util.py

"""Miscellaneous utility functions and classes.

This module is used internally by Tornado.  It is not necessarily expected
that the functions and classes defined here will be useful to other
applications, but they are documented here in case they are.

The one public-facing part of this module is the `Configurable` class
and its `~Configurable.configure` method, which becomes a part of the
interface of its subclasses, including `.AsyncHTTPClient`, `.IOLoop`,
and `.Resolver`.
"""

from __future__ import absolute_import, division, print_function, with_statement

import inspect
import sys
import zlib


class ObjectDict(dict):
    """Makes a dictionary behave like an object, with attribute-style access.
    """
    def __getattr__(self, name):
        try:
            return self[name]
        except KeyError:
            raise AttributeError(name)

    def __setattr__(self, name, value):
        self[name] = value


class GzipDecompressor(object):
    """Streaming gzip decompressor.

    The interface is like that of `zlib.decompressobj` (without the
    optional arguments, but it understands gzip headers and checksums.
    """
    def __init__(self):
        # Magic parameter makes zlib module understand gzip header
        # http://stackoverflow.com/questions/1838699/how-can-i-decompress-a-gzip-stream-with-zlib
        # This works on cpython and pypy, but not jython.
        self.decompressobj = zlib.decompressobj(16 + zlib.MAX_WBITS)

    def decompress(self, value):
        """Decompress a chunk, returning newly-available data.

        Some data may be buffered for later processing; `flush` must
        be called when there is no more input data to ensure that
        all data was processed.
        """
        return self.decompressobj.decompress(value)

    def flush(self):
        """Return any remaining buffered data not yet returned by decompress.

        Also checks for errors such as truncated input.
        No other methods may be called on this object after `flush`.
        """
        return self.decompressobj.flush()


def import_object(name):
    """Imports an object by name.

    import_object('x.y.z') is equivalent to 'from x.y import z'.

    >>> import tornado.escape
    >>> import_object('tornado.escape') is tornado.escape
    True
    >>> import_object('tornado.escape.utf8') is tornado.escape.utf8
    True
    """
    parts = name.split('.')
    obj = __import__('.'.join(parts[:-1]), None, None, [parts[-1]], 0)
    return getattr(obj, parts[-1])

# Fake unicode literal support:  Python 3.2 doesn't have the u'' marker for
# literal strings, and alternative solutions like "from __future__ import
# unicode_literals" have other problems (see PEP 414).  u() can be applied
# to ascii strings that include \u escapes (but they must not contain
# literal non-ascii characters).
if type('') is not type(b''):
    def u(s):
        return s
    bytes_type = bytes
    unicode_type = str
    basestring_type = str
else:
    def u(s):
        return s.decode('unicode_escape')
    bytes_type = str
    unicode_type = unicode
    basestring_type = basestring


if sys.version_info > (3,):
    exec("""
def raise_exc_info(exc_info):
    raise exc_info[1].with_traceback(exc_info[2])

def exec_in(code, glob, loc=None):
    if isinstance(code, str):
        code = compile(code, '<string>', 'exec', dont_inherit=True)
    exec(code, glob, loc)
""")
else:
    exec("""
def raise_exc_info(exc_info):
    raise exc_info[0], exc_info[1], exc_info[2]

def exec_in(code, glob, loc=None):
    if isinstance(code, basestring):
        # exec(string) inherits the caller's future imports; compile
        # the string first to prevent that.
        code = compile(code, '<string>', 'exec', dont_inherit=True)
    exec code in glob, loc
""")


class Configurable(object):
    """Base class for configurable interfaces.

    A configurable interface is an (abstract) class whose constructor
    acts as a factory function for one of its implementation subclasses.
    The implementation subclass as well as optional keyword arguments to
    its initializer can be set globally at runtime with `configure`.

