path: root/blessings/terminal.py

# encoding: utf-8
"""This module contains :class:`Terminal`, the primary API entry point."""

import codecs
import collections
import contextlib
import curses
import functools
import io
import locale
import os
import platform
import select
import struct
import sys
import time
import warnings

try:
    import termios
    import fcntl
    import tty
except ImportError:
    _TTY_METHODS = ('setraw', 'cbreak', 'kbhit', 'height', 'width')
    _MSG_NOSUPPORT = (
        "One or more of the modules: 'termios', 'fcntl', and 'tty' "
        "are not found on your platform '{0}'. The following methods "
        "of Terminal are dummy/no-op unless a deriving class overrides "
        "them: {1}".format(sys.platform.lower(), ', '.join(_TTY_METHODS)))
    warnings.warn(_MSG_NOSUPPORT)
    HAS_TTY = False
else:
    HAS_TTY = True

try:
    InterruptedError
except NameError:
    # alias py2 exception to py3
    InterruptedError = select.error

# local imports
from .formatters import (
    ParameterizingString,
    NullCallableString,
    resolve_capability,
    resolve_attribute,
)

from .sequences import (
    init_sequence_patterns,
    SequenceTextWrapper,
    Sequence,
)

from .keyboard import (
    get_keyboard_sequences,
    get_keyboard_codes,
    resolve_sequence,
)


class Terminal(object):
    """
    An abstraction for color, style, positioning, and input in the terminal

    This keeps the endless calls to ``tigetstr()`` and ``tparm()`` out of your
    code, acts intelligently when somebody pipes your output to a non-terminal,
    and abstracts over the complexity of unbuffered keyboard input. It uses the
    terminfo database to remain portable across terminal types.
    """
    #: Sugary names for commonly-used capabilities
    _sugar = dict(
        save='sc',
        restore='rc',
        # 'clear' clears the whole screen.
        clear_eol='el',
        clear_bol='el1',
        clear_eos='ed',
        position='cup',  # deprecated
        enter_fullscreen='smcup',
        exit_fullscreen='rmcup',
        move='cup',
        move_x='hpa',
        move_y='vpa',
        move_left='cub1',
        move_right='cuf1',
        move_up='cuu1',
        move_down='cud1',
        hide_cursor='civis',
        normal_cursor='cnorm',
        reset_colors='op',  # oc doesn't work on my OS X terminal.
        normal='sgr0',
        reverse='rev',
        italic='sitm',
        no_italic='ritm',
        shadow='sshm',
        no_shadow='rshm',
        standout='smso',
        no_standout='rmso',
        subscript='ssubm',
        no_subscript='rsubm',
        superscript='ssupm',
        no_superscript='rsupm',
        underline='smul',
        no_underline='rmul')

    def __init__(self, kind=None, stream=None, force_styling=False):
        """
        Initialize the terminal.

        :param str kind: A terminal string as taken by
            :func:`curses.setupterm`. Defaults to the value of the ``TERM``
            environment variable.

            .. note:: Terminals withing a single process must share a common
                ``kind``. See :obj:`_CUR_TERM`.

        :param file stream: A file-like object representing the Terminal
            output. Defaults to the original value of :obj:`sys.__stdout__`,
            like :func:`curses.initscr` does.

            If ``stream`` is not a tty, empty Unicode strings are returned for
            all capability values, so things like piping your program output to
            a pipe or file does not emit terminal sequences.
        :param bool force_styling: Whether to force the emission of
            capabilities even if :obj:`sys.__stdout__` does not seem to be
            connected to a terminal. If you want to force styling to not
            happen, use ``force_styling=None``.

