path: root/pkg_resources.py

"""\
Package resource API
--------------------

A resource is a logical file contained within a package, or a logical
subdirectory thereof.  The package resource API expects resource names
to have their path parts separated with ``/``, *not* whatever the local
path separator is.  Do not use os.path operations to manipulate resource
names being passed into the API.

The package resource API is designed to work with normal filesystem packages,
.egg files, and unpacked .egg files.  It can also work in a limited way with
.zip files and with custom PEP 302 loaders that support the ``get_data()``
method.
"""
__all__ = [
    'register_loader_type', 'get_provider', 'IResourceProvider', 
    'ResourceManager', 'AvailableDistributions', 'require', 'resource_string',
    'resource_stream', 'resource_filename', 'set_extraction_path',
    'cleanup_resources', 'parse_requirements', 'parse_version',
    'compatible_platforms', 'get_platform', 'IMetadataProvider',
    'ResolutionError', 'VersionConflict', 'DistributionNotFound',
    'InvalidOption', 'Distribution', 'Requirement', 'yield_lines',
    'split_sections', # 'glob_resources'
]

import sys, os, zipimport, time, re

class ResolutionError(Exception):
    """Abstract base for dependency resolution errors"""

class VersionConflict(ResolutionError):
    """An already-installed version conflicts with the requested version"""

class DistributionNotFound(ResolutionError):
    """A requested distribution was not found"""

class InvalidOption(ResolutionError):
    """Invalid or unrecognized option name for a distribution"""


_provider_factories = {}

def register_loader_type(loader_type, provider_factory):
    """Register `provider_factory` to make providers for `loader_type`

    `loader_type` is the type or class of a PEP 302 ``module.__loader__``,
    and `provider_factory` is a function that, passed a *module* object,
    returns an ``IResourceProvider`` for that module.
    """
    _provider_factories[loader_type] = provider_factory

def get_provider(moduleName):
    """Return an IResourceProvider for the named module"""
    module = sys.modules[moduleName]
    loader = getattr(module, '__loader__', None)
    return _find_adapter(_provider_factories, loader)(module)

def get_platform():
    """Return this platform's string for platform-specific distributions

    XXX Currently this is the same as ``distutils.util.get_platform()``, but it
    needs some hacks for Linux and Mac OS X.
    """
    from distutils.util import get_platform
    return get_platform()

def compatible_platforms(provided,required):
    """Can code for the `provided` platform run on the `required` platform?

    Returns true if either platform is ``None``, or the platforms are equal.

    XXX Needs compatibility checks for Linux and Mac OS X.
    """
    if provided is None or required is None or provided==required:
        return True     # easy case

    # XXX all the tricky cases go here

    return False


class IMetadataProvider:

    def has_metadata(name):
        """Does the package's distribution contain the named metadata?"""

    def get_metadata(name):
        """The named metadata resource as a string"""

    def get_metadata_lines(name):
        """Yield named metadata resource as list of non-blank non-comment lines

       Leading and trailing whitespace is stripped from each line, and lines
       with ``#`` as the first non-blank character are omitted.
       """

class IResourceProvider(IMetadataProvider):

    """An object that provides access to package resources"""

    def get_resource_filename(manager, resource_name):
        """Return a true filesystem path for `resource_name`

        `manager` must be an ``IResourceManager``"""

    def get_resource_stream(manager, resource_name):
        """Return a readable file-like object for `resource_name`

        `manager` must be an ``IResourceManager``"""

    def get_resource_string(manager, resource_name):
        """Return a string containing the contents of `resource_name`

        `manager` must be an ``IResourceManager``"""

    def has_resource(resource_name):
        """Does the package contain the named resource?"""

    # XXX list_resources?  glob_resources?



class AvailableDistributions(object):
    """Searchable snapshot of distributions on a search path"""

    def __init__(self, search_path=None, platform=get_platform()):
        """Snapshot distributions available on a search path

        `search_path` should be a sequence of ``sys.path`` items.  If not
        supplied, ``sys.path`` is used.

