Add a basic tutorial

doc/source/changelog.rst

@@ -3,7 +3,11 @@
 Changelog
 =========
 
-0.2.0 (DD-MM-YYYY)
+0.2.1 (DD-MM-YYYY)
+------------------
+* Add a basic tutorial (:pr:`13`)
+
+0.2.0 (11-09-2024)
 ------------------
 
 * Initial release

doc/source/conf.py

@@ -24,6 +24,8 @@
   "sphinx.ext.doctest",
   "sphinx.ext.napoleon",
   "sphinx.ext.intersphinx",
+  "IPython.sphinxext.ipython_directive",
+  "IPython.sphinxext.ipython_console_highlighting",
 ]
 
 templates_path = ["_templates"]
@@ -52,8 +54,8 @@
 html_static_path = ["_static"]
 
 extlinks = {
-  "issue": ("https://github.com/ratt-ru/xarray-ms/issues/%s", "GH#"),
-  "pr": ("https://github.com/ratt-ru/xarray-ms/pull/%s", "GH#"),
+  "issue": ("https://github.com/ratt-ru/xarray-ms/issues/%s", "GH%s"),
+  "pr": ("https://github.com/ratt-ru/xarray-ms/pull/%s", "PR%s"),
 }
 
 # Example configuration for intersphinx: refer to the Python standard library.
@@ -63,3 +65,12 @@
   "python": ("https://docs.python.org/3/", None),
   "xarray": ("https://docs.xarray.dev/en/stable", None),
 }
+
+# Exclude link file
+exclude_patterns = ["_build", "links.rst"]
+
+# make rst_epilog a variable, so you can add other epilog parts to it
+rst_epilog = ""
+# Read link all targets from file
+with open("links.rst") as f:
+  rst_epilog += f.read()

doc/source/index.rst

@@ -12,5 +12,6 @@ xarray-ms documentation
 
    readme
    install
+   tutorial
    api
    changelog

doc/source/links.rst

@@ -0,0 +1,19 @@
+..
+  This file should not be included in the standard project
+  but is included in the rst_epilog in conf.py
+
+.. _SKAO: https://www.skao.int/
+.. _NRAO: https://public.nrao.edu/
+.. _msv2-spec: https://casa.nrao.edu/Memos/229.html
+.. _msv4-spec: https://docs.google.com/spreadsheets/d/14a6qMap9M5r_vjpLnaBKxsR9TF4azN5LVdOxLacOX-s/
+.. _xradio: https://github.com/casangi/xradio
+.. _dask-ms: https://github.com/ratt-ru/dask-ms
+.. _arcae: https://github.com/ratt-ru/arcae
+.. _dask: https://www.dask.org/
+.. _python-casacore: https://github.com/casacore/python-casacore/
+.. _xarray: https://xarray.dev/
+.. _xarray_backend: https://docs.xarray.dev/en/stable/internals/how-to-add-new-backend.html
+.. _xarray_lazy: https://docs.xarray.dev/en/latest/internals/internal-design.html#lazy-indexing-classes
+.. _xarray_indexing_and_selecting: https://docs.xarray.dev/en/latest/user-guide/indexing.html
+.. _xarray_chunked_arrays: https://docs.xarray.dev/en/latest/internals/chunked-arrays.html
+.. _zarr: https://zarr.dev/

