I/O Registry (astropy.io.registry
)¶
Note
The I/O registry is only meant to be used directly by users who want
to define their own custom readers/writers. Users who want to find
out more about what built-in formats are supported by
Table
by default should see
Unified file read/write interface. No built-in formats are currently defined for
NDData
, but this will be added in
future).
Introduction¶
The I/O registry is a sub-module used to define the readers/writers available
for the Table
and
NDData
classes.
Using astropy.io.registry
¶
This section demonstrates how to create a custom reader/writer. A reader is
written as a function that can take any arguments except format
(which is
needed when manually specifying the format - see below) and returns an
instance of the Table
or
NDData
classes (or sub-classes). For demonstration,
let’s assume that we are trying to write a reader/writer for the
Table
class:
from astropy.table import Table
def my_table_reader(filename, some_option=1):
# Read in the table by any means necessary
return table # should be an instance of Table
Such a function can then be registered with the I/O registry:
from astropy.io import registry
registry.register_reader('my-table-format', Table, my_table_reader)
where the first argument is the name of the format, the second argument is the class that the function returns an instance for, and the third argument is the reader itself.
We can then read in a table with:
d = Table.read('my_table_file.mtf', format='my-table-format')
In practice, it would be nice to have the read
method automatically
identify that this file is in the my-table-format
format, so we can
construct a function that can recognize these files, which we refer to here as
an identifier function.
An identifier function should take a first argument that is a string
which indicates whether the identifier is being called from read
or
write
, and should then accept arbitrary number of positional and keyword
arguments via *args
and **kwargs
, which are the arguments passed to
the read
method.
In the above case, we can write a simplistic function that only looks at
filenames (but in practice, this function could even look at the first few
bytes of the file for example). The only requirement for the identifier
function is that it return a boolean indicating whether the input matches that
expected for the format. In our example, we want to automatically recognize
files with filenames ending in .mtf
as being in the my-table-format
format:
import os
from astropy.extern import six
def identify_mtf(origin, *args, **kwargs):
return (isinstance(args[0], six.string_types) and
os.path.splitext(args[0].lower())[1] == '.mtf')
Note
Identifier functions should be prepared for arbitrary input - in particular, the first argument may not be a filename or file object, so it should not assume that this is the case.
We then register this identifier function, similarly to the reader function:
registry.register_identifier('my-table-format', Table, identify_mtf)
Having registered this function, we can then do:
t = Table.read('catalog.mtf')
If multiple formats match the current input, then an exception is
raised, and similarly if no format matches the current input. In that
case, the format should be explicitly given with the format=
keyword argument.
It is also possible to create custom writers. To go with our custom reader above, we can write a custom writer:
def my_table_writer(table, filename, overwrite=False):
... # Write the table out to a file
Writer functions should take a dataset object (either an instance of the
Table
or NDData
classes or sub-classes), and any number of subsequent positional and keyword
arguments - although as for the reader, the format
keyword argument cannot
be used.
We then register the writer:
registry.register_writer('my-custom-format', Table, my_table_writer)
We can write the table out to a file:
t.write('catalog_new.mtf', format='my-table-format')
Since we have already registered the identifier function, we can also simply do:
t.write('catalog_new.mtf')
Reference/API¶
astropy.io.registry Module¶
Functions¶
register_reader (data_format, data_class, …) |
Register a reader function. |
register_writer (data_format, data_class, …) |
Register a table writer function. |
register_identifier (data_format, data_class, …) |
Associate an identifier function with a specific data type. |
identify_format (origin, data_class_required, …) |
Loop through identifiers to see which formats match. |
get_reader (data_format, data_class) |
Get reader for data_format . |
get_writer (data_format, data_class) |
Get writer for data_format . |
read (cls, *args[, format]) |
Read in data. |
write (data, *args[, format]) |
Write out data. |
get_formats ([data_class, readwrite]) |
Get the list of registered I/O formats as a Table. |
delay_doc_updates (cls) |
Contextmanager to disable documentation updates when registering reader and writer. |
Classes¶
IORegistryError |
Custom error for registry clashes. |