        The `platform` is an optional string specifying the name of the
        platform that platform-specific distributions must be compatible
        with.  If not specified, it defaults to the current platform
        (as defined by the result of ``get_platform()`` when ``pkg_resources``
        was first imported.)

        You may explicitly set `platform` to ``None`` if you wish to map *all*
        distributions, not just those compatible with a single platform.
        """

        self._distmap = {}
        self._cache = {}
        self.scan(search_path,platform)

    def __iter__(self):
        """Iterate over distribution keys"""
        return iter(self._distmap.keys())

    def __contains__(self,name):
        """Has a distribution named `name` ever been added to this map?"""
        return name.lower() in self._distmap

    def __len__(self):
        return len(self._distmap)

    def get(self,key,default=None):
        """Return ``self[key]`` if `key` in self, otherwise return `default`"""
        if key in self:
            return self[key]
        else:
            return default

    def scan(self, search_path=None, platform=get_platform()):
        """Scan `search_path` for distributions usable on `platform`

        Any distributions found are added to the distribution map.
        `search_path` should be a sequence of ``sys.path`` items.  If not
        supplied, ``sys.path`` is used.  `platform` is an optional string
        specifying the name of the platform that platform-specific
        distributions must be compatible with.  If unspecified, it defaults to
        the current platform.

        You may explicitly set `platform` to ``None`` if you wish to map *all*
        distributions, not just those compatible with the running platform.
        """
        if search_path is None:
            search_path = sys.path
        add = self.add
        for item in search_path:
            source = get_distro_source(item)
            for dist in source.iter_distributions(requirement):
                if compatible_platforms(dist.platform, platform):
                    add(dist)   # XXX should also check python version!

    def __getitem__(self,key):
        """Return a newest-to-oldest list of distributions for the given key

        The returned list may be modified in-place, e.g. for narrowing down
        usable distributions.
        """
        try:
            return self._cache[key]
        except KeyError:
            key = key.lower()
            if key not in self._distmap:
                raise

        if key not in self._cache:
            dists = self._cache[key] = self._distmap[key]
            _sort_dists(dists)

        return self._cache[key]

    def add(self,dist):
        """Add `dist` to the distribution map"""
        self._distmap.setdefault(dist.key,[]).append(dist)
        if dist.key in self._cache:
            _sort_dists(self._cache[dist.key])

    def remove(self,dist):
        """Remove `dist` from the distribution map"""
        self._distmap[dist.key].remove(dist)

    def best_match(self,requirement,path=None):
        """Find distribution best matching `requirement` and usable on `path`

        If a distribution that's already installed on `path` is unsuitable,
        a VersionConflict is raised.  If one or more suitable distributions are
        already installed, the leftmost distribution (i.e., the one first in
        the search path) is returned.  Otherwise, the available distribution
        with the highest version number is returned, or a deferred distribution
        object is returned if a suitable ``obtain()`` method exists.  If there
        is no way to meet the requirement, None is returned.
        """
        if path is None:
            path = sys.path

        distros = self.get(requirement.key, ())
        find = dict([(dist.path,dist) for dist in distros]).get

        for item in path:
            dist = find(item)
            if dist is not None:
                if dist in requirement:
                    return dist
                else:
                    raise VersionConflict(dist,requirement) # XXX add more info

        for dist in distros:
            if dist in requirement:
                return dist

        return self.obtain(requirement) # as a last resort, try and download

    def resolve(self, requirements, path=None):
        """List all distributions needed to (recursively) meet requirements"""

        if path is None:
            path = sys.path

        requirements = list(requirements)[::1]  # set up the stack
        processed = {}  # set of processed requirements
        best = {}       # key -> dist

        while requirements:

            req = requirements.pop()
            if req in processed:
                # Ignore cyclic or redundant dependencies
                continue

            dist = best.get(req.key)