            This comes in handy if users are trying to pipe your output through
            something like ``less -r`` or build systems which support decoding
            of terminal sequences.
        """
        # pylint: disable=global-statement
        #         Using the global statement (col 8)

        global _CUR_TERM
        self._keyboard_fd = None

        # Default stream is stdout, keyboard valid as stdin only when
        # output stream is stdout is a tty.
        if stream is None or stream == sys.__stdout__:
            stream = sys.__stdout__
            self._keyboard_fd = sys.__stdin__.fileno()

        try:
            stream_fd = (stream.fileno() if hasattr(stream, 'fileno') and
                         callable(stream.fileno) else None)
        except io.UnsupportedOperation:
            stream_fd = None

        self._is_a_tty = stream_fd is not None and os.isatty(stream_fd)
        self._does_styling = ((self.is_a_tty or force_styling) and
                              force_styling is not None)

        # _keyboard_fd only non-None if both stdin and stdout is a tty.
        self._keyboard_fd = (self._keyboard_fd
                             if self._keyboard_fd is not None and
                             self.is_a_tty and os.isatty(self._keyboard_fd)
                             else None)
        self._normal = None  # cache normal attr, preventing recursive lookups

        # The descriptor to direct terminal initialization sequences to.
        self._init_descriptor = (stream_fd is None and
                                 sys.__stdout__.fileno() or
                                 stream_fd)
        self._kind = kind or os.environ.get('TERM', 'unknown')

        if self.does_styling:
            # Make things like tigetstr() work. Explicit args make setupterm()
            # work even when -s is passed to nosetests. Lean toward sending
            # init sequences to the stream if it has a file descriptor, and
            # send them to stdout as a fallback, since they have to go
            # somewhere.
            try:
                if (platform.python_implementation() == 'PyPy' and
                        isinstance(self._kind, unicode)):
                    # pypy/2.4.0_2/libexec/lib_pypy/_curses.py, line 1131
                    # TypeError: initializer for ctype 'char *' must be a str
                    curses.setupterm(self._kind.encode('ascii'),
                                     self._init_descriptor)
                else:
                    curses.setupterm(self._kind, self._init_descriptor)
            except curses.error as err:
                warnings.warn('Failed to setupterm(kind={0!r}): {1}'
                              .format(self._kind, err))
                self._kind = None
                self._does_styling = False
            else:
                if _CUR_TERM is None or self._kind == _CUR_TERM:
                    _CUR_TERM = self._kind
                else:
                    warnings.warn(
                        'A terminal of kind "%s" has been requested; due to an'
                        ' internal python curses bug, terminal capabilities'
                        ' for a terminal of kind "%s" will continue to be'
                        ' returned for the remainder of this process.' % (
                            self._kind, _CUR_TERM,))

        for re_name, re_val in init_sequence_patterns(self).items():
            setattr(self, re_name, re_val)

        # Build database of int code <=> KEY_NAME.
        self._keycodes = get_keyboard_codes()

        # Store attributes as: self.KEY_NAME = code.
        for key_code, key_name in self._keycodes.items():
            setattr(self, key_name, key_code)

        # Build database of sequence <=> KEY_NAME.
        self._keymap = get_keyboard_sequences(self)

        self._keyboard_buf = collections.deque()
        if self._keyboard_fd is not None:
            locale.setlocale(locale.LC_ALL, '')
            self._encoding = locale.getpreferredencoding() or 'ascii'
            try:
                self._keyboard_decoder = codecs.getincrementaldecoder(
                    self._encoding)()
            except LookupError as err:
                warnings.warn('LookupError: %s, fallback to ASCII for '
                              'keyboard.' % (err,))
                self._encoding = 'ascii'
                self._keyboard_decoder = codecs.getincrementaldecoder(
                    self._encoding)()

        self._stream = stream

    def __getattr__(self, attr):
        r"""
        Return a terminal capability as Unicode string.

        For example, ``term.bold`` is a unicode string that may be prepended
        to text to set the video attribute for bold, which should also be
        terminated with the pairing :attr:`normal`. This capability
        returns a callable, so you can use ``term.bold("hi")`` which
        results in the joining of ``(term.bold, "hi", term.normal)``.

        Compound formatters may also be used. For example...

