Reading and writing Table and EventTable objects

Note

This document complements the upstream Astropy documentation on reading/writing Table objects, please refer to Reading and Writing Table Objects.

Astropy provides an excellent unified input/output system for the Table object, and GWpy extends upon that to include common gravitational-wave file types, as well as providing event-specific input/output registrations for event data. In the most general case you can read a table of data as follows:

Reading an EventTable from an ASCII file
>>> from gwpy.table import EventTable
>>> table = EventTable.read('mydata.txt')

EventTable.read() will attempt to automatically identify the file format based on the file extension and/or the contents of the file, however, the format keyword argument can be used to manually identify the input file-format.

The read() and write() methods take different arguments and keywords based on the input/output file format, see Accessing GravitySpy events for details on reading/writing for each of the built-in formats.

Accessing Open Data event catalogues

The Gravitational-Wave Open Science Centre (GWOSC) publishes the Gravitational-Wave Transient Event Catalogues (GWTCs), allowing public access to tables of gravitational wave events and their parameters.

GWpy’s EventTable class comes with a fetch_open_data() method that queries the GWOSC database to download data for the relevant catalogue.

Simple queries

The simplest query just requires the catalogue name, and will return all parameters for all events in the catalogue:

Fetching the GWTC-2 catalogue using EventTable.fetch_open_data() (output correct as of Jul 19, 2024)
>>> from gwpy.table import EventTable
>>> events = EventTable.fetch_open_data("GWTC-2")
>>> print(events)
       name        final_mass_source_lower ... mass_2_source_upper mass_1_source_upper
                                           ...
------------------ ----------------------- ... ------------------- -------------------
GW190408_181802-v1                    -2.8 ...                 3.3                 5.1
       GW190412-v3                    -3.8 ...                 1.6                 4.7
GW190413_052954-v1                    -9.2 ...                 7.3                12.6
GW190413_134308-v1                   -11.4 ...                11.7                13.5
GW190421_213856-v1                    -8.7 ...                 8.0                10.4
GW190424_180648-v1                   -10.1 ...                 7.6                11.1
       GW190425-v2                    None ...                 0.3                 0.6
GW190426_152155-v1                    None ...                 0.8                 3.9
GW190503_185404-v1                    -7.7 ...                 7.7                 9.2
GW190512_180714-v1                    -3.5 ...                 3.6                 5.3
GW190513_205428-v1                    -5.8 ...                 7.7                 9.5
GW190514_065416-v1                   -10.4 ...                 9.3                14.7
GW190517_055101-v1                    -8.9 ...                 7.0                11.7
GW190519_153544-v1                   -13.8 ...                11.0                10.7
       GW190521-v3                   -22.4 ...                22.7                28.7
GW190521_074359-v1                    -4.4 ...                 5.4                 5.9
GW190527_092055-v1                    -9.3 ...                10.5                16.4
GW190602_175927-v1                   -14.9 ...                14.3                15.7
GW190620_030421-v1                   -12.1 ...                12.2                16.0
GW190630_185205-v1                    -4.6 ...                 5.2                 6.9
GW190701_203306-v1                    -8.9 ...                 8.7                11.8
GW190706_222641-v1                   -13.5 ...                14.6                14.6
GW190707_093326-v1                    -1.3 ...                 1.4                 3.3
GW190708_232457-v1                    -1.8 ...                 2.0                 4.7
GW190719_215514-v1                   -10.2 ...                 9.0                18.0
GW190720_000836-v1                    -2.2 ...                 2.3                 6.7
GW190727_060333-v1                    -7.5 ...                 7.1                 9.5
GW190728_064510-v1                    -1.3 ...                 1.7                 7.2
GW190731_140936-v1                   -10.8 ...                 9.7                12.2
GW190803_022701-v1                    -8.5 ...                 7.8                10.6
       GW190814-v2                    -0.9 ...                0.08                 1.1
GW190828_063405-v1                    -4.3 ...                 4.6                 5.8
GW190828_065509-v1                    -4.5 ...                 3.6                 7.0
GW190909_114149-v1                   -16.8 ...                13.4                52.7
GW190910_112807-v1                    -8.6 ...                 6.3                 7.6
GW190915_235702-v1                    -6.0 ...                 5.6                 9.5
GW190924_021846-v1                    -1.0 ...                 1.4                 7.0
GW190929_012149-v1                   -25.3 ...                19.3                33.0
GW190930_133541-v1                    -1.5 ...                 1.7                12.4

