Use dask map_blocks() in _get_array()

This includes:
- Extracting the `_get_array()` method so that it is now a function in the module and not a class method.
- Introduction of `NativeMSGFileHandler_make_dask_array_with_map_blocks()` method to utilize the dask `map_blocks()`.
- Introduction of a new method, namely `NativeMSGFileHandler._number_of_visir_channels` to facilitate testing and mock patching.
- Adapting the mock patches in tests accordingly.

satpy/readers/seviri_l1b_native.py

@@ -193,10 +193,27 @@ def __init__(self, filename, filename_info, filetype_info,
         # Available channels are known only after the header has been read
         self.header_type = get_native_header(has_archive_header(self.filename))
         self._read_header()
-        self.dask_array = da.from_array(self._get_array(), chunks=(CHUNK_SIZE,))
+        self._make_dask_array_with_map_blocks()
         self._read_trailer()
         self.image_boundaries = ImageBoundaries(self.header, self.trailer, self.mda)
 
+    def _make_dask_array_with_map_blocks(self):
+        """Makes the dask array using the ``da.map_blocks()`` functionality."""
+        dtype = self._get_data_dtype()
+        chunks = da.core.normalize_chunks(
+            "auto",
+            shape=(self.mda["number_of_lines"],),
+            dtype=dtype)
+        self.dask_array = da.map_blocks(
+            _get_array,
+            dtype=dtype,
+            chunks=chunks,
+            meta=np.zeros(1, dtype=dtype),
+            # The following will be passed as keyword arguments to the `_get_array()` function.
+            filename=self.filename,
+            hdr_size=self.header_type.itemsize
+        )
+
     @property
     def _repeat_cycle_duration(self):
         """Get repeat cycle duration from the trailer."""
@@ -266,21 +283,17 @@ def get_lrec(cols):
         # each pixel is 10-bits -> one line of data has 25% more bytes
         # than the number of columns suggest (10/8 = 1.25)
         visir_rec = get_lrec(int(self.mda["number_of_columns"] * 1.25))
-        number_of_visir_channels = len(
-            [s for s in self.mda["channel_list"] if not s == "HRV"])
-        drec = [("visir", (visir_rec, number_of_visir_channels))]
+        drec = [("visir", (visir_rec, self._number_of_visir_channels()))]
 
         if self.mda["available_channels"]["HRV"]:
             hrv_rec = get_lrec(int(self.mda["hrv_number_of_columns"] * 1.25))
             drec.append(("hrv", (hrv_rec, 3)))
 
         return np.dtype(drec)
 
-    def _get_array(self):
-        """Get the numpy array for the SEVIRI data."""
-        data_dtype = self._get_data_dtype()
-        hdr_size = self.header_type.itemsize
-        return fromfile(self.filename, dtype=data_dtype, offset=hdr_size, count=self.mda["number_of_lines"])
+    def _number_of_visir_channels(self):
+        """Returns the number of visir channels, i.e. all channels excluding ``HRV``."""
+        return len([s for s in self.mda["channel_list"] if not s == "HRV"])
 
     def _read_header(self):
         """Read the header info."""
@@ -891,3 +904,14 @@ def read_header(filename):
     dtype = get_native_header(has_archive_header(filename))
     hdr = fromfile(filename, dtype=dtype, count=1)
     return recarray2dict(hdr)
+
+
+def _get_array(filename=None, hdr_size=None, block_info=None):
+    """Get the numpy array for the SEVIRI data."""
+    output_block_info = block_info[None]
+    data_dtype = output_block_info["dtype"]
+    return fromfile(
+        filename,
+        dtype=data_dtype,
+        offset=hdr_size + output_block_info["array-location"][0][0] * data_dtype.itemsize,
+        count=output_block_info["chunk-shape"][0])

satpy/tests/reader_tests/test_seviri_l1b_native.py

@@ -638,7 +638,9 @@ def prepare_area_definitions(test_dict):
 