            if dist is None:
                # Find the best distribution and add it to the map
                dist = best[req.key] = self.best_match(req,path)
                if dist is None:
                    raise DistributionNotFound(req)  # XXX put more info here

            elif dist not in requirement:
                # Oops, the "best" so far conflicts with a dependency
                raise VersionConflict(req,dist) # XXX put more info here

            requirements.extend(dist.depends(req.options)[::-1])
            processed[req] = True

        return best.values()    # return list of distros to install


    def obtain(self, requirement):
        """Obtain a distro that matches requirement (e.g. via download)"""
        return None     # override this in subclasses



class ResourceManager:
    """Manage resource extraction and packages"""

    extraction_path = None

    def __init__(self):
        self.cached_files = []

    def resource_exists(self, package_name, resource_name):
        """Does the named resource exist in the named package?"""
        return get_provider(package_name).has_resource(self, resource_name)

    def resource_filename(self, package_name, resource_name):
        """Return a true filesystem path for specified resource"""
        return get_provider(package_name).get_resource_filename(
            self,resource_name
        )

    def resource_stream(self, package_name, resource_name):
        """Return a readable file-like object for specified resource"""
        return get_provider(package_name).get_resource_stream(
            self, resource_name
        )

    def resource_string(self, package_name, resource_name):
        """Return specified resource as a string"""
        return get_provider(package_name).get_resource_string(
            self, resource_name
        )












    def get_cache_path(self, archive_name, names=()):
        """Return absolute location in cache for `archive_name` and `names`

        The parent directory of the resulting path will be created if it does
        not already exist.  `archive_name` should be the base filename of the
        enclosing egg (which may not be the name of the enclosing zipfile!),
        including the ".egg" extension.  `names`, if provided, should be a
        sequence of path name parts "under" the egg's extraction location.

        This method should only be called by resource providers that need to
        obtain an extraction location, and only for names they intend to
        extract, as it tracks the generated names for possible cleanup later.
        """
        extract_path = self.extraction_path
        extract_path = extract_path or os.path.expanduser('~/.python-eggs')
        target_path = os.path.join(extract_path, archive_name, *names)
        _ensure_directory(target_path)
        self.cached_files.append(target_path)
        return target_path


    def postprocess(self, filename):
        """Perform any platform-specific postprocessing of file `filename`

        This is where Mac header rewrites should be done; other platforms don't
        have anything special they should do.

        Resource providers should call this method ONLY after successfully
        extracting a compressed resource.  They must NOT call it on resources
        that are already in the filesystem.
        """
        # XXX









    def set_extraction_path(self, path):
        """Set the base path where resources will be extracted to, if needed.

        If not set, this defaults to ``os.expanduser("~/.python-eggs")``.
        Resources are extracted to subdirectories of this path based upon
        information given by the ``IResourceProvider``.  You may set this to a
        temporary directory, but then you must call ``cleanup_resources()`` to
        delete the extracted files when done.  There is no guarantee that
        ``cleanup_resources()`` will be able to remove all extracted files.

        (Note: you may not change the extraction path for a given resource
        manager once resources have been extracted, unless you first call
        ``cleanup_resources()``.)
        """
        if self.cached_files:
            raise ValueError(
                "Can't change extraction path, files already extracted"
            )

        self.extraction_path = path

    def cleanup_resources(self, force=False):
        """
        Delete all extracted resource files and directories, returning a list
        of the file and directory names that could not be successfully removed.
        This function does not have any concurrency protection, so it should
        generally only be called when the extraction path is a temporary
        directory exclusive to a single process.  This method is not
        automatically called; you must call it explicitly or register it as an
        ``atexit`` function if you wish to ensure cleanup of a temporary
        directory used for extractions.
        """
        # XXX








def require(*requirements):
    """Ensure that distributions matching `requirements` are on ``sys.path``

    `requirements` must be a string or a (possibly-nested) sequence
    thereof, specifying the distributions and versions required.

