Lib/functools.py

"""functools.py - Tools for working with functions and callable objects
"""
# Python module wrapper for _functools C module
# to allow utilities written in Python to be added
# to the functools module.
# Written by Nick Coghlan <ncoghlan at gmail.com>,
# Raymond Hettinger <python at rcn.com>,
# and Łukasz Langa <lukasz at langa.pl>.
#   Copyright (C) 2006-2013 Python Software Foundation.
# See C source code for _functools credits/copyright

__all__ = ['update_wrapper', 'wraps', 'WRAPPER_ASSIGNMENTS', 'WRAPPER_UPDATES',
           'total_ordering', 'cmp_to_key', 'lru_cache', 'reduce', 'partial',
           'singledispatch']

try:
    from _functools import reduce
except ImportError:
    pass
from abc import get_cache_token
from collections import namedtuple
from types import MappingProxyType
from weakref import WeakKeyDictionary
try:
    from _thread import RLock
except:
    class RLock:
        'Dummy reentrant lock for builds without threads'
        def __enter__(self): pass
        def __exit__(self, exctype, excinst, exctb): pass


################################################################################
### update_wrapper() and wraps() decorator
################################################################################

# update_wrapper() and wraps() are tools to help write
# wrapper functions that can handle naive introspection

WRAPPER_ASSIGNMENTS = ('__module__', '__name__', '__qualname__', '__doc__',
                       '__annotations__')
WRAPPER_UPDATES = ('__dict__',)
def update_wrapper(wrapper,
                   wrapped,
                   assigned = WRAPPER_ASSIGNMENTS,
                   updated = WRAPPER_UPDATES):
    """Update a wrapper function to look like the wrapped function

       wrapper is the function to be updated
       wrapped is the original function
       assigned is a tuple naming the attributes assigned directly
       from the wrapped function to the wrapper function (defaults to
       functools.WRAPPER_ASSIGNMENTS)
       updated is a tuple naming the attributes of the wrapper that
       are updated with the corresponding attribute from the wrapped
       function (defaults to functools.WRAPPER_UPDATES)
    """
    wrapper.__wrapped__ = wrapped
    for attr in assigned:
        try:
            value = getattr(wrapped, attr)
        except AttributeError:
            pass
        else:
            setattr(wrapper, attr, value)
    for attr in updated:
        getattr(wrapper, attr).update(getattr(wrapped, attr, {}))
    # Return the wrapper so this can be used as a decorator via partial()
    return wrapper

def wraps(wrapped,
          assigned = WRAPPER_ASSIGNMENTS,
          updated = WRAPPER_UPDATES):
    """Decorator factory to apply update_wrapper() to a wrapper function

       Returns a decorator that invokes update_wrapper() with the decorated
       function as the wrapper argument and the arguments to wraps() as the
       remaining arguments. Default arguments are as for update_wrapper().
       This is a convenience function to simplify applying partial() to
       update_wrapper().
    """
    return partial(update_wrapper, wrapped=wrapped,
                   assigned=assigned, updated=updated)


################################################################################
### total_ordering class decorator
################################################################################

def total_ordering(cls):
    """Class decorator that fills in missing ordering methods"""
    convert = {
        '__lt__': [('__gt__', lambda self, other: not (self < other or self == other)),
                   ('__le__', lambda self, other: self < other or self == other),
                   ('__ge__', lambda self, other: not self < other)],
        '__le__': [('__ge__', lambda self, other: not self <= other or self == other),
                   ('__lt__', lambda self, other: self <= other and not self == other),
                   ('__gt__', lambda self, other: not self <= other)],
        '__gt__': [('__lt__', lambda self, other: not (self > other or self == other)),
                   ('__ge__', lambda self, other: self > other or self == other),
                   ('__le__', lambda self, other: not self > other)],
        '__ge__': [('__le__', lambda self, other: (not self >= other) or self == other),
                   ('__gt__', lambda self, other: self >= other and not self == other),
                   ('__lt__', lambda self, other: not self >= other)]
    }
    # Find user-defined comparisons (not those inherited from object).
    roots = [op for op in convert if getattr(cls, op, None) is not getattr(object, op, None)]
    if not roots:
        raise ValueError('must define at least one ordering operation: < > <= >=')
    root = max(roots)       # prefer __lt__ to __le__ to __gt__ to __ge__
    for opname, opfunc in convert[root]:
        if opname not in roots:
            opfunc.__name__ = opname
            opfunc.__doc__ = getattr(int, opname).__doc__
            setattr(cls, opname, opfunc)
    return cls