The full list of available columns can be queried as follows:

Printing the columns of an EventTable (output correct as of Jul 19, 2024)
>>> print(events.info)
<EventTable masked=True length=39>
              name                dtype    unit            description
-------------------------------- ------- ------- --------------------------------
                            name   str18
         final_mass_source_lower  object                  final_mass_source_lower
         chirp_mass_source_lower float64                  chirp_mass_source_lower
         total_mass_source_upper float64                  total_mass_source_upper
                chirp_mass_lower float64                         chirp_mass_lower
...

GWOSC catalogues

For more details on the GWOSC catalogues, see https://gwosc.org/eventapi/html/.

Filtered queries

The columns returned can be selected using the column keyword, and mathematical selection filters can be applied on-the-fly using the selection keyword:

Downloading a sub-set of a catalogue from GWOSC (output correct as of Jul 19, 2024)
>>> t = EventTable.fetch_open_data(
...     "GWTC-2",
...     selection="mass_1_source < 4",
...     columns=["name", "mass_1_source", "mass_2_source", "luminosity_distance"],
... )
>>> print(t)
    name    mass_1_source mass_2_source luminosity_distance
               solMass       solMass            Mpc
----------- ------------- ------------- -------------------
GW190425-v2           2.0           1.4               160.0

More on filtering an EventTable

For more information on filtering, see Filtering tables.

Accessing GravitySpy events

Gravity Spy is a citizen-science project that enables the public to characterize and classify glitches in IGWN detector data. The GravitySpyTable subclass of EventTable provides methods to query GravitySpy for various tables of classified events.

Full database queries

The GravitySpyTable.fetch() method (inherited directly from EventTable) enables querying the Gravity Spy database directly:

>>> from gwpy.table import GravitySpyTable
>>> blips = GravitySpyTable.fetch(
...     "gravityspy",
...     "glitches",
...     selection="Label=Blip",
... )

Warning

Login credentials are required to support this query. IGWN members with LIGO.ORG credentials can find the required credentials at https://secrets.ligo.org/secrets/144/.

Similarity searches

The GravitySpyTable.search() method enables performing a Similarity Search given the ID of a Gravity Spy event:

>>> from gwpy.table import GravitySpyTable
>>> similar = GravitySpyTable.search("8FHTgA8MEu", howmany=5)
>>> print(similar)
ifo  peak_frequency  links_subjects ml_label searchedID ...
--- ---------------- -------------- -------- ---------- ...
 H1 84.4759674072266      5740011.0 Scratchy 8FHTgA8MEu ...
 L1   128.8896484375     20892636.0 Scratchy 8FHTgA8MEu ...
 L1 73.4049224853516     20892632.0 Scratchy 8FHTgA8MEu ...
 L1 75.5168914794922     20892526.0 Scratchy 8FHTgA8MEu ...
 L1 144.991333007812      8644242.0 Scratchy 8FHTgA8MEu ...

This has download 5 similarly Scratchy glitches from the LIGO-Hanford ('H1') and LIGO-Livingston ('L1') observatories.

Built-in file formats

GWpy extends the Astropy functionality with readers for the following file formats:

Each of the sub-sections below outlines how to read and write in these file formats, include the custom keyword arguments to pass to EventTable.read() and EventTable.write().

Listing all available formats

To list all available formats, consult the documentation for EventTable.read().

Multi-processed file reading

The EventTable.read() method accepts the nproc keyword argument, allowing multi-processed reading of lists of files. This argument can be used with any file-format, not just those defined below, but is not backported to for use with Table.read().