     with mock.patch("satpy.readers.seviri_l1b_native.fromfile") as fromfile, \
             mock.patch("satpy.readers.seviri_l1b_native.recarray2dict") as recarray2dict, \
-            mock.patch("satpy.readers.seviri_l1b_native.NativeMSGFileHandler._get_array") as _get_array, \
+            mock.patch("satpy.readers.seviri_l1b_native._get_array") as _get_array, \
+            mock.patch(
+                "satpy.readers.seviri_l1b_native.NativeMSGFileHandler._number_of_visir_channels") as _n_visir_ch, \
             mock.patch("satpy.readers.seviri_l1b_native.NativeMSGFileHandler._read_trailer"), \
             mock.patch(
                 "satpy.readers.seviri_l1b_native.has_archive_header"
@@ -647,6 +649,7 @@ def prepare_area_definitions(test_dict):
         fromfile.return_value = header
         recarray2dict.side_effect = (lambda x: x)
         _get_array.return_value = np.arange(3)
+        _n_visir_ch.return_value = 11
         fh = NativeMSGFileHandler(filename=None, filename_info={}, filetype_info=None)
         fh.fill_disk = fill_disk
         fh.header = header
@@ -722,7 +725,9 @@ def prepare_is_roi(test_dict):
 
     with mock.patch("satpy.readers.seviri_l1b_native.fromfile") as fromfile, \
             mock.patch("satpy.readers.seviri_l1b_native.recarray2dict") as recarray2dict, \
-            mock.patch("satpy.readers.seviri_l1b_native.NativeMSGFileHandler._get_array") as _get_array, \
+            mock.patch("satpy.readers.seviri_l1b_native._get_array") as _get_array, \
+            mock.patch(
+                "satpy.readers.seviri_l1b_native.NativeMSGFileHandler._number_of_visir_channels") as _n_visir_ch, \
             mock.patch("satpy.readers.seviri_l1b_native.NativeMSGFileHandler._read_trailer"), \
             mock.patch(
                 "satpy.readers.seviri_l1b_native.has_archive_header"
@@ -731,6 +736,7 @@ def prepare_is_roi(test_dict):
         fromfile.return_value = header
         recarray2dict.side_effect = (lambda x: x)
         _get_array.return_value = np.arange(3)
+        _n_visir_ch.return_value = 11
         fh = NativeMSGFileHandler(filename=None, filename_info={}, filetype_info=None)
         fh.header = header
         fh.trailer = trailer
@@ -1172,12 +1178,15 @@ def test_header_type(file_content, exp_header_size):
         header.pop("15_SECONDARY_PRODUCT_HEADER")
     with mock.patch("satpy.readers.seviri_l1b_native.fromfile") as fromfile, \
             mock.patch("satpy.readers.seviri_l1b_native.recarray2dict") as recarray2dict, \
-            mock.patch("satpy.readers.seviri_l1b_native.NativeMSGFileHandler._get_array") as _get_array, \
+            mock.patch("satpy.readers.seviri_l1b_native._get_array") as _get_array, \
+            mock.patch(
+                "satpy.readers.seviri_l1b_native.NativeMSGFileHandler._number_of_visir_channels") as _n_visir_ch, \
             mock.patch("satpy.readers.seviri_l1b_native.NativeMSGFileHandler._read_trailer"), \
             mock.patch("satpy.readers.seviri_l1b_native.generic_open", mock.mock_open(read_data=file_content)):
         fromfile.return_value = header
         recarray2dict.side_effect = (lambda x: x)
         _get_array.return_value = np.arange(3)
+        _n_visir_ch.return_value = 11
         fh = NativeMSGFileHandler(filename=None, filename_info={}, filetype_info=None)
         assert fh.header_type.itemsize == exp_header_size
         assert "15_SECONDARY_PRODUCT_HEADER" in fh.header
@@ -1202,7 +1211,9 @@ def test_header_warning():
 
     with mock.patch("satpy.readers.seviri_l1b_native.fromfile") as fromfile, \
             mock.patch("satpy.readers.seviri_l1b_native.recarray2dict") as recarray2dict, \
-            mock.patch("satpy.readers.seviri_l1b_native.NativeMSGFileHandler._get_array") as _get_array, \
+            mock.patch("satpy.readers.seviri_l1b_native._get_array") as _get_array, \
+            mock.patch(
+                "satpy.readers.seviri_l1b_native.NativeMSGFileHandler._number_of_visir_channels") as _n_visir_ch, \
             mock.patch("satpy.readers.seviri_l1b_native.NativeMSGFileHandler._read_trailer"), \
             mock.patch("satpy.readers.seviri_l1b_native.generic_open", mock.mock_open(read_data=ASCII_STARTSWITH)):
         recarray2dict.side_effect = (lambda x: x)
@@ -1211,6 +1222,8 @@ def test_header_warning():
         exp_warning = "The quality flag for this file indicates not OK. Use this data with caution!"
 
         fromfile.return_value = header_good
+        _n_visir_ch.return_value = 11
+
         with warnings.catch_warnings():
             warnings.simplefilter("error")
             NativeMSGFileHandler(filename=None, filename_info={}, filetype_info=None)