doc/source/tutorial.rst

@@ -0,0 +1,97 @@
+Tutorial
+========
+
+The `Measurement Set v2.0 <msv2-spec_>`_ is a tabular format that
+includes notions of regularity or, the shape of the data, in the MAIN table.
+This is accomplished through the ``DATA_DESC_ID`` column which defines the
+Spectral Window and Polarisation Configuration associated with each row:
+the shape of the visibility in each row of the ``DATA`` column can
+vary per-row.
+
+By contrast `Measurement Set v4.0 <msv4-spec_>`_ specifies a
+collection of Datasets of ndarrays on a regular grid.
+To move data between the two formats, it is necessary to partition
+or group MSv2 rows by the same shape and configuration.
+
+In xarray-ms, this is accomplished by specifying ``partition_columns``
+when opening a Measurement Set.
+Different columns may be used to define the partition, but
+:code:`[DATA_DESC_ID, FIELD_ID, OBSERVATION_ID]` is a reasonable choice.
+
+Opening a Measurement Set
+-------------------------
+
+As xarray-ms implements an `xarray backend <xarray_backend_>`_,
+it is possible to use the standard :func:`xarray.open_dataset`
+to open up a single partition of a Measurement Set.
+
+.. ipython:: python
+  :okwarning:
+
+  import xarray_ms
+  from xarray_ms.testing.simulator import simulate
+  import xarray
+
+  # Simulate a Measurement Set with 3
+  # channel and polarisation configurations
+  ms = simulate("test.ms", data_description=[
+    (8, ("XX", "XY", "YX", "YY")),
+    (4, ("RR", "LL")),
+    (16, ("RR", "RL", "LR", "LL"))])
+
+  ds = xarray.open_dataset(ms,
+    partition_columns=["DATA_DESC_ID", "FIELD_ID", "OBSERVATION_ID"])
+
+  ds
+
+Opening a specific partition
+++++++++++++++++++++++++++++++
+
+Because we've simulated multiple Data Description values in
+our Measurement Set, xarray-ms has automatically opened the first partition
+containing 8 frequencies and 4 linear polarisations.
+To open the second partition a ``partition_key`` can be also be
+passed to :func:`xarray.open_dataset`.
+
+.. ipython:: python
+
+  ds = xarray.open_dataset(ms,
+    partition_columns=["DATA_DESC_ID", "FIELD_ID", "OBSERVATION_ID"],
+    partition_key=(("DATA_DESC_ID", 1), ("FIELD_ID", 0), ("OBSERVATION_ID", 0)))
+
+  ds
+
+and it can be seen that the dataset refers to the second partition
+containing 4 frequencies and 2 circular polarisations.
+
+Selecting a subset of the data
+++++++++++++++++++++++++++++++
+
+By default, :func:`xarray.open_dataset` will return a dataset
+with a lazy view over the data.
+xarray has extensive functionality for
+`indexing and selecting data <xarray_indexing_and_selecting_>`_.
+
+For example, one could select select some specific dimensions out:
+
+.. ipython:: python
+
+  ds = xarray.open_dataset(ms,
+    partition_columns=["DATA_DESC_ID", "FIELD_ID", "OBSERVATION_ID"],
+    partition_key=(("DATA_DESC_ID", 1), ("FIELD_ID", 0), ("OBSERVATION_ID", 0)))
+
+  subds = ds.isel(time=slice(1, 3), baseline=[1, 3, 5], frequency=slice(2, 4))
+  subds
+
+At this point, the dataset is still lazy -- no Data variables have been loaded
+into memory.
+
+Loading in a lazy dataset
++++++++++++++++++++++++++
+
+By calling load on the lazy dataset, all the Data Variables are loaded onto the
+dataset as numpy arrays.
+
+.. ipython:: python
+
+  subds.load()

pyproject.toml

@@ -37,6 +37,7 @@ sphinx = "^8.0.2"
 pygments = "^2.18.0"
 sphinx-copybutton = "^0.5.2"
 pydata-sphinx-theme = "^0.15.4"
+ipython = "^8.27.0"
 
 [tool.ruff]
 line-length = 88

tests/conftest.py

@@ -4,9 +4,7 @@
 
 from xarray_ms.backend.msv2.structure import MSv2StructureFactory
 from xarray_ms.backend.msv2.table_factory import TableFactory
-from xarray_ms.testing.simulator import MSStructureSimulator
-
-DEFAULT_SIM_PARAMS = {"ntime": 5, "data_description": [(8, ["XX", "XY", "YX", "YY"])]}
+from xarray_ms.testing.simulator import DEFAULT_SIM_PARAMS, MSStructureSimulator
 
 
 @pytest.fixture(autouse=True)

xarray_ms/backend/msv2/array.py

@@ -14,6 +14,11 @@
 
 
 def slice_length(s, max_len):
+  if isinstance(s, np.ndarray):
+    if s.ndim != 1:
+      raise NotImplementedError("Slicing with non-1D numpy arrays")
+    return len(s)
+
   start, stop, step = s.indices(max_len)
   if step != 1:
     raise NotImplementedError(f"Slicing with steps {s} other than 1 not supported")

xarray_ms/testing/simulator.py

@@ -1,4 +1,6 @@
 import dataclasses
+import os
+import tempfile
 import typing
 from typing import (
   Any,
@@ -17,6 +19,8 @@
 # First of February 2023
 FIRST_FEB_2023_MJDS = 2459976.50000 * 86400
 
+# Default simulation parameters
+DEFAULT_SIM_PARAMS = {"ntime": 5, "data_description": [(8, ["XX", "XY", "YX", "YY"])]}
 
 # Additional Columns to add
 ADDITIONAL_COLUMNS = {
@@ -383,3 +387,16 @@ def data_factory(
       )
 
     return {column: (dims, data) for column, dims, data in np_arrays}
+
+
+def simulate(name=None, **sim_params) -> str:
+  """
+  Create a Measurement Set in a temporary directory,
+  with the given simulation parameters.
+  Return the directory
+  """
+  tmpdir = tempfile.mkdtemp()
+  ms_path = os.path.join(tmpdir, name or "simulated.ms")
+  simulator = MSStructureSimulator(**{**DEFAULT_SIM_PARAMS, **sim_params})
+  simulator.simulate_ms(ms_path)
+  return ms_path