LIGO_LW XML

Additional dependencies: python-ligo-lw

The LIGO Scientific Collaboration uses a custom scheme of XML in which to store tabular data, called LIGO_LW. Complementing the scheme is a python library - python-ligo-lw - which allows users to read and write all of the different types of tabular data produced by gravitational-wave searches.

Reading and writing tables in LIGO_LW XML format is supported with format='ligolw', tablename=<tablename>' where <tablename> can be any of the supported LSC table names (see ligo.lw.lsctables).

Reading

When reading, the tablename keyword argument should be given to identify the table in the file, as follows:

Reading an EventTable from LIGO_LW XML.
>>> t = EventTable.read('H1-LDAS_STRAIN-968654552-10.xml.gz', tablename='sngl_burst')

The result should be something similar to this:

Result of reading an EventTable from LIGO_LW XML.
>>> print(t)
ifo peak_time peak_time_ns start_time ... confidence chisq chisq_dof bandwidth
--- --------- ------------ ---------- ... ---------- ----- --------- ---------
 H1 968654557    783203126  968654557 ...  16.811825   0.0     512.0     256.0
 H1 968654557    781250001  968654557 ...  16.816761   0.0     512.0     256.0
 H1 968654557    779296876  968654557 ...  16.696106   0.0     512.0     256.0
 H1 968654557    777343751  968654557 ...  16.739489   0.0     512.0     256.0
 H1 968654557    775390626  968654557 ...  16.802326   0.0     512.0     256.0
 H1 968654557    773437501  968654557 ...   16.30731   0.0     512.0     256.0
 H1 968654557    771484376  968654557 ...  16.307253   0.0     512.0     256.0
 H1 968654557    769531251  968654557 ...   16.35647   0.0     512.0     256.0
 H1 968654557    767578126  968654557 ...  16.561176   0.0     512.0     256.0
 H1 968654557    765625001  968654557 ...  16.393112   0.0     512.0     256.0
 H1 968654557    763671876  968654557 ...  16.404041   0.0     512.0     256.0
 H1 968654557    761718751  968654557 ...  16.405825   0.0     512.0     256.0
 H1 968654557    759765626  968654557 ...  16.715092   0.0     512.0     256.0
 H1 968654557    757812501  968654557 ...  17.512749   0.0     512.0     256.0
 H1 968654557    755859376  968654557 ...  17.347675   0.0     512.0     256.0
 H1 968654557    753906251  968654557 ...  17.077478   0.0     512.0     256.0
 H1 968654557    751953126  968654557 ...  16.742907   0.0     512.0     256.0
...       ...          ...        ... ...        ...   ...       ...       ...
 H1 968654560    342773438  968654559 ...  11.029792   0.0      16.0       8.0
 H1 968654559    280273438  968654558 ...  12.363036   0.0      16.0       8.0
 H1 968654559    217773438  968654558 ...  13.985101   0.0      16.0       8.0
 H1 968654559    155273438  968654558 ...  14.662391   0.0      16.0       8.0
 H1 968654559     92773438  968654558 ...  15.864924   0.0      16.0       8.0
 H1 968654559     30273438  968654558 ...  16.321821   0.0      16.0       8.0
 H1 968654558    967773438  968654558 ...  16.975931   0.0      16.0       8.0
 H1 968654558    905273438  968654558 ...  19.160393   0.0      16.0       8.0
 H1 968654560    811523438  968654560 ...  11.270205   0.0       8.0       4.0
 H1 968654560    686523438  968654560 ...  15.839205   0.0       8.0       4.0
 H1 968654560    561523438  968654560 ...  15.944695   0.0       8.0       4.0
 H1 968654560    436523438  968654559 ...  14.384432   0.0       8.0       4.0
 H1 968654560    311523438  968654559 ...  14.465309   0.0       8.0       4.0
 H1 968654560    186523438  968654559 ...  13.045853   0.0       8.0       4.0
 H1 968654560    561523438  968654560 ...  11.636543   0.0       4.0       4.0
 H1 968654560    436523438  968654560 ...  15.344837   0.0       4.0       4.0
 H1 968654560    311523438  968654560 ...  11.367717   0.0       4.0       4.0
Length = 2052 rows

Single-table files

The tablename keyword can be omitted if there is only a single table in the file.

