Support multiple readers in group_files and MultiScene.from_files

doc/source/multiscene.rst

@@ -231,3 +231,51 @@ multiple Scenes use:
     >>> mscn = MultiScene.from_files(glob('/data/abi/day_1/*C0[12]*.nc'), reader='abi_l1b')
     >>> mscn.load(['C01', 'C02'])
     >>> mscn.save_datasets(base_dir='/path/for/output')
+
+Combining multiple readers
+--------------------------
+
+.. versionadded:: 0.23
+
+The :meth:`~satpy.multiscene.MultiScene.from_files` constructor allows to
+automatically combine multiple readers into a single MultiScene.  It is no
+longer necessary for the user to create the :class:`~satpy.scene.Scene`
+objects themselves.  For example, you can combine Advanced Baseline
+Imager (ABI) and Global Lightning Mapper (GLM) measurements.
+Constructing a multi-reader MultiScene requires more parameters than a
+single-reader MultiScene, because Satpy can poorly guess how to group
+files belonging to different instruments.  For an example creating
+a video with lightning superimposed on ABI channel 14 (11.2 µm)
+using the built-in composite ``C14_flash_extent_density``,
+which superimposes flash extent density from GLM (read with the
+:class:`~satpy.readers.glm_l2.NCGriddedGLML2` or ``glm_l2`` reader) on ABI
+channel 14 data (read with the :class:`~satpy.readers.abi_l1b.NC_ABI_L1B`
+or ``abi_l1b`` reader), and therefore needs Scene objects that combine
+both readers:
+
+    >>> glm_dir = "/path/to/GLMC/"
+    >>> abi_dir = "/path/to/ABI/"
+    >>> ms = satpy.MultiScene.from_files(
+    ...        glob.glob(glm_dir + "OR_GLM-L2-GLMC-M3_G16_s202010418*.nc") +
+    ...        glob.glob(abi_dir + "C*/OR_ABI-L1b-RadC-M6C*_G16_s202010418*_e*_c*.nc"),
+    ...        reader=["glm_l2", "abi_l1b"],
+    ...        ensure_all_readers=True,
+    ...        group_keys=["start_time"],
+    ...        time_threshold=30)
+    >>> ms.load(["C14_flash_extent_density"])
+    >>> ms = ms.resample(ms.first_scene["C14"].attrs["area"])
+    >>> ms.save_animation("/path/for/output/{name:s}_{start_time:%Y%m%d_%H%M}.mp4")
+
+In this example, we pass to
+:meth:`~satpy.multiscene.MultiScene.from_files` the additional parameters
+``ensure_all_readers=True, group_keys=["start_time"], time_threshold=30``
+so we only get scenes at times that both ABI and GLM have a file starting
+within 30 seconds from each other, and ignore all other differences for
+the purposes of grouping the two.  For this example, the ABI files occur
+every 5 minutes but the GLM files (processed with glmtools) every minute.
+Scenes where there is a GLM file without an ABI file starting within at
+most ±30 seconds are skipped.  The ``group_keys`` and ``time_threshold``
+keyword arguments are processed by the :func:`~satpy.readers.group_files`
+function.  The heavy work of blending the two instruments together is
+performed by the :class:`~satpy.composites.BackgroundCompositor` class
+through the `"C14_flash_extent_density"` composite.

satpy/multiscene.py

@@ -174,18 +174,29 @@ def first_scene(self):
         return self._scene_gen.first
 
     @classmethod
-    def from_files(cls, files_to_sort, reader=None, **kwargs):
+    def from_files(cls, files_to_sort, reader=None,
+                   ensure_all_readers=False, **kwargs):
         """Create multiple Scene objects from multiple files.
 
+        Args:
+            files_to_sort (Collection[str]): files to read
+            reader (str or Collection[str]): reader or readers to use
+            ensure_all_readers (bool): If True, limit to scenes where all
+                readers have at least one file.  If False (default), include
+                all scenes where at least one reader has at least one file.
+
         This uses the :func:`satpy.readers.group_files` function to group
-        files. See this function for more details on possible keyword
-        arguments.
+        files. See this function for more details on additional possible
+        keyword arguments.  In particular, it is strongly recommended to pass
+        `"group_keys"` when using multiple instruments.
 