    XXX This doesn't work yet, because:

        * get_distro_source() isn't implemented
        * Distribution.install_on() isn't implemented
        * Requirement.options isn't implemented
        * AvailableDistributions.resolve() is untested
        * AvailableDistributions.scan() is untested

    There may be other things missing as well, but this definitely won't work
    as long as any of the above items remain unimplemented.
    """

    requirements = parse_requirements(requirements)

    for dist in AvailableDistributions().resolve(requirements):
        dist.install_on(sys.path)



















class DefaultProvider:
    """Provides access to package resources in the filesystem"""

    egg_info = None

    def __init__(self, module):
        self.module = module
        self.loader = getattr(module, '__loader__', None)
        self.module_path = os.path.dirname(module.__file__)

    def get_resource_filename(self, manager, resource_name):
        return self._fn(resource_name)

    def get_resource_stream(self, manager, resource_name):
        return open(self._fn(resource_name), 'rb')

    def get_resource_string(self, manager, resource_name):
        return self._get(self._fn(resource_name))

    def has_resource(self, resource_name):
        return self._has(self._fn(resource_name))

    def has_metadata(self, name):
        if not self.egg_info:
            raise NotImplementedError("Only .egg supports metadata")
        return self._has(os.path.join(self.egg_info, *name.split('/')))

    def get_metadata(self, name):
        if not self.egg_info:
            raise NotImplementedError("Only .egg supports metadata")
        return self._get(os.path.join(self.egg_info, *name.split('/')))

    def get_metadata_lines(self, name):
        return yield_lines(self.get_metadata(name))







    def _has(self, path):
        return os.path.exists(path)

    def _get(self, path):
        stream = open(path, 'rb')
        try:
            return stream.read()
        finally:
            stream.close()

    def _fn(self, resource_name):
        return os.path.join(self.module_path, *resource_name.split('/'))


register_loader_type(type(None), DefaultProvider)



class NullProvider(DefaultProvider):
    """Try to implement resource support for arbitrary PEP 302 loaders"""

    def _has(self, path):
        raise NotImplementedError(
            "Can't perform this operation for unregistered loader type"
        )

    def _get(self, path):
        if hasattr(self.loader, 'get_data'):
            return self.loader.get_data(path)
        raise NotImplementedError(
            "Can't perform this operation for loaders without 'get_data()'"
        )


register_loader_type(object, NullProvider)






class ZipProvider(DefaultProvider):
    """Resource support for zips and eggs"""

    egg_name = None
    eagers   = None

    def __init__(self, module):
        self.module = module
        self.loader = module.__loader__
        self.zipinfo = zipimport._zip_directory_cache[self.loader.archive]
        self.zip_pre = self.loader.archive+os.sep

        path = self.module_path = os.path.dirname(module.__file__)
        old = None
        self.prefix = []
        while path!=old:
            if path.lower().endswith('.egg'):
                self.egg_name = os.path.basename(path)
                self.egg_info = os.path.join(path, 'EGG-INFO')
                break
            old = path
            path, base = os.path.split(path)
            self.prefix.append(base)

    def _short_name(self, path):
        if path.startswith(self.zip_pre):
            return path[len(self.zip_pre):]
        return path

    def _has(self, path):
        return self._short_name(path) in self.zipinfo

    def _get(self, path):
        return self.loader.get_data(path)

    def get_resource_stream(self, manager, resource_name):
        return StringIO(self.get_resource_string(manager, resource_name))




    def _extract_resource(self, manager, resource_name):
        parts = resource_name.split('/')
        zip_path = os.path.join(self.module_path, *parts)
        zip_stat = self.zipinfo[os.path.join(*self.prefix+parts)]
        t,d,size = zip_stat[5], zip_stat[6], zip_stat[3]
        date_time = (
            (d>>9)+1980, (d>>5)&0xF, d&0x1F,                      # ymd
            (t&0xFFFF)>>11, (t>>5)&0x3F, (t&0x1F) * 2, 0, 0, -1   # hms, etc.
        )
        timestamp = time.mktime(date_time)
        real_path = manager.get_cache_path(self.egg_name, self.prefix+parts)

        if os.path.isfile(real_path):
            stat = os.stat(real_path)
            if stat.st_size==size and stat.st_mtime==timestamp:
                # size and stamp match, don't bother extracting
                return real_path