To restrict the columns returned in the new EventTable, use the columns keyword argument:

Reading specific columns into an EventTable from LIGO_LW XML.
>>> t = EventTable.read(
...     "H1-LDAS_STRAIN-968654552-10.xml.gz",
...     tablename="sngl_burst",
...     columns=["peak_time", "peak_time_ns", "snr", "peak_frequency"],
... )

Many LIGO_LW table objects (as defined in ligo.lw.lsctables) include utility functions to create new columns by combining others, e.g. to calculate the Q of a sine-Gaussian pulse from the duration and central frequency. These ‘columns’ can be requested directly, providing the ligo.lw.table.Table representation of the data has a get_ method for that name:

Reading get_ columns into an EventTable from LIGO_LW XML.
>>> t = EventTable.read(
...     "H1-LDAS_STRAIN-968654552-10.xml.gz",
...     tablename="sngl_burst",
...     columns=["snr", "q", "duration", "central_freq"],
... )

Note

When reading a processed column in this manner, all required input columns for a processed column must be included in the columns keyword list. To exclude these columns from the returned data, use the ligolw_columns= keyword to specify the columns required to provide the output columns:

>>> t = EventTable.read(
...     "H1-LDAS_STRAIN-968654552-10.xml.gz",
...     tablename="sngl_burst",
...     columns=["snr", "q"],
...     ligolw_columns=["snr", "duration", "central_freq"],
... )

By default, the returned EventTable uses the dtypes returned by the ligol.lw library, and functions therein, which often end up as numpy.object_ arrays in the table. To force all columns to have real numpy data types, use the use_numpy_dtypes=True keyword, which will cast (known) custom object types to a standard numpy.dtype, e.g:

Example of using use_numpy_dtypes=True when reading an EventTable from LIGO_LW XML.
>>> t = EventTable.read(
...     "H1-LDAS_STRAIN-968654552-10.xml.gz",
...     tablename="sngl_burst",
...     columns=["peak"],
...     ligolw_columns=["peak_time", "peak_time_ns"])
>>> print(type(t[0]["peak"]))
<type 'lal.LIGOTimeGPS'>
>>> t = EventTable.read(
...     "H1-LDAS_STRAIN-968654552-10.xml.gz",
...     tablename="sngl_burst",
...     columns=["peak"],
...     ligolw_columns=["peak_time", "peak_time_ns"],
...     use_numpy_dtypes=True)
>>> print(type(t[0]["peak"]))
<type 'numpy.float64'>

Writing

A table can be written as follows:

>>> t.write('new-table.xml', format='ligolw', tablename='sngl_burst')

Because LIGO_LW isn’t the only scheme of XML, the format keyword is required for all Table.write() operations.

If the target file already exists, an IOError will be raised, use overwrite=True to force a new file to be written.

To write a table to an existing file, use append=True:

>>> t.write('new-table.xml', format='ligolw', tablename='sngl_burst', append=True)

To replace an existing table of the given type in an existing file, while preserving other tables, use both append=True and overwrite=True:

>>> t.write('new-table.xml', format='ligolw', tablename='sngl_burst', append=True, overwrite=True)

Note

The python-ligo-lw library reads and writes files using an updated version of the LIGO_LW format compared to glue.ligolw used to. GWpy should support both format versions natively when _reading_, but only supports writing using the updated format.

Coherence WaveBurst ASCII (aka EVENTS.txt)

coherent WaveBurst (cWB) is an analysis pipeline is used to detect generic gravitational-wave bursts, without using a signal model to restrict the analysis, and runs in both low-latency (online) and offline modes over current GWO data. The analysis uses the ROOT framework for most data products, but also produces ASCII data in a custom format commonly written in a file called EVENTS.txt.