         .. versionadded:: 0.12
 
         """
         from satpy.readers import group_files
         file_groups = group_files(files_to_sort, reader=reader, **kwargs)
+        if ensure_all_readers:
+            file_groups = [fg for fg in file_groups if all(fg.values())]
         scenes = (Scene(filenames=fg) for fg in file_groups)
         return cls(scenes)
 

satpy/readers/__init__.py

@@ -20,6 +20,7 @@
 import logging
 import numbers
 import os
+import warnings
 from datetime import datetime, timedelta
 
 import yaml
@@ -392,9 +393,9 @@ def group_files(files_to_sort, reader=None, time_threshold=10,
 
     Args:
         files_to_sort (iterable): File paths to sort in to group
-        reader (str): Reader whose file patterns should be used to sort files.
-            This is currently a required keyword argument, but may be optional
-            in the future (see inline code comments for details).
+        reader (str or Collection[str]): Reader or readers whose file patterns
+            should be used to sort files.  If not given, try all readers (slow,
+            adding a list of readers is strongly recommended).
         time_threshold (int): Number of seconds used to consider time elements
             in a group as being equal. For example, if the 'start_time' item
             is used to group files then any time within `time_threshold`
@@ -407,7 +408,9 @@ def group_files(files_to_sort, reader=None, time_threshold=10,
             the first key in ``group_keys``. Otherwise, there is a good chance
             that files will not be grouped properly (datetimes being barely
             unequal). Defaults to a reader's ``group_keys`` configuration (set
-            in YAML), otherwise ``('start_time',)``.
+            in YAML), otherwise ``('start_time',)``.  When passing multiple
+            readers, passing group_keys is strongly recommended as the
+            behaviour without doing so is undefined.
         ppp_config_dir (str): Root usser configuration directory for Satpy.
             This will be deprecated in the future, but is here for consistency
             with other Satpy features.
@@ -420,40 +423,131 @@ def group_files(files_to_sort, reader=None, time_threshold=10,
         a `Scene` object.
 
     """
-    # FUTURE: Find the best reader for each filename using `find_files_and_readers`
-    if reader is None:
-        raise ValueError("'reader' keyword argument is required.")
-    elif not isinstance(reader, (list, tuple)):
+
+    if reader is not None and not isinstance(reader, (list, tuple)):
         reader = [reader]
 
-    # FUTURE: Handle multiple readers
-    reader = reader[0]
-    reader_configs = list(configs_for_reader(reader, ppp_config_dir))[0]
     reader_kwargs = reader_kwargs or {}
-    try:
-        reader_instance = load_reader(reader_configs, **reader_kwargs)
-    except (KeyError, IOError, yaml.YAMLError) as err:
-        LOG.info('Cannot use %s', str(reader_configs))
-        LOG.debug(str(err))
-        # if reader and (isinstance(reader, str) or len(reader) == 1):
-        #     # if it is a single reader then give a more usable error
-        #     raise
-        raise
-
-    if group_keys is None:
-        group_keys = reader_instance.info.get('group_keys', ('start_time',))
-    file_keys = []
-    # make a copy because filename_items_for_filetype will modify inplace
+
+    reader_files = _assign_files_to_readers(
+            files_to_sort, reader, ppp_config_dir, reader_kwargs)
+
+    if reader is None:
+        reader = reader_files.keys()
+
+    file_keys = _get_file_keys_for_reader_files(
+            reader_files, group_keys=group_keys)
+
+    file_groups = _get_sorted_file_groups(file_keys, time_threshold)
+
+    return [{rn: file_groups[group_key].get(rn, []) for rn in reader} for group_key in file_groups]
+
+
+def _assign_files_to_readers(files_to_sort, reader_names, ppp_config_dir,
+                             reader_kwargs):
+    """Assign files to readers.
+
+    Given a list of file names (paths), match those to reader instances.
+
+    Internal helper for group_files.
+
+    Args:
+        files_to_sort (Collection[str]): Files to assign to readers.
+        reader_names (Collection[str]): Readers to consider
+        ppp_config_dir (str):
+        reader_kwargs (Mapping):
+
+    Returns:
+        Mapping[str, Tuple[reader, Set[str]]]
+        Mapping where the keys are reader names and the values are tuples of
+        (reader_configs, filenames).
+    """
+
     files_to_sort = set(files_to_sort)
-    for _, filetype_info in reader_instance.sorted_filetype_items():
-        for f, file_info in reader_instance.filename_items_for_filetype(files_to_sort, filetype_info):
-            group_key = tuple(file_info.get(k) for k in group_keys)
-            file_keys.append((group_key, f))
+    reader_dict = {}
+    for reader_configs in configs_for_reader(reader_names, ppp_config_dir):
+        try:
+            reader = load_reader(reader_configs, **reader_kwargs)
+        except yaml.constructor.ConstructorError:
+            LOG.exception(
+                    f"ConstructorError loading {reader_configs!s}, "
+                    "probably a missing dependency, skipping "
+                    "corresponding reader (if you did not explicitly "
+                    "specify the reader, Satpy tries all; performance "
+                    "will improve if you pass readers explicitly).")
+            continue
+        reader_name = reader.info["name"]
+        files_matching = set(reader.filter_selected_filenames(files_to_sort))
+        files_to_sort -= files_matching
+        if files_matching or reader_names is not None:
+            reader_dict[reader_name] = (reader, files_matching)
+    if files_to_sort:
+        raise ValueError("No matching readers found for these files: " +
+                         ", ".join(files_to_sort))
+    return reader_dict
+
+
+def _get_file_keys_for_reader_files(reader_files, group_keys=None):
+    """From a mapping from _assign_files_to_readers, get file keys.
 