        # print "extracting", zip_path

        data = self.loader.get_data(zip_path)
        open(real_path, 'wb').write(data)
        os.utime(real_path, (timestamp,timestamp))
        manager.postprocess(real_path)
        return real_path

    def _get_eager_resources(self):
        if self.eagers is None:
            eagers = []
            for name in ('native_libs.txt', 'eager_resources.txt'):
                if self.has_metadata(name):
                    eagers.extend(self.get_metadata_lines(name))
            self.eagers = eagers
        return self.eagers







    def get_resource_filename(self, manager, resource_name):
        if not self.egg_name:
            raise NotImplementedError(
                "resource_filename() only supported for .egg, not .zip"
            )

        # should lock for extraction here
        eagers = self._get_eager_resources()
        if resource_name in eagers:
            for name in eagers:
                self._extract_resource(manager, name)

        return self._extract_resource(manager, resource_name)


register_loader_type(zipimport.zipimporter, ZipProvider)


def StringIO(*args, **kw):
    """Thunk to load the real StringIO on demand"""
    global StringIO
    try:
        from cStringIO import StringIO
    except ImportError:
        from StringIO import StringIO
    return StringIO(*args,**kw)


def get_distro_source(path_item):
    pass    # XXX











def yield_lines(strs):
    """Yield non-empty/non-comment lines of a ``basestring`` or sequence"""
    if isinstance(strs,basestring):
        for s in strs.splitlines():
            s = s.strip()
            if s and not s.startswith('#'):     # skip blank lines/comments
                yield s
    else:
        for ss in strs:
            for s in yield_lines(ss):
                yield s

LINE_END = re.compile(r"\s*(#.*)?$").match         # whitespace and comment
CONTINUE = re.compile(r"\s*\\\s*(#.*)?$").match    # line continuation
DISTRO   = re.compile(r"\s*(\w+)").match           # Distribution name
VERSION  = re.compile(r"\s*(<=?|>=?|==|!=)\s*((\w|\.)+)").match  # version info
COMMA    = re.compile(r"\s*,").match               # comma between items

EGG_NAME = re.compile(
    r"(?P<name>[^-]+)"
    r"( -(?P<ver>[^-]+) (-py(?P<pyver>[^-]+) (-(?P<plat>.+))? )? )?",
    re.VERBOSE | re.IGNORECASE
).match

component_re = re.compile(r'(\d+ | [a-z]+ | \.| -)', re.VERBOSE)
replace = {'pre':'c', 'preview':'c','-':'final-','rc':'c'}.get

def _parse_version_parts(s):
    for part in component_re.split(s):
        part = replace(part,part)
        if not part or part=='.':
            continue
        if part[:1] in '0123456789':
            yield part.zfill(8)    # pad for numeric comparison
        else:
            yield '*'+part

    yield '*final'  # ensure that alpha/beta/candidate are before final



def parse_version(s):
    """Convert a version string to a sortable key

    This is a rough cross between distutils' StrictVersion and LooseVersion;
    if you give it versions that would work with StrictVersion, then it behaves
    the same; otherwise it acts like a slightly-smarter LooseVersion.

    The returned value will be a tuple of strings.  Numeric portions of the
    version are padded to 8 digits so they will compare numerically, but
    without relying on how numbers compare relative to strings.  Dots are
    dropped, but dashes are retained.  Trailing zeros between alpha segments
    or dashes are suppressed, so that e.g. 2.4.0 is considered the same as 2.4.
    Alphanumeric parts are lower-cased.

    The algorithm assumes that strings like '-' and any alpha string > "final"
    represents a "patch level".  So, "2.4-1" is assumed to be a branch or patch
    of "2.4", and therefore "2.4.1" is considered newer than "2.4-1".