################################################################################
### cmp_to_key() function converter
################################################################################

def cmp_to_key(mycmp):
    """Convert a cmp= function into a key= function"""
    class K(object):
        __slots__ = ['obj']
        def __init__(self, obj):
            self.obj = obj
        def __lt__(self, other):
            return mycmp(self.obj, other.obj) < 0
        def __gt__(self, other):
            return mycmp(self.obj, other.obj) > 0
        def __eq__(self, other):
            return mycmp(self.obj, other.obj) == 0
        def __le__(self, other):
            return mycmp(self.obj, other.obj) <= 0
        def __ge__(self, other):
            return mycmp(self.obj, other.obj) >= 0
        def __ne__(self, other):
            return mycmp(self.obj, other.obj) != 0
        __hash__ = None
    return K

try:
    from _functools import cmp_to_key
except ImportError:
    pass


################################################################################
### partial() argument application
################################################################################

def partial(func, *args, **keywords):
    """new function with partial application of the given arguments
    and keywords.
    """
    def newfunc(*fargs, **fkeywords):
        newkeywords = keywords.copy()
        newkeywords.update(fkeywords)
        return func(*(args + fargs), **newkeywords)
    newfunc.func = func
    newfunc.args = args
    newfunc.keywords = keywords
    return newfunc

try:
    from _functools import partial
except ImportError:
    pass


################################################################################
### LRU Cache function decorator
################################################################################

_CacheInfo = namedtuple("CacheInfo", ["hits", "misses", "maxsize", "currsize"])

class _HashedSeq(list):
    """ This class guarantees that hash() will be called no more than once
        per element.  This is important because the lru_cache() will hash
        the key multiple times on a cache miss.

    """

    __slots__ = 'hashvalue'

    def __init__(self, tup, hash=hash):
        self[:] = tup
        self.hashvalue = hash(tup)

    def __hash__(self):
        return self.hashvalue

def _make_key(args, kwds, typed,
             kwd_mark = (object(),),
             fasttypes = {int, str, frozenset, type(None)},
             sorted=sorted, tuple=tuple, type=type, len=len):
    """Make a cache key from optionally typed positional and keyword arguments

    The key is constructed in a way that is flat as possible rather than
    as a nested structure that would take more memory.

    If there is only a single argument and its data type is known to cache
    its hash value, then that argument is returned without a wrapper.  This
    saves space and improves lookup speed.

    """
    key = args
    if kwds:
        sorted_items = sorted(kwds.items())
        key += kwd_mark
        for item in sorted_items:
            key += item
    if typed:
        key += tuple(type(v) for v in args)
        if kwds:
            key += tuple(type(v) for k, v in sorted_items)
    elif len(key) == 1 and type(key[0]) in fasttypes:
        return key[0]
    return _HashedSeq(key)

def lru_cache(maxsize=128, typed=False):
    """Least-recently-used cache decorator.

    If *maxsize* is set to None, the LRU features are disabled and the cache
    can grow without bound.

    If *typed* is True, arguments of different types will be cached separately.
    For example, f(3.0) and f(3) will be treated as distinct calls with
    distinct results.

    Arguments to the cached function must be hashable.

    View the cache statistics named tuple (hits, misses, maxsize, currsize)
    with f.cache_info().  Clear the cache and statistics with f.cache_clear().
    Access the underlying function with f.__wrapped__.

    See:  http://en.wikipedia.org/wiki/Cache_algorithms#Least_Recently_Used

    """

    # Users should only access the lru_cache through its public API:
    #       cache_info, cache_clear, and f.__wrapped__
    # The internals of the lru_cache are encapsulated for thread safety and
    # to allow the implementation to change (including a possible C version).

    # Constants shared by all lru cache instances:
    sentinel = object()          # unique object used to signal cache misses
    make_key = _make_key         # build a key from the function arguments
    PREV, NEXT, KEY, RESULT = 0, 1, 2, 3   # names for the link fields

    def decorating_function(user_function):
        cache = {}
        hits = misses = 0
        full = False
        cache_get = cache.get    # bound method to lookup a key or return None
        lock = RLock()           # because linkedlist updates aren't threadsafe
        root = []                # root of the circular doubly linked list
        root[:] = [root, root, None, None]     # initialize by pointing to self

        if maxsize == 0:

            def wrapper(*args, **kwds):
                # No caching -- just a statistics update after a successful call
                nonlocal misses
                result = user_function(*args, **kwds)
                misses += 1
                return result

        elif maxsize is None:

            def wrapper(*args, **kwds):
                # Simple caching without ordering or size limit
                nonlocal hits, misses
                key = make_key(args, kwds, typed)
                result = cache_get(key, sentinel)
                if result is not sentinel:
                    hits += 1
                    return result
                result = user_function(*args, **kwds)
                cache[key] = result
                misses += 1
                return result