+    Given a mapping where each key is a reader name and each value is a
+    tuple of reader instance (typically FileYAMLReader) and a collection
+    of files, return a mapping with the same keys, but where the values are
+    lists of tuples of (keys, filename), where keys are extracted from the filenames
+    according to group_keys and filenames are the names those keys were
+    extracted from.
+
+    Internal helper for group_files.
+
+    Returns:
+        Mapping[str, List[Tuple[Tuple, str]]], as described.
+    """
+
+    file_keys = {}
+    for (reader_name, (reader_instance, files_to_sort)) in reader_files.items():
+        if group_keys is None:
+            group_keys = reader_instance.info.get('group_keys', ('start_time',))
+        file_keys[reader_name] = []
+        # make a copy because filename_items_for_filetype will modify inplace
+        files_to_sort = set(files_to_sort)
+        for _, filetype_info in reader_instance.sorted_filetype_items():
+            for f, file_info in reader_instance.filename_items_for_filetype(files_to_sort, filetype_info):
+                group_key = tuple(file_info.get(k) for k in group_keys)
+                if all(g is None for g in group_key):
+                    warnings.warn(
+                            f"Found matching file {f:s} for reader "
+                            "{reader_name:s}, but none of group keys found. "
+                            "Group keys requested: " + ", ".join(group_keys),
+                            UserWarning)
+                file_keys[reader_name].append((group_key, f))
+    return file_keys
+
+
+def _get_sorted_file_groups(all_file_keys, time_threshold):
+    """Get sorted file groups.
+
+    Get a list of dictionaries, where each list item consists of a dictionary
+    mapping a tuple of keys to a mapping of reader names to files.  The files
+    listed in each list item are considered to be grouped within the same time.
+
+    Args:
+        all_file_keys, as returned by _get_file_keys_for_reader_files
+        time_threshold: temporal threshold
+
+    Returns:
+        List[Mapping[Tuple, Mapping[str, List[str]]]], as described
+
+    Internal helper for group_files.
+    """
+    # flatten to get an overall sorting; put the name in the middle in the
+    # interest of sorting
+    flat_keys = ((v[0], rn, v[1]) for (rn, vL) in all_file_keys.items() for v in vL)
     prev_key = None
     threshold = timedelta(seconds=time_threshold)
+    # file_groups is sorted, because dictionaries are sorted by insertion
+    # order in Python 3.7+
     file_groups = {}
-    for gk, f in sorted(file_keys):
+    for gk, rn, f in sorted(flat_keys):
         # use first element of key as time identifier (if datetime type)
         if prev_key is None:
             is_new_group = True
@@ -471,13 +565,14 @@ def group_files(files_to_sort, reader=None, time_threshold=10,
                           if this_val is not None and prev_val is not None)
         # if this is a new group based on the first element
         if is_new_group or any(vals_not_equal):
-            file_groups[gk] = [f]
+            file_groups[gk] = {rn: [f]}
             prev_key = gk
         else:
-            file_groups[prev_key].append(f)
-    sorted_group_keys = sorted(file_groups)
-    # passable to Scene as 'filenames'
-    return [{reader: file_groups[group_key]} for group_key in sorted_group_keys]
+            if rn not in file_groups[prev_key]:
+                file_groups[prev_key][rn] = [f]
+            else:
+                file_groups[prev_key][rn].append(f)
+    return file_groups
 
