iris.io.format_picker

Provide convenient file format identification.

A module to provide convenient file format identification through a combination of filename extension and file based magic numbers.

To manage a collection of FormatSpecifications for loading:

import iris.io.format_picker as fp
import matplotlib.pyplot as plt
fagent = fp.FormatAgent()
png_spec = fp.FormatSpecification('PNG image', fp.MagicNumber(8),
                                  0x89504E470D0A1A0A,
                                  handler=lambda filename: plt.imread(filename),
                                  priority=5
                                  )
fagent.add_spec(png_spec)

To identify a specific format from a file:

with open(png_filename, 'rb') as png_fh:
    handling_spec = fagent.get_spec(png_filename, png_fh)

In the example, handling_spec will now be the png_spec previously added to the agent.

Now that a specification has been found, if a handler has been given with the specification, then the file can be handled:

handler = handling_spec.handler
if handler is None:
   raise ValueError('File cannot be handled.')
else:
   result = handler(filename)

The calling sequence of handler is dependent on the function given in the original specification and can be customised to your project’s needs.

class iris.io.format_picker.DataSourceObjectProtocol

Bases: FileElement

A FileElement that simply returns the URI entry itself.

This enables a arbitrary non-string data object to be passed, subject to subsequent checks on the object itself (specified in the handler).

Construct a new file element, which may require a file buffer.

Parameters:

requires_fh (bool, default=True) – Whether this FileElement needs a file buffer.

get_element(basename, file_handle)

In this context, there should not be a file opened by the handler.

Just return ‘basename’, which in this case is not a name, or even a string, but a passed ‘data object’.

class iris.io.format_picker.FileElement(requires_fh=True)

Bases: object

Represents a specific aspect of a FileFormat.

Represents a specific aspect of a FileFormat which can be identified using the given element getter function.

Construct a new file element, which may require a file buffer.

Parameters:

requires_fh (bool, default=True) – Whether this FileElement needs a file buffer.

get_element(basename, file_handle)

Identify the element of a file that this FileElement is representing.

class iris.io.format_picker.FileExtension(requires_fh=True)

Bases: FileElement

A FileElement that returns the extension from the filename.

Construct a new file element, which may require a file buffer.

Parameters:

requires_fh (bool, default=True) – Whether this FileElement needs a file buffer.

get_element(basename, file_handle)

Identify the element of a file that this FileElement is representing.

class iris.io.format_picker.FormatAgent(format_specs=None)

Bases: object

Identifies format of a given file by interrogating its children instances.

The FormatAgent class is the containing object which is responsible for identifying the format of a given file by interrogating its children FormatSpecification instances.

Typically a FormatAgent will be created empty and then extended with the FormatAgent.add_spec() method:

agent = FormatAgent()
agent.add_spec(NetCDF_specification)

Less commonly, this can also be written:

agent = FormatAgent(NetCDF_specification)

add_spec(format_spec)

Add a FormatSpecification instance to this agent for format.

get_spec(basename, buffer_obj)

Pick the first FormatSpecification.

Pick the first FormatSpecification which can handle the given filename and file/buffer object.

Parameters:

• basename (TBD) –

• buffer_obj (TBD) –

Notes

buffer_obj may be None when a seekable file handle is not feasible (such as over the http protocol). In these cases only the format specifications which do not require a file handle are tested.

class iris.io.format_picker.FormatSpecification(format_name, file_element, file_element_value, handler=None, priority=0, constraint_aware_handler=False)

Bases: object

Provides the base class for file type definition.

Every FormatSpecification instance has a name which can be accessed with the FormatSpecification.name property and a FileElement, such as filename extension or 32-bit magic number, with an associated value for format identification.

Construct a new FormatSpecification.

Parameters:

• format_name (str) – String name of fileformat being described.

• file_element – FileElement instance of the element which identifies this FormatSpecification.

• file_element_value – The value that the file_element should take if a file matches this FormatSpecification.

• handler (optional) – Function which will be called when the specification has been identified and is required to handler a format. If None, then the file can still be identified but no handling can be done.

• priority (int) – Integer giving a priority for considering this specification where higher priority means sooner consideration.

• constraint_aware_handler (default=False) –

property file_element

property file_element_value

property handler

The handler function of this FileFormat. (Read only).

property name

The name of this FileFormat. (Read only).

class iris.io.format_picker.LeadingLine(requires_fh=True)

Bases: FileElement

A FileElement that returns the first line from the file.

Construct a new file element, which may require a file buffer.

Parameters:

requires_fh (bool, default=True) – Whether this FileElement needs a file buffer.

get_element(basename, file_handle)

Identify the element of a file that this FileElement is representing.

class iris.io.format_picker.MagicNumber(num_bytes, offset=None)

Bases: FileElement

A FileElement that returns a byte sequence in the file.

Construct a new file element, which may require a file buffer.

Parameters:

requires_fh (bool, default=True) – Whether this FileElement needs a file buffer.

get_element(basename, file_handle)

Identify the element of a file that this FileElement is representing.

len_formats = {4: '>L', 8: '>Q'}

class iris.io.format_picker.UriProtocol

Bases: FileElement

Return the scheme and part from a URI, using decode_uri().

A FileElement that returns the “scheme” and “part” from a URI, using decode_uri().

Construct a new file element, which may require a file buffer.

Parameters:

requires_fh (bool, default=True) – Whether this FileElement needs a file buffer.

get_element(basename, file_handle)

Identify the element of a file that this FileElement is representing.