Reading tables

The majority of commonly encountered ASCII tables can be easily read with the read() function:

>>> from astropy.io import ascii
>>> data = ascii.read(table)

where table is the name of a file, a string representation of a table, or a list of table lines. By default read() will try to guess the table format by trying all the supported formats. If this does not work (for unusually formatted tables) then one needs give astropy.io.ascii additional hints about the format, for example:

>>> data = astropy.io.ascii.read('t/nls1_stackinfo.dbout', data_start=2, delimiter='|')
>>> data = astropy.io.ascii.read('t/simple.txt', quotechar="'")
>>> data = astropy.io.ascii.read('t/simple4.txt', Reader=ascii.NoHeader, delimiter='|')

The read() function accepts a number of parameters that specify the detailed table format. Different Reader classes can define different defaults, so the descriptions below sometimes mention “typical” default values. This refers to the Basic reader and other similar Reader classes.

Parameters for read()

table : input table

There are four ways to specify the table to be read:

• Name of a file (string)
• Single string containing all table lines separated by newlines
• File-like object with a callable read() method
• List of strings where each list element is a table line

The first two options are distinguished by the presence of a newline in the string. This assumes that valid file names will not normally contain a newline.

Reader : Reader class (default= Basic)

This specifies the top-level format of the ASCII table, for example if it is a basic character delimited table, fixed format table, or a CDS-compatible table, etc. The value of this parameter must be a Reader class. For basic usage this means one of the built-in Extension Reader classes.

guess: try to guess table format (default=True)

If set to True then read() will try to guess the table format by cycling through a number of possible table format permuations and attemping to read the table in each case. See the Guess table format section for further details.

delimiter : column delimiter string

A one-character string used to separate fields which typically defaults to the space character. Other common values might be “\s” (whitespace), ”,” or “|” or “\t” (tab). A value of “\s” allows any combination of the tab and space characters to delimit columns.

comment : regular expression defining a comment line in table

If the comment regular expression matches the beginning of a table line then that line will be discarded from header or data processing. For the Basic Reader this defaults to “\s*#” (any whitespace followed by #).

quotechar : one-character string to quote fields containing special characters

This specifies the quote character and will typically be either the single or double quote character. This is can be useful for reading text fields with spaces in a space-delimited table. The default is typically the double quote.

header_start : line index for the header line not counting comment lines

This specifies in the line index where the header line will be found. Comment lines are not included in this count and the counting starts from 0 (first non-comment line has index=0). If set to None this indicates that there is no header line and the column names will be auto-generated. The default is dependent on the Reader.

data_start: line index for the start of data not counting comment lines

This specifies in the line index where the data lines begin where the counting starts from 0 and does not include comment lines. The default is dependent on the Reader.

data_end: line index for the end of data (can be negative to count from end)

If this is not None then it allows for excluding lines at the end that are not valid data lines. A negative value means to count from the end, so -1 would exclude the last line, -2 the last two lines, and so on.

converters: dict of data type converters

See the Converters section for more information.

names: list of names corresponding to each data column

Define the complete list of names for each data column. This will override names found in the header (if it exists). If not supplied then use names from the header or auto-generated names if there is no header.

include_names: list of names to include in output

From the list of column names found from the header 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 exlude 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: fill value specifier of lists

This can be used to fill missing values in the table or replace strings with special meaning. See the Replace bad or missing values section for more information and examples.

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.

Outputter: Outputter class

This converts the raw data tables value into the output object that gets returned by read(). The default is TableOutputter, which returns a Table object. The other option is NumpyOutputter which returns a numpy record array. If fill_values are specified then NumpyOutputter is used and a masked array is returned.

Inputter: Inputter class

This is generally not specified.

data_Splitter: Splitter class to split data columns

header_Splitter: Splitter class to split header columns

Replace bad or missing values

astropy.io.ascii can replace string values in the input data before they are converted. The most common use case is probably a table which contains string values that are not a valid representation of a number, e.g. "..." for a missing value or "". If astropy.io.ascii cannot convert all elements in a column to a numeric type, it will format the column as strings. To avoid this, fill_values can be used at the string level to fill missing values with the following syntax, which replaces <old> with <new> before the type conversion is done:

fill_values = <fill_spec> | [<fill_spec1>, <fill_spec2>, ...]
<fill_spec> = (<old>, <new>, <optional col name 1>, <optional col name 2>, ...)

Within the <fill_spec> tuple the <old> and <new> values must be strings. These two values are then followed by zero or more column names. If column names are included the replacement is limited to those columns listed. If no columns are specified then the replacement is done in every column, subject to filtering by fill_include_names and fill_exclude_names (see below).

The fill_values parameter in read() takes a single <fill_spec> or a list of <fill_spec> tuples. If several <fill_spec> apply to a single occurence of <old> then the first one determines the <new> value. For instance the following will replace an empty data value in the x or y columns with “1e38” while empty values in any other column will get “-999”:

>>> ascii.read(table, fill_values=[('', '1e38', 'x', 'y'), ('', '-999')])

The following shows an example where string information needs to be exchanged before the conversion to float values happens. Here no_rain and no_snow is replaced by 0.0:

>>> table = ['day  rain     snow',    # column names
             #---  -------  --------
             'Mon  3.2      no_snow',
             'Tue  no_rain  1.1',
             'Wed  0.3      no_snow']
>>> print ascii.read(table, fill_values=[('no_rain', '0.0'), ('no_snow', '0.0')])
[('Mon', 3.2, --) ('Tue', --, 1.1) ('Wed', 0.3, --)]

Sometimes these rules apply only to specific columns in the table. Columns can be selected with fill_include_names or excluded with fill_exclude_names. Also, column names can be given directly with fill_values:

>>> asciidata = ['text,no1,no2', 'text1,1,1.',',2,']
>>> print ascii.read(asciidata, fill_values = ('', 'nan','no1','no2'), delimiter = ',')
[('text1', 1, 1.0) ('', 2, --)]

Here, the empty value '' in column no2 is replaced by nan, but the text column remains unaltered.