        else:

            def wrapper(*args, **kwds):
                # Size limited caching that tracks accesses by recency
                nonlocal root, hits, misses, full
                key = make_key(args, kwds, typed)
                with lock:
                    link = cache_get(key)
                    if link is not None:
                        # Move the link to the front of the circular queue
                        link_prev, link_next, _key, result = link
                        link_prev[NEXT] = link_next
                        link_next[PREV] = link_prev
                        last = root[PREV]
                        last[NEXT] = root[PREV] = link
                        link[PREV] = last
                        link[NEXT] = root
                        hits += 1
                        return result
                result = user_function(*args, **kwds)
                with lock:
                    if key in cache:
                        # Getting here means that this same key was added to the
                        # cache while the lock was released.  Since the link
                        # update is already done, we need only return the
                        # computed result and update the count of misses.
                        pass
                    elif full:
                        # Use the old root to store the new key and result.
                        oldroot = root
                        oldroot[KEY] = key
                        oldroot[RESULT] = result
                        # Empty the oldest link and make it the new root.
                        # Keep a reference to the old key and old result to
                        # prevent their ref counts from going to zero during the
                        # update. That will prevent potentially arbitrary object
                        # clean-up code (i.e. __del__) from running while we're
                        # still adjusting the links.
                        root = oldroot[NEXT]
                        oldkey = root[KEY]
                        oldresult = root[RESULT]
                        root[KEY] = root[RESULT] = None
                        # Now update the cache dictionary.
                        del cache[oldkey]
                        # Save the potentially reentrant cache[key] assignment
                        # for last, after the root and links have been put in
                        # a consistent state.
                        cache[key] = oldroot
                    else:
                        # Put result in a new link at the front of the queue.
                        last = root[PREV]
                        link = [last, root, key, result]
                        last[NEXT] = root[PREV] = cache[key] = link
                        full = (len(cache) >= maxsize)
                    misses += 1
                return result

        def cache_info():
            """Report cache statistics"""
            with lock:
                return _CacheInfo(hits, misses, maxsize, len(cache))

        def cache_clear():
            """Clear the cache and cache statistics"""
            nonlocal hits, misses, full
            with lock:
                cache.clear()
                root[:] = [root, root, None, None]
                hits = misses = 0
                full = False

        wrapper.cache_info = cache_info
        wrapper.cache_clear = cache_clear
        return update_wrapper(wrapper, user_function)

    return decorating_function


################################################################################
### singledispatch() - single-dispatch generic function decorator
################################################################################

def _c3_merge(sequences):
    """Merges MROs in *sequences* to a single MRO using the C3 algorithm.

    Adapted from http://www.python.org/download/releases/2.3/mro/.

    """
    result = []
    while True:
        sequences = [s for s in sequences if s]   # purge empty sequences
        if not sequences:
            return result
        for s1 in sequences:   # find merge candidates among seq heads
            candidate = s1[0]
            for s2 in sequences:
                if candidate in s2[1:]:
                    candidate = None
                    break      # reject the current head, it appears later
            else:
                break
        if not candidate:
            raise RuntimeError("Inconsistent hierarchy")
        result.append(candidate)
        # remove the chosen candidate
        for seq in sequences:
            if seq[0] == candidate:
                del seq[0]

def _c3_mro(cls, abcs=None):
    """Computes the method resolution order using extended C3 linearization.

    If no *abcs* are given, the algorithm works exactly like the built-in C3
    linearization used for method resolution.

    If given, *abcs* is a list of abstract base classes that should be inserted
    into the resulting MRO. Unrelated ABCs are ignored and don't end up in the
    result. The algorithm inserts ABCs where their functionality is introduced,
    i.e. issubclass(cls, abc) returns True for the class itself but returns
    False for all its direct base classes. Implicit ABCs for a given class
    (either registered or inferred from the presence of a special method like
    __len__) are inserted directly after the last ABC explicitly listed in the
    MRO of said class. If two implicit ABCs end up next to each other in the
    resulting MRO, their ordering depends on the order of types in *abcs*.