    By using the constructor as the factory method, the interface
    looks like a normal class, `isinstance` works as usual, etc.  This
    pattern is most useful when the choice of implementation is likely
    to be a global decision (e.g. when `~select.epoll` is available,
    always use it instead of `~select.select`), or when a
    previously-monolithic class has been split into specialized
    subclasses.

    Configurable subclasses must define the class methods
    `configurable_base` and `configurable_default`, and use the instance
    method `initialize` instead of ``__init__``.
    """
    __impl_class = None
    __impl_kwargs = None

    def __new__(cls, **kwargs):
        base = cls.configurable_base()
        args = {}
        if cls is base:
            impl = cls.configured_class()
            if base.__impl_kwargs:
                args.update(base.__impl_kwargs)
        else:
            impl = cls
        args.update(kwargs)
        instance = super(Configurable, cls).__new__(impl)
        # initialize vs __init__ chosen for compatiblity with AsyncHTTPClient
        # singleton magic.  If we get rid of that we can switch to __init__
        # here too.
        instance.initialize(**args)
        return instance

    @classmethod
    def configurable_base(cls):
        """Returns the base class of a configurable hierarchy.

        This will normally return the class in which it is defined.
        (which is *not* necessarily the same as the cls classmethod parameter).
        """
        raise NotImplementedError()

    @classmethod
    def configurable_default(cls):
        """Returns the implementation class to be used if none is configured."""
        raise NotImplementedError()

    def initialize(self):
        """Initialize a `Configurable` subclass instance.

        Configurable classes should use `initialize` instead of ``__init__``.
        """

    @classmethod
    def configure(cls, impl, **kwargs):
        """Sets the class to use when the base class is instantiated.

        Keyword arguments will be saved and added to the arguments passed
        to the constructor.  This can be used to set global defaults for
        some parameters.
        """
        base = cls.configurable_base()
        if isinstance(impl, (unicode_type, bytes_type)):
            impl = import_object(impl)
        if impl is not None and not issubclass(impl, cls):
            raise ValueError("Invalid subclass of %s" % cls)
        base.__impl_class = impl
        base.__impl_kwargs = kwargs

    @classmethod
    def configured_class(cls):
        """Returns the currently configured class."""
        base = cls.configurable_base()
        if cls.__impl_class is None:
            base.__impl_class = cls.configurable_default()
        return base.__impl_class

    @classmethod
    def _save_configuration(cls):
        base = cls.configurable_base()
        return (base.__impl_class, base.__impl_kwargs)

    @classmethod
    def _restore_configuration(cls, saved):
        base = cls.configurable_base()
        base.__impl_class = saved[0]
        base.__impl_kwargs = saved[1]


class ArgReplacer(object):
    """Replaces one value in an ``args, kwargs`` pair.

    Inspects the function signature to find an argument by name
    whether it is passed by position or keyword.  For use in decorators
    and similar wrappers.
    """
    def __init__(self, func, name):
        self.name = name
        try:
            self.arg_pos = inspect.getargspec(func).args.index(self.name)
        except ValueError:
            # Not a positional parameter
            self.arg_pos = None

    def replace(self, new_value, args, kwargs):
        """Replace the named argument in ``args, kwargs`` with ``new_value``.

        Returns ``(old_value, args, kwargs)``.  The returned ``args`` and
        ``kwargs`` objects may not be the same as the input objects, or
        the input objects may be mutated.

        If the named argument was not found, ``new_value`` will be added
        to ``kwargs`` and None will be returned as ``old_value``.
        """
        if self.arg_pos is not None and len(args) > self.arg_pos:
            # The arg to replace is passed positionally
            old_value = args[self.arg_pos]
            args = list(args)  # *args is normally a tuple
            args[self.arg_pos] = new_value
        else:
            # The arg to replace is either omitted or passed by keyword.
            old_value = kwargs.get(self.name)
            kwargs[self.name] = new_value
        return old_value, args, kwargs


def doctests():
    import doctest
    return doctest.DocTestSuite()