ASCII Tables (`astropy.io.ascii`)#

Introduction#

astropy.io.ascii provides methods for reading and writing a wide range of ASCII data table formats via built-in Extension Reader Classes. The emphasis is on flexibility and convenience of use, although readers can optionally use a less flexible C-based engine for reading and writing for improved performance. This subpackage was originally developed as asciitable.

The following shows a few of the ASCII formats that are available, while the section on Supported formats contains the full list.

Basic: basic table with customizable delimiters and header configurations
Cds: CDS format table (also Vizier)
Daophot: table from the IRAF DAOphot package
Ecsv: ECSV Format for lossless round-trip of data tables (recommended)
FixedWidth: table with fixed-width columns (see also Fixed-Width Gallery)
Ipac: IPAC format table
HTML: HTML format table contained in a <table> tag
Latex: LaTeX table with datavalue in the tabular environment
Mrt: AAS Machine-Readable Tables (MRT))
SExtractor: SExtractor format table

The strength of astropy.io.ascii is the support for astronomy-specific formats (often with metadata) and specialized data types such as SkyCoord, Time, and Quantity. For reading or writing large data tables in a generic format such as CSV, using the Table - Pandas interface is an option to consider.

Note

It is strongly encouraged to use the functionality from astropy.io.ascii through a higher level interface in the Data Tables package. See Unified File Read/Write Interface for more details.

Getting Started#

Reading Tables#

The majority of commonly encountered ASCII tables can be read with the read() function. Assume you have a file named sources.dat with the following contents:

obsid redshift  X      Y     object
3102  0.32      4167  4085   Q1250+568-A
877   0.22      4378  3892   "Source 82"

This table can be read with the following:

>>> from astropy.io import ascii
>>> data = ascii.read("sources.dat")  
>>> print(data)                       
obsid redshift  X    Y      object
----- -------- ---- ---- -----------
 3102     0.32 4167 4085 Q1250+568-A
  877     0.22 4378 3892   Source 82

The first argument to the read() function can be the name of a file, a string representation of a table, or a list of table lines. The return value (data in this case) is a Table object.

By default, read() will try to guess the table format by trying all of the supported formats.

Warning

Guessing the file format is often slow for large files because the reader tries parsing the file with every allowed format until one succeeds. For large files it is recommended to disable guessing with guess=False.

If guessing the format does not work, as in the case for unusually formatted tables, you may need to give astropy.io.ascii additional hints about the format.

To specify specific data types for one or more columns, use the converters argument (see Converters for Specifying Dtype for details). For instance if the obsid is actually a string identifier (instead of an integer) you can read the table with the code below. This also illustrates using the preferred Table interface for reading:

>>> from astropy.table import Table
>>> sources = """
... target observatory obsid
... TW_Hya Chandra     22178
... MP_Mus XMM         0406030101"""
>>> data = Table.read(sources, format='ascii', converters={'obsid': str})
>>> data
<Table length=2>
target observatory   obsid
 str6      str7      str10
------ ----------- ----------
TW_Hya     Chandra      22178
MP_Mus         XMM 0406030101

Writing Tables#

The write() function provides a way to write a data table as a formatted ASCII table. Most of the input table Supported Formats for reading are also available for writing. This provides a great deal of flexibility in the format for writing.

The following shows how to write a formatted ASCII table using the write() function:

>>> import numpy as np
>>> from astropy.io import ascii
>>> from astropy.table import Table
>>> data = Table()
>>> data['x'] = np.array([1, 2, 3], dtype=np.int32)
>>> data['y'] = data['x'] ** 2
>>> ascii.write(data, 'values.dat', overwrite=True)

The values.dat file will then contain:

x y
1
4
9

It is also possible and encouraged to use the write functionality from astropy.io.ascii through a higher level interface in the Data Tables package (see Unified File Read/Write Interface for more details). For example:

>>> data.write('values.dat', format='ascii', overwrite=True)

Attention

ECSV is recommended

For a reproducible ASCII version of your table, we recommend using the ECSV Format. This stores all the table meta-data (in particular the column types and units) to a comment section at the beginning while maintaining compatibility with most plain CSV readers. It also allows storing richer data like SkyCoord or multidimensional or variable-length columns. ECSV is also supported in Java by STIL and TOPCAT (see ECSV Format).

To write our simple example table to ECSV we use:

>>> data.write('values.ecsv', overwrite=True)  

The .ecsv extension is recognized and implies using ECSV (equivalent to format='ascii.ecsv'). The values.ecsv file will then contain:

# %ECSV 1.0
# ---
# datatype:
# - {name: x, datatype: int32}
# - {name: y, datatype: int32}
# schema: astropy-2.0
x y
1 1
2 4
3 9

Supported Formats#

A full list of the supported format values and corresponding format types for ASCII tables is given below. The Write column indicates which formats support write functionality, and the Fast column indicates which formats are compatible with the fast Cython/C engine for reading and writing.

Format	Write	Fast	Description
`aastex`	Yes		`AASTex`: AASTeX deluxetable used for AAS journals
`basic`	Yes	Yes	`Basic`: Basic table with custom delimiters
`cds`	Yes		`Cds`: CDS format table
`commented_header`	Yes	Yes	`CommentedHeader`: Column names in a commented line
`csv`	Yes	Yes	`Csv`: Basic table with comma-separated values
`daophot`			`Daophot`: IRAF DAOphot format table
`ecsv`	Yes		`Ecsv`: Enhanced CSV format (recommended)
`fixed_width`	Yes		`FixedWidth`: Fixed width
`fixed_width_no_header`	Yes		`FixedWidthNoHeader`: Fixed-width with no header
`fixed_width_two_line`	Yes		`FixedWidthTwoLine`: Fixed-width with second header line
`html`	Yes		`HTML`: HTML format table
`ipac`	Yes		`Ipac`: IPAC format table
`latex`	Yes		`Latex`: LaTeX table
`mrt`	Yes		`Mrt`: AAS Machine-Readable Table format
`no_header`	Yes	Yes	`NoHeader`: Basic table with no headers
`qdp`	Yes		`QDP`: Quick and Dandy Plotter files
`rdb`	Yes	Yes	`Rdb`: Tab-separated with a type definition header line
`rst`	Yes		`RST`: reStructuredText simple format table
`sextractor`			`SExtractor`: SExtractor format table
`tab`	Yes	Yes	`Tab`: Basic table with tab-separated values

Using `astropy.io.ascii`#

The details of using astropy.io.ascii are provided in the following sections:

Performance Tips#

By default, when trying to read a file the reader will guess the format, which involves trying to read it with many different readers. For better performance when dealing with large tables, it is recommended to specify the format and any options explicitly, and turn off guessing as well.

Example#

If you are reading a simple CSV file with a one-line header with column names, the following:

read('example.csv', format='basic', delimiter=',', guess=False)  # doctest: +SKIP

can be at least an order of magnitude faster than:

read('example.csv')  # doctest: +SKIP

Reference/API#

Reference/API
- astropy.io.ascii Package

ASCII Tables (astropy.io.ascii)#

Introduction#

Getting Started#

Reading Tables#

Writing Tables#

Supported Formats#

Using astropy.io.ascii#

Reading tables#

Writing tables#

ECSV Format#

Fixed-Width Gallery#

Fast ASCII Engine#

Base Class Elements#

Extension Reader Classes#

Performance Tips#

Example#

Reference/API#

ASCII Tables (`astropy.io.ascii`)#

Using `astropy.io.ascii`#