    Strings like "a", "b", "c", "alpha", "beta", "candidate" and so on (that
    come before "final" alphabetically) are assumed to be pre-release versions,
    and so the version "2.4" is considered newer than "2.4a1".

    Finally, to handle miscellaneous cases, the strings "pre", "preview", and
    "rc" are treated as if they were "c", i.e. as though they were release
    candidates, and therefore are not as new as a version string that does not
    contain them.
    """
    parts = []
    for part in _parse_version_parts(s.lower()):
        if part.startswith('*'):
            # remove trailing zeros from each series of numeric parts
            while parts and parts[-1]=='00000000':
                parts.pop()
        parts.append(part)
    return tuple(parts)






class Distribution(object):
    """Wrap an actual or potential sys.path entry w/metadata"""

    def __init__(self,
        path_str, metadata=None, name=None, version=None,
        py_version=sys.version[:3], platform=None
    ):
        if name:
            self.name = name.replace('_','-')
        if version:
            self._version = version.replace('_','-')

        self.py_version = py_version
        self.platform = platform
        self.path = path_str
        self.metadata = metadata

    def installed_on(self,path=None):
        """Is this distro installed on `path`? (defaults to ``sys.path``)"""
        if path is None:
            path = sys.path
        return self.path in path

    #@classmethod
    def from_filename(cls,filename,metadata=None):
        name,version,py_version,platform = [None]*4
        basename,ext = os.path.splitext(os.path.basename(filename))
        if ext.lower()==".egg":
            match = EGG_NAME(basename)
            if match:
                name,version,py_version,platform = match.group(
                    'name','ver','pyver','plat'
                )
        return cls(
            filename, metadata, name=name, version=version,
            py_version=py_version, platform=platform
        )
    from_filename = classmethod(from_filename)



    # These properties have to be lazy so that we don't have to load any
    # metadata until/unless it's actually needed.  (i.e., some distributions
    # may not know their name or version without loading PKG-INFO)

    #@property
    def key(self):
        try:
            return self._key
        except AttributeError:
            self._key = key = self.name.lower()
            return key
    key = property(key)

    #@property
    def parsed_version(self):
        try:
            return self._parsed_version
        except AttributeError:
            self._parsed_version = pv = parse_version(self.version)
            return pv

    parsed_version = property(parsed_version)

    #@property
    def version(self):
        try:
            return self._version
        except AttributeError:
            for line in self.metadata.get_metadata_lines('PKG-INFO'):
                if line.lower().startswith('version:'):
                    self._version = line.split(':',1)[1].strip()
                    return self._version
            else:
                raise AttributeError(
                    "Missing Version: header in PKG-INFO", self
                )
    version = property(version)




    #@property
    def _dep_map(self):
        try:
            return self.__dep_map
        except AttributeError:
            dm = self.__dep_map = {None: []}
            if self.metadata.has_metadata('depends.txt'):
                for section,contents in split_sections(
                    self.metadata.get_metadata_lines('depends.txt')
                ):
                    dm[section] = list(parse_requirements(contents))
            return dm

    _dep_map = property(_dep_map)

    def depends(self,options=()):
        """List of Requirements needed for this distro if `options` are used"""
        dm = self._dep_map
        deps = []
        deps.extend(dm.get(None,()))

        for opt in options:
            try:
                deps.extend(dm[opt.lower()])
            except KeyError:
                raise InvalidOption("No such option", self, opt)
        return deps



def _sort_dists(dists):
    tmp = [(dist.version,dist) for dist in dists]
    tmp.sort()
    dists[::-1] = [d for v,d in tmp]







def parse_requirements(strs):
    """Yield ``Requirement`` objects for each specification in `strs`