 
 def read_reader_config(config_files, loader=UnsafeLoader):

satpy/tests/test_multiscene.py

@@ -124,20 +124,57 @@ def test_properties(self):
     def test_from_files(self):
         """Test creating a multiscene from multiple files."""
         from satpy import MultiScene
-        input_files = [
+        input_files_abi = [
             "OR_ABI-L1b-RadC-M3C01_G16_s20171171502203_e20171171504576_c20171171505018.nc",
             "OR_ABI-L1b-RadC-M3C01_G16_s20171171507203_e20171171509576_c20171171510018.nc",
             "OR_ABI-L1b-RadC-M3C01_G16_s20171171512203_e20171171514576_c20171171515017.nc",
             "OR_ABI-L1b-RadC-M3C01_G16_s20171171517203_e20171171519577_c20171171520019.nc",
             "OR_ABI-L1b-RadC-M3C01_G16_s20171171522203_e20171171524576_c20171171525020.nc",
             "OR_ABI-L1b-RadC-M3C01_G16_s20171171527203_e20171171529576_c20171171530017.nc",
+            ]
+        input_files_glm = [
+            "OR_GLM-L2-GLMC-M3_G16_s20171171500000_e20171171501000_c20380190314080.nc",
+            "OR_GLM-L2-GLMC-M3_G16_s20171171501000_e20171171502000_c20380190314080.nc",
+            "OR_GLM-L2-GLMC-M3_G16_s20171171502000_e20171171503000_c20380190314080.nc",
+            "OR_GLM-L2-GLMC-M3_G16_s20171171503000_e20171171504000_c20380190314080.nc",
+            "OR_GLM-L2-GLMC-M3_G16_s20171171504000_e20171171505000_c20380190314080.nc",
+            "OR_GLM-L2-GLMC-M3_G16_s20171171505000_e20171171506000_c20380190314080.nc",
+            "OR_GLM-L2-GLMC-M3_G16_s20171171506000_e20171171507000_c20380190314080.nc",
+            "OR_GLM-L2-GLMC-M3_G16_s20171171507000_e20171171508000_c20380190314080.nc",
         ]
         with mock.patch('satpy.multiscene.Scene') as scn_mock:
-            mscn = MultiScene.from_files(input_files, reader='abi_l1b')
-            self.assertEqual(len(mscn.scenes), 6)
-            calls = [mock.call(filenames={'abi_l1b': [in_file]}) for in_file in input_files]
+            mscn = MultiScene.from_files(
+                    input_files_abi,
+                    reader='abi_l1b')
+            assert len(mscn.scenes) == 6
+            calls = [mock.call(
+                filenames={'abi_l1b': [in_file_abi]})
+                for in_file_abi in input_files_abi]
             scn_mock.assert_has_calls(calls)
 
+            scn_mock.reset_mock()
+            mscn = MultiScene.from_files(
+                    input_files_abi + input_files_glm,
+                    reader=('abi_l1b', "glm_l2"),
+                    group_keys=["start_time"],
+                    ensure_all_readers=True,
+                    time_threshold=30)
+            assert len(mscn.scenes) == 2
+            calls = [mock.call(
+                filenames={'abi_l1b': [in_file_abi], 'glm_l2': [in_file_glm]})
+                for (in_file_abi, in_file_glm) in
+                zip(input_files_abi[0:2],
+                    [input_files_glm[2]] + [input_files_glm[7]])]
+            scn_mock.assert_has_calls(calls)
+            scn_mock.reset_mock()
+            mscn = MultiScene.from_files(
+                    input_files_abi + input_files_glm,
+                    reader=('abi_l1b', "glm_l2"),
+                    group_keys=["start_time"],
+                    ensure_all_readers=False,
+                    time_threshold=30)
+            assert len(mscn.scenes) == 12
+
     def test_group(self):
         from satpy import Scene, MultiScene, DatasetID