAX_THREADS limit to avoid warning, and not
+    # over the thread count that the user gave numexpr with NUMEXPR_NUM_THREADS
     numexpr_max_env = os.environ.get("NUMEXPR_MAX_THREADS")
     numexpr_max: int | None = None
     if numexpr_max_env is not None:
         with contextlib.suppress(ValueError):
             numexpr_max = int(numexpr_max_env)
-    if numexpr_max is None or nthreads <= numexpr_max:
-        numexpr.set_num_threads(nthreads)
+    if not os.environ.get("NUMEXPR_NUM_THREADS") and (numexpr_max is None or nthreads <= numexpr_max):
+        # BLOSC_NTHREADS can go beyond numexpr's own maximum, which it refuses with an error
+        numexpr.set_num_threads(min(nthreads, numexpr.MAX_THREADS))
 
 # This import must be before ndarray and schunk
 from .storage import (  # noqa: I001