Writing Tables

astropy.io.ascii is able to write ASCII tables out to a file or file-like object using the same class structure and basic user interface as for reading tables.

The write() function provides a way to write a data table as a formatted ASCII table.


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 1
2 4
3 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)  

For a more 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 still maintaining compatibility with most plain CSV readers. It also allows storing richer data like SkyCoord or multidimensional or variable-length columns. For our simple example:

>>> 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

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 example below writes the data as a LaTeX table, using the option to send the output to sys.stdout instead of a file:

>>> ascii.write(data, format='latex')  
x & y \\
1 & 1 \\
2 & 4 \\
3 & 9 \\

There is also a faster Cython engine for writing simple formats, which is enabled by default for these formats (see Fast ASCII I/O). To disable this engine, use the parameter fast_writer:

>>> ascii.write(data, 'values.csv', format='csv', fast_writer=False)  


For most supported formats one can write a masked table and then read it back without losing information about the masked table entries. This is accomplished by using a blank string entry to indicate a masked (missing) value. See the Bad or Missing Values section for more information.

Parameters for write()

The write() function accepts a number of parameters that specify the detailed output table format. Each of the Supported Formats is handled by a corresponding Writer class that can define different defaults, so the descriptions below sometimes mention “typical” default values. This refers to the Basic writer and other similar Writer classes.

Some output format Writer classes (e.g., Latex or AASTex) accept additional keywords that can customize the output further. See the documentation of these classes for details.

output: output specifier

There are two ways to specify the output for the write operation:

  • Name of a file (string)

  • File-like object (from open(), StringIO, etc.)

table: input table

Any value that is supported for initializing a Table object (see Constructing a Table). This includes a table with a list of columns, a dictionary of columns, or from numpy arrays (either structured or homogeneous).

format: output format (default=’basic’)

This specifies the format of the ASCII table to be written, such as a basic character delimited table, fixed-format table, or a CDS-compatible table, etc. The value of this parameter must be one of the Supported Formats.

delimiter: column delimiter string

A one-character string used to separate fields which typically defaults to the space character. Other common values might be “,” or “|” or “\t”.

comment: string defining start of a comment line in output table

For the Basic Writer this defaults to “# “. Which comments are written and how depends on the format chosen. The comments are defined as a list of strings in the input table meta['comments'] element. Comments in the metadata of the given Table will normally be written before the header, although CommentedHeader writes table comments after the commented header. To disable writing comments, set comment=False.

formats: dict of data type converters

For each key (column name) use the given value to convert the column data to a string. If the format value is string-like, then it is used as a Python format statement (e.g., ‘%0.2f’ % value). If it is a callable function, then that function is called with a single argument containing the column value to be converted. Example:

astropy.io.ascii.write(table, sys.stdout, formats={'XCENTER': '%12.1f',
                                             'YCENTER': lambda x: round(x, 1)},
names: list of names corresponding to each data column

Define the complete list of names for each data column. This will override names determined from the data table (if available). If not supplied then use names from the data table or auto-generated names.

include_names: list of names to include in output

From the list of column names found from the data table or the names parameter, select for output only columns within this list. If not supplied then include all names.

exclude_names: list of names to exclude from output

Exclude these names from the list of output columns. This is applied after the include_names filtering. If not specified then no columns are excluded.

fill_values: list of fill value specifiers

This can be used to fill missing values in the table or replace values with special meaning.

See the Bad or Missing Values section for more information on the syntax. The syntax is almost the same as when reading a table. There is a special value astropy.io.ascii.masked that is used to say “output this string for all masked values in a masked table” (the default is to use an empty string ""):

>>> import sys
>>> from astropy.table import Table, Column, MaskedColumn
>>> from astropy.io import ascii
>>> t = Table([(1, 2), (3, 4)], names=('a', 'b'), masked=True)
>>> t['a'].mask = [True, False]
>>> ascii.write(t, sys.stdout)
a b
"" 3
2 4
>>> ascii.write(t, sys.stdout, fill_values=[(ascii.masked, 'N/A')])
a b
N/A 3
2 4

Note that when writing a table, all values are converted to strings before any value is replaced. Because fill_values only replaces cells that are an exact match to the specification, you need to provide the string representation (stripped of whitespace) for each value. For example, in the following commands -99 is formatted with two digits after the comma, so we need to replace -99.00 and not -99:

>>> t = Table([(-99, 2), (3, 4)], names=('a', 'b'))
>>> ascii.write(t, sys.stdout, fill_values = [('-99.00', 'no data')],
...             formats={'a': '%4.2f'})
a b
"no data" 3
2.00 4

Similarly, if you replace a value in a column that has a fixed length format (e.g., 'f4.2'), then the string you want to replace must have the same number of characters. In the example above, fill_values=[(' nan',' N/A')] would work.

fill_include_names: list of column names, which are affected by fill_values

If not supplied, then fill_values can affect all columns.

fill_exclude_names: list of column names, which are not affected by fill_values

If not supplied, then fill_values can affect all columns.

fast_writer: whether to use the fast Cython writer

If this parameter is None (which it is by default), write() will attempt to use the faster writer (described in Fast ASCII I/O) if possible. Specifying fast_writer=False disables this behavior.

WriterWriter class (deprecated in favor of format)

This specifies the top-level format of the ASCII table to be written, such as a basic character delimited table, fixed-format table, or a CDS-compatible table, etc. The value of this parameter must be a Writer class. For basic usage this means one of the built-in Extension Reader Classes. Note that Reader classes and Writer classes are synonymous; in other words, Reader classes can also write, but for historical reasons they are often called Reader classes.