When fill_values is specified then read() returns a NumPy masked array instead of the default Table object. See the description of the NumpyOutputter class for information on disabling masked arrays.

Guess table format

If the guess parameter in read() is set to True (which is the default) then read() will try to guess the table format by cycling through a number of possible table format permutations and attemping to read the table in each case. The first format which succeeds and will be used to read the table. To succeed the table must be successfully parsed by the Reader and satisfy the following column requirements:

• At least two table columns
• No column names are a float or int number
• No column names begin or end with space, comma, tab, single quote, double quote, or a vertical bar (|).

These requirements reduce the chance for a false positive where a table is successfully parsed with the wrong format. A common situation is a table with numeric columns but no header row, and in this case astropy.io.ascii will auto-assign column names because of the restriction on column names that look like a number.

The order of guessing is shown by this Python code:

for Reader in (Rdb, Tab, Cds, Daophot, SExtractor, Ipac):
    read(Reader=Reader)
for Reader in (CommentedHeader, Basic, NoHeader):
    for delimiter in ("|", ",", " ", "\\s"):
        for quotechar in ('"', "'"):
            read(Reader=Reader, delimiter=delimiter, quotechar=quotechar)

Note that the FixedWidth derived-readers are not included in the default guess sequence (this causes problems), so to read such tables one must explicitly specify the reader class with the Reader keyword.

If none of the guesses succeed in reading the table (subject to the column requirements) a final try is made using just the user-supplied parameters but without checking the column requirements. In this way a table with only one column or column names that look like a number can still be successfully read.

The guessing process respects any values of the Reader, delimiter, and quotechar parameters that were supplied to the read() function. Any guesses that would conflict are skipped. For example the call:

>>> data = astropy.io.ascii.read(table, Reader=NoHeader, quotechar="'")

would only try the four delimiter possibilities, skipping all the conflicting Reader and quotechar combinations.

Guessing can be disabled in two ways:

import astropy.io.ascii
data = astropy.io.ascii.read(table)               # guessing enabled by default
data = astropy.io.ascii.read(table, guess=False)  # disable for this call
astropy.io.ascii.set_guess(False)                 # set default to False globally
data = astropy.io.ascii.read(table)               # guessing disabled

Converters

astropy.io.ascii converts the raw string values from the table into numeric data types by using converter functions such as the Python int and float functions. For example int("5.0") will fail while float(“5.0”) will succeed and return 5.0 as a Python float.

The default converters are:

default_converters = [astropy.io.ascii.convert_numpy(numpy.int),
                      astropy.io.ascii.convert_numpy(numpy.float),
                      astropy.io.ascii.convert_numpy(numpy.str)]

These take advantage of the convert_numpy() function which returns a 2-element tuple (converter_func, converter_type) as described in the previous section. The type provided to convert_numpy() must be a valid numpy type, for example numpy.int, numpy.uint, numpy.int8, numpy.int64, numpy.float, numpy.float64, numpy.str.

The default converters for each column can be overridden with the converters keyword:

>>> converters = {'col1': [astropy.io.ascii.convert_numpy(numpy.uint)],
                  'col2': [astropy.io.ascii.convert_numpy(numpy.float32)]}
>>> ascii.read('file.dat', converters=converters)

Advanced customization

Here we provide a few examples that demonstrate how to extend the base functionality to handle special cases. To go beyond these simple examples the best reference is to read the code for the existing Extension Reader classes.

Define a custom reader functionally

def read_rdb_table(table):
    reader = astropy.io.ascii.Basic()
    reader.header.splitter.delimiter = '\t'
    reader.data.splitter.delimiter = '\t'
    reader.header.splitter.process_line = None
    reader.data.splitter.process_line = None
    reader.data.start_line = 2

    return reader.read(table)

Define custom readers by class inheritance

# Note: Tab and Rdb are already included in astropy.io.ascii for convenience.
class Tab(astropy.io.ascii.Basic):
    def __init__(self):
        astropy.io.ascii.Basic.__init__(self)
        self.header.splitter.delimiter = '\t'
        self.data.splitter.delimiter = '\t'
        # Don't strip line whitespace since that includes tabs
        self.header.splitter.process_line = None
        self.data.splitter.process_line = None
        # Don't strip data value spaces since that is significant in TSV tables
        self.data.splitter.process_val = None
        self.data.splitter.skipinitialspace = False

class Rdb(astropy.io.ascii.Tab):
    def __init__(self):
        astropy.io.ascii.Tab.__init__(self)
        self.data.start_line = 2

Create a custom splitter.process_val function

# The default process_val() normally just strips whitespace.
# In addition have it replace empty fields with -999.
def process_val(x):
    """Custom splitter process_val function: Remove whitespace at the beginning
    or end of value and substitute -999 for any blank entries."""
    x = x.strip()
    if x == '':
        x = '-999'
    return x

# Create an RDB reader and override the splitter.process_val function
rdb_reader = astropy.io.ascii.get_reader(Reader=astropy.io.ascii.Rdb)
rdb_reader.data.splitter.process_val = process_val

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