    `strs` must be an instance of ``basestring``, or a (possibly-nested)
    iterable thereof.
    """
    # create a steppable iterator, so we can handle \-continuations
    lines = iter(yield_lines(strs))
    for line in lines:
        line = line.replace('-','_')
        match = DISTRO(line)
        if not match:
            raise ValueError("Missing distribution spec", line)
        distname = match.group(1)
        p = match.end()
        specs = []
        while not LINE_END(line,p):
            if CONTINUE(line,p):
                try:
                    line = lines.next().replace('-','_'); p = 0
                except StopIteration:
                    raise ValueError(
                        "\\ must not appear on the last nonblank line"
                    )
            match = VERSION(line,p)
            if not match:
                raise ValueError("Expected version spec in",line,"at",line[p:])
            op,val = match.group(1,2)
            specs.append((op,val.replace('_','-')))
            p = match.end()
            match = COMMA(line,p)
            if match:
                p = match.end() # skip the comma
            elif not LINE_END(line,p):
                raise ValueError("Expected ',' or EOL in",line,"at",line[p:])

        yield Requirement(distname.replace('_','-'), specs)




class Requirement:

    def __init__(self, distname, specs=()):
        self.distname = distname
        self.key = distname.lower()
        index = [(parse_version(v),state_machine[op],op,v) for op,v in specs]
        index.sort()
        self.specs = [(op,ver) for parsed,trans,op,ver in index]
        self.index = index

    def __str__(self):
        return self.distname + ','.join([''.join(s) for s in self.specs])

    def __repr__(self):
        return "Requirement(%r, %r)" % (self.distname, self.specs)

    def __eq__(self,other):
        return isinstance(other,Requirement) \
            and self.key==other.key and self.specs==other.specs

    def __contains__(self,item):
        if isinstance(item,Distribution):
            if item.key <> self.key:
                return False
            item = item.parsed_version
        elif isinstance(item,basestring):
            item = parse_version(item)
        last = True
        for parsed,trans,op,ver in self.index:
            action = trans[cmp(item,parsed)]
            if action=='F':
                return False
            elif action=='T':
                return True
            elif action=='+':
                last = True
            elif action=='-':
                last = False
        return last


    #@staticmethod
    def parse(s):
        reqs = list(parse_requirements(s))
        if reqs:
            if len(reqs)==1:
                return reqs[0]
            raise ValueError("Expected only one requirement", s)
        raise ValueError("No requirements found", s)            

    parse = staticmethod(parse)

state_machine = {
    #       =><
    '<' :  '--T',
    '<=':  'T-T',
    '>' :  'F+F',
    '>=':  'T+F',
    '==':  'T..',
    '!=':  'F..',
}

def _get_mro(cls):
    """Get an mro for a type or classic class"""
    if not isinstance(cls,type):
        class cls(cls,object): pass
        return cls.__mro__[1:]
    return cls.__mro__

def _find_adapter(registry, ob):
    """Return an adapter factory for `ob` from `registry`"""
    for t in _get_mro(getattr(ob, '__class__', type(ob))):
        if t in registry:
            return registry[t]


def _ensure_directory(path):
    dirname = os.path.dirname(path)
    if not os.path.isdir(dirname):
        os.makedirs(dirname)


def split_sections(s):
    """Split a string or iterable thereof into (section,content) pairs

    Each ``section`` is a lowercase version of the section header ("[section]")
    and each ``content`` is a list of stripped lines excluding blank lines and
    comment-only lines.  If there are any such lines before the first section
    header, they're returned in a first ``section`` of ``None``.
    """
    section = None
    content = []
    for line in yield_lines(s):
        if line.startswith("["):
            if line.endswith("]"):
                if content:
                    yield section, content
                section = line[1:-1].strip().lower()
                content = []
            else:
                raise ValueError("Invalid section heading", line)
        else:
            content.append(line)

    # wrap up last segment
    if content:
        yield section, content


# Set up global resource manager

_manager = ResourceManager()

def _initialize(g):
    for name in dir(_manager):
        if not name.startswith('_'):
            g[name] = getattr(_manager, name)
_initialize(globals())