Reading

To read a cWB ASCII file:

Reading an EventTable from cWB-format ASCII.
>>> t = EventTable.read('EVENTS.txt', format='ascii.cwb')

See the astropy.io.ascii.read() documentation for full details on keyword arguments when reading ascii.cwb files.

Writing

To write a table using the cWB ASCII format:

Writing an EventTable to a cWB-format ASCII file.
>>> t.write('EVENTS.txt', format='ascii.cwb')

[the output file name is not required to be 'EVENTS.txt', this is simply the convention used in the cWB analysis.]

ROOT

Additional dependencies: uproot

Reading

To read a ROOT tree into a Table (or EventTable), specify the relevant tree via the treename keyword argument:

Reading an EventTable from a ROOT file.
>>> t = Table.read('my-data.root', treename='triggers')

If treename=None is given (default), a single tree will be read if only one exists in the file, otherwise a ValueError will be raised.

Any other keyword arguments will be passed directly to uproot.tree.TTreeMethods.arrays().

Writing

To write a Table as a ROOT tree:

Writing an EventTable to a ROOT file.
>>> t.write('new-table.root')

As with reading, the treename keyword argument can be used to specify the tree, the default is treename='tree'.

By default, an existing file with an existing tree of the given name will be appended to, to overwrite use the mode='recreate' keyword argument:

Writing an EventTable over an existing tree in a ROOT file.
>>> t.write('new-table.root', treename='triggers', mode='recreate')

Any other keyword arguments will be passed directly to uproot.newtree.

PyCBC Live (HDF5)

PyCBC Live is a low-latency search for gravitational waves from compact binary coalescences, built from the PyCBC analysis package. This search writes files on the LIGO Data Grid (LIGO.ORG-authenticated users only) in HDF5 format, containing tables of events; each column in the table is recorded as a separate HDF5 Dataset.

Identifying

PyCBC Live HDF5 files are identified automatically if the file is identified as an HDF5 file (see astropy.io.misc.hdf5.is_hdf5()) and the file _name_ matches the following regular expression:

([A-Z][0-9])+-Live-[0-9.]+-[0-9.]+.(h5|hdf|hdf5)

e.g.

H1-Live-1234567890-4.h5

If the format cannot be auto-identifed (because the filename doesn’t match the above regular expression), the format can be specified via

format="hdf5.pycbc_live"

Reading

To read an EventTable from a pycbc_live format HDF5 file:

Reading an EventTable from a PyCBC-Live HDF5 file.
>>> t = EventTable.read("H1-Live-1234567890-4.h5")

To restrict the returned columns, use the columns keyword argument:

Reading specific columns into an EventTable from PyCBC-Live HDF5.
>>> t = EventTable.read(
...     "H1-MY_DATA-1234567890-4.h5",
...     format="hdf5.pycbc_live",
...     columns=["end_time", "snr", "chisq"],
... )

Similarly to the LIGO_LW XML format, some processed columns can be specified that are not included in the HDF5 files, but are created on-the-fly. Supported processed columns are:

  • mchirp

  • new_snr

These can be specified without having to specify any of the input columns.

Additionally, PyCBC HDF5 table Groups include extra datasets that aren’t part of the table, e.g. 'psd'. These can be included in the returned EventTable.meta dict via the keyword extended_metadata=True (default), or excluded with extended_metadata=False.

Writing

Writing tables in PyCBC Live HDF5 format is not supported at this time.

GstLAL (LIGO_LW XML)

GstLAL is a low-latency search for gravitational waves from compact binary coalescences, built using |GStreamer|_ elements and tools from the LALSuite library. This search writes files on the LIGO Data Grid (LIGO.ORG-authenticated users only) in LIGO_LW XML format, containing tables of events.

