source: sasview/src/sas/sascalc/data_util/registry.py

"""
File extension registry.

This provides routines for opening files based on extension,
and registers the built-in file extensions.
"""
from __future__ import print_function

from sas.sascalc.dataloader.loader_exceptions import NoKnownLoaderException


class ExtensionRegistry(object):
    """
    Associate a file loader with an extension.

    Note that there may be multiple loaders for the same extension.

    Example: ::

        registry = ExtensionRegistry()

        # Add an association by setting an element
        registry['.zip'] = unzip

        # Multiple extensions for one loader
        registry['.tgz'] = untar
        registry['.tar.gz'] = untar

        # Generic extensions to use after trying more specific extensions;
        # these will be checked after the more specific extensions fail.
        registry['.gz'] = gunzip

        # Multiple loaders for one extension
        registry['.cx'] = cx1
        registry['.cx'] = cx2
        registry['.cx'] = cx3

        # Show registered extensions
        print registry.extensions()

        # Can also register a format name for explicit control from caller
        registry['cx3'] = cx3
        print registry.formats()

        # Retrieve loaders for a file name
        registry.lookup('hello.cx') -> [cx3,cx2,cx1]

        # Run loader on a filename
        registry.load('hello.cx') ->
            try:
                return cx3('hello.cx')
            except:
                try:
                    return cx2('hello.cx')
                except:
                    return cx1('hello.cx')

        # Load in a specific format ignoring extension
        registry.load('hello.cx',format='cx3') ->
            return cx3('hello.cx')
    """
    def __init__(self, **kw):
        self.loaders = {}

    def __setitem__(self, ext, loader):
        if ext not in self.loaders:
            self.loaders[ext] = []
        self.loaders[ext].insert(0,loader)

    def __getitem__(self, ext):
        return self.loaders[ext]

    def __contains__(self, ext):
        return ext in self.loaders

    def formats(self):
        """
        Return a sorted list of the registered formats.
        """
        names = [a for a in self.loaders.keys() if not a.startswith('.')]
        names.sort()
        return names

    def extensions(self):
        """
        Return a sorted list of registered extensions.
        """
        exts = [a for a in self.loaders.keys() if a.startswith('.')]
        exts.sort()
        return exts

    def lookup(self, path):
        """
        Return the loader associated with the file type of path.

        :param path: Data file path
        :raises ValueError: When no loaders are found for the file.
        :return: List of available readers for the file extension
        """
        # Find matching extensions
        extlist = [ext for ext in self.extensions() if path.endswith(ext)]
        # Sort matching extensions by decreasing order of length
        extlist.sort(key=len)
        # Combine loaders for matching extensions into one big list
        loaders = []
        for L in [self.loaders[ext] for ext in extlist]:
            loaders.extend(L)
        # Remove duplicates if they exist
        if len(loaders) != len(set(loaders)):
            result = []
            for L in loaders:
                if L not in result: result.append(L)
            loaders = result
            #loaders = L
        # Raise an error if there are no matching extensions
        if len(loaders) == 0:
            raise ValueError("Unknown file type for "+path)
        return loaders

    def load(self, path, format=None):
        """
        Call the loader for the file type of path.

        :raises ValueError: if no loader is available.
        :raises KeyError: if format is not available.

        May raise a loader-defined exception if loader fails.
        """
        loaders = []
        if format is None:
            try:
                loaders = self.lookup(path)
            except ValueError as e:
                pass
        else:
            try:
                loaders = self.loaders[format]
            except KeyError as e:
                pass
        last_exc = None
        for fn in loaders:
            try:
                return fn(path)
            except Exception as e:
                last_exc = e
                pass  # give other loaders a chance to succeed
        # If we get here it is because all loaders failed
        if last_exc is not None and len(loaders) != 0:
            # If file has associated loader(s) and they;ve failed
            raise last_exc
        raise NoKnownLoaderException(e.message)  # raise generic exception