    """
    for i, base in enumerate(reversed(cls.__bases__)):
        if hasattr(base, '__abstractmethods__'):
            boundary = len(cls.__bases__) - i
            break   # Bases up to the last explicit ABC are considered first.
    else:
        boundary = 0
    abcs = list(abcs) if abcs else []
    explicit_bases = list(cls.__bases__[:boundary])
    abstract_bases = []
    other_bases = list(cls.__bases__[boundary:])
    for base in abcs:
        if issubclass(cls, base) and not any(
                issubclass(b, base) for b in cls.__bases__
            ):
            # If *cls* is the class that introduces behaviour described by
            # an ABC *base*, insert said ABC to its MRO.
            abstract_bases.append(base)
    for base in abstract_bases:
        abcs.remove(base)
    explicit_c3_mros = [_c3_mro(base, abcs=abcs) for base in explicit_bases]
    abstract_c3_mros = [_c3_mro(base, abcs=abcs) for base in abstract_bases]
    other_c3_mros = [_c3_mro(base, abcs=abcs) for base in other_bases]
    return _c3_merge(
        [[cls]] +
        explicit_c3_mros + abstract_c3_mros + other_c3_mros +
        [explicit_bases] + [abstract_bases] + [other_bases]
    )

def _compose_mro(cls, types):
    """Calculates the method resolution order for a given class *cls*.

    Includes relevant abstract base classes (with their respective bases) from
    the *types* iterable. Uses a modified C3 linearization algorithm.

    """
    bases = set(cls.__mro__)
    # Remove entries which are already present in the __mro__ or unrelated.
    def is_related(typ):
        return (typ not in bases and hasattr(typ, '__mro__')
                                 and issubclass(cls, typ))
    types = [n for n in types if is_related(n)]
    # Remove entries which are strict bases of other entries (they will end up
    # in the MRO anyway.
    def is_strict_base(typ):
        for other in types:
            if typ != other and typ in other.__mro__:
                return True
        return False
    types = [n for n in types if not is_strict_base(n)]
    # Subclasses of the ABCs in *types* which are also implemented by
    # *cls* can be used to stabilize ABC ordering.
    type_set = set(types)
    mro = []
    for typ in types:
        found = []
        for sub in typ.__subclasses__():
            if sub not in bases and issubclass(cls, sub):
                found.append([s for s in sub.__mro__ if s in type_set])
        if not found:
            mro.append(typ)
            continue
        # Favor subclasses with the biggest number of useful bases
        found.sort(key=len, reverse=True)
        for sub in found:
            for subcls in sub:
                if subcls not in mro:
                    mro.append(subcls)
    return _c3_mro(cls, abcs=mro)

def _find_impl(cls, registry):
    """Returns the best matching implementation from *registry* for type *cls*.

    Where there is no registered implementation for a specific type, its method
    resolution order is used to find a more generic implementation.

    Note: if *registry* does not contain an implementation for the base
    *object* type, this function may return None.

    """
    mro = _compose_mro(cls, registry.keys())
    match = None
    for t in mro:
        if match is not None:
            # If *match* is an implicit ABC but there is another unrelated,
            # equally matching implicit ABC, refuse the temptation to guess.
            if (t in registry and t not in cls.__mro__
                              and match not in cls.__mro__
                              and not issubclass(match, t)):
                raise RuntimeError("Ambiguous dispatch: {} or {}".format(
                    match, t))
            break
        if t in registry:
            match = t
    return registry.get(match)

def singledispatch(func):
    """Single-dispatch generic function decorator.

    Transforms a function into a generic function, which can have different
    behaviours depending upon the type of its first argument. The decorated
    function acts as the default implementation, and additional
    implementations can be registered using the register() attribute of the
    generic function.

    """
    registry = {}
    dispatch_cache = WeakKeyDictionary()
    cache_token = None

    def dispatch(cls):
        """generic_func.dispatch(cls) -> <function implementation>

        Runs the dispatch algorithm to return the best available implementation
        for the given *cls* registered on *generic_func*.

        """
        nonlocal cache_token
        if cache_token is not None:
            current_token = get_cache_token()
            if cache_token != current_token:
                dispatch_cache.clear()
                cache_token = current_token
        try:
            impl = dispatch_cache[cls]
        except KeyError:
            try:
                impl = registry[cls]
            except KeyError:
                impl = _find_impl(cls, registry)
            dispatch_cache[cls] = impl
        return impl

    def register(cls, func=None):
        """generic_func.register(cls, func) -> func

        Registers a new implementation for the given *cls* on a *generic_func*.

        """
        nonlocal cache_token
        if func is None:
            return lambda f: register(cls, f)
        registry[cls] = func
        if cache_token is None and hasattr(cls, '__abstractmethods__'):
            cache_token = get_cache_token()
        dispatch_cache.clear()
        return func

    def wrapper(*args, **kw):
        return dispatch(args[0].__class__)(*args, **kw)

    registry[object] = func
    wrapper.register = register
    wrapper.dispatch = dispatch
    wrapper.registry = MappingProxyType(registry)
    wrapper._clear_cache = dispatch_cache.clear
    update_wrapper(wrapper, func)
    return wrapper