Reading

To read an EventTable from a gstlal format LIGO_LW XML file, use the format='ligolw.gstlal' keyword:

Reading an EventTable from a GstLAL LIGO_LW XML file.
>>> t = EventTable.read("H1L1-GstLAL-1234567890-4.xml.gz", format="ligolw.gstlal")

GstLAL LIGO_LW XML files contain information about triggers from each detector separately as well as from a combination of detectors. Accessing these different sets of information can be done using the triggers keyword. By default, information about triggers from each detector separately is read in. This is equivalent to using triggers='sngl'. To instead read information about triggers from multiple detectors, you can instead use the triggers='coinc' keyword:

Reading coincident triggers into an EventTable GstLAL LIGO_LW XML HDF5.
>>> t = EventTable.read(
...     "H1L1-GstLAL-1234567890-4.xml.gz",
...     format="gstlal.gstlal",
...     triggers='coinc',
... )

To restrict the returned columns, use the columns keyword argument:

Reading specific columns into an EventTable GstLAL LIGO_LW XML HDF5.
>>> t = EventTable.read(
...     "H1L1-GstLAL-1234567890-4.xml.gz",
...     format="gstlal.gstlal",
...     columns=["end_time", "snr", "chisq"],
... )

Similarly to the LIGO_LW XML format, some processed columns can be specified that are not included in the XML files, but are created on-the-fly. In addition to processed columns support by LIGO_LW XML, the additional supported processed columns are:

  • mchirp

  • snr_chi

  • chi_snr

These can be specified without having to specify any of the input columns.

Writing

Writing tables in GstLAL LIGO_LW XML format is not supported at this time.

SNAX (HDF5)

The SNAX (Signal-based Noise Acquisition and eXtraction) analysis pipeline is a low-latency search for identifying glitches in h(t) and auxiliary channel data using glitch waveforms, operating in low-latency (online) and offline modes. This search writes files on the LIGO Data Grid (LIGO.ORG-authenticated users only) in HDF5 format containing regularly-sampled features; each channel in the table is recorded as a separate HDF5 Dataset.

Reading

To read an EventTable from a snax format HDF5 file, use the format='hdf5.snax' keyword:

Reading an EventTable from a SNAX-format HDF5 file.
>>> t = EventTable.read("H-SNAX_FEATURES-1255853400-20.h5", format="hdf5.snax")

By default, all channels contained in the file are read in. To restrict the returned channels, use the channels keyword argument:

Reading specific channels into an EventTable from a SNAX-format HDF5 file.
>>> t = EventTable.read(
...     "H-SNAX_FEATURES-1255853400-20.h5",
...     format="hdf5.snax",
...     channels="H1:CAL-DELTAL_EXTERNAL_DQ",
... )

To restrict the returned columns, use the columns keyword argument:

Reading specific columns into an EventTable from SNAX-format HDF5.
>>> t = EventTable.read(
...     "H-GSTLAL_IDQ_FEATURES-1255853400-20.h5",
...     format="hdf5.snax",
...     columns=["channel", "time", "snr"],
... )

Writing

Writing tables in SNAX HDF5 format is not supported at this time.

GWF

Additional dependencies: FrameCPP

The Gravitational-Wave Frame file format supports tabular data via FrEvent structures, which allow storage of arbitrary event information.

Reading

To read an EventTable from a GWF-format file, specify the filename and the name of the FrEvent structures to read:

Reading an EventTable from GWF.
>>> t = EventTable.read('events.gwf', 'my-event-name')

To restrict the returned columns, use the columns keyword argument:

Reading specific columns into an EventTable from GWF.
>>> t = EventTable.read('events.gwf', 'my-event-name', columns=['time', 'amplitude'])

All FrEvent structures contain the following columns, any other columns are use-specific:

Column name

Description (from LIGO-T970130)

time

Reference time of event, as defined by the search algorithm

amplitude

Continuous output amplitude returned by event

probability

Likelihood estimate of event, if available (probability = -1 if cannot be estimated)

timeBefore

Signal duration before reference time (seconds)

timeAfter

Signal duration after reference time (seconds)

comment

Descriptor of event

Writing

Writing tables in GWF format is not supported at this time.