        >>> term.bold_blink_red_on_green("merry x-mas!").
        u'\x1b[1m\x1b[5m\x1b[31m\x1b[42mmerry x-mas!\x1b[m'

        For a parametrized capability such as ``move`` (or ``cup``), pass the
        parameters as positional arguments:
        
        >>> term.move(line, column)
        
        See the manual page terminfo(5) for a complete list of capabilities and
        their arguments.
        """
        if not self.does_styling:
            return NullCallableString()
        val = resolve_attribute(self, attr)
        # Cache capability codes.
        setattr(self, attr, val)
        return val

    @property
    def kind(self):
        """The terminal type this instance was initialized with"""
        return self._kind

    @property
    def does_styling(self):
        """Whether this instance will emit terminal sequences"""
        return self._does_styling

    @property
    def is_a_tty(self):
        """Whether :attr:`~.stream` is a terminal"""
        return self._is_a_tty

    @property
    def height(self):
        """The height of the terminal (in number of lines)"""
        return self._height_and_width().ws_row

    @property
    def width(self):
        """The width of the terminal (in number of columns)"""
        return self._height_and_width().ws_col

    @staticmethod
    def _winsize(fdesc):
        """
        Return named tuple describing size of the terminal by ``fdesc``.

        If the given platform does not have modules :mod:`termios`,
        :mod:`fcntl`, or :mod:`tty`, window size of 80 columns by 24
        rows is always returned.

        :param int fdesc: file descriptor queries for its window size.
        :raises IOError: the file descriptor ``fdesc`` is not a terminal.
        :rtype: WINSZ

        WINSZ is a :class:`collections.namedtuple` instance, whose structure
        directly maps to the return value of the :const:`termios.TIOCGWINSZ`
        ioctl return value. The return parameters are:

            - ``ws_row``: width of terminal by its number of character cells.
            - ``ws_col``: height of terminal by its number of character cells.
            - ``ws_xpixel``: width of terminal by pixels (not accurate).
            - ``ws_ypixel``: height of terminal by pixels (not accurate).
        """
        if HAS_TTY:
            data = fcntl.ioctl(fdesc, termios.TIOCGWINSZ, WINSZ._BUF)
            return WINSZ(*struct.unpack(WINSZ._FMT, data))
        return WINSZ(ws_row=24, ws_col=80, ws_xpixel=0, ws_ypixel=0)

    def _height_and_width(self):
        """
        Return a tuple of (terminal height, terminal width).

        If :attr:`stream` or :obj:`sys.__stdout__` is not a tty or does not
        support :func:`fcntl.ioctl` of :const:`termios.TIOCGWINSZ`, a window
        size of 80 columns by 24 rows is returned.

        :rtype: WINSZ

        WINSZ is a :class:`collections.namedtuple` instance, whose structure
        directly maps to the return value of the :const:`termios.TIOCGWINSZ`
        ioctl return value. The return parameters are:

            - ``ws_row``: width of terminal by its number of character cells.
            - ``ws_col``: height of terminal by its number of character cells.
            - ``ws_xpixel``: width of terminal by pixels (not accurate).
            - ``ws_ypixel``: height of terminal by pixels (not accurate).

        """
        for fdesc in (self._init_descriptor, sys.__stdout__):
            # pylint: disable=pointless-except
            #         Except doesn't do anything
            try:
                if fdesc is not None:
                    return self._winsize(fdesc)
            except IOError:
                pass

        return WINSZ(ws_row=int(os.getenv('LINES', '25')),
                     ws_col=int(os.getenv('COLUMNS', '80')),
                     ws_xpixel=None,
                     ws_ypixel=None)

    @contextlib.contextmanager
    def location(self, x=None, y=None):
        """
        Return a context manager for temporarily moving the cursor.

        Move the cursor to a certain position on entry, let you print stuff
        there, then return the cursor to its original position::

            term = Terminal()
            with term.location(2, 5):
                for x in xrange(10):
                    print('I can do it %i times!' % x)
            print('We're back to the original location.')

        Specify ``x`` to move to a certain column, ``y`` to move to a certain
        row, both, or neither. If you specify neither, only the saving and
        restoration of cursor position will happen. This can be useful if you
        simply want to restore your place after doing some manual cursor
        movement.

        .. note:: The store- and restore-cursor capabilities used internally
            provide no stack. This means that :meth:`location` calls cannot be
            nested: only one should be entered at a time.
        """
        # pylint: disable=invalid-name
        #         Invalid argument name "x"

        # Save position and move to the requested column, row, or both:
        self.stream.write(self.save)
        if x is not None and y is not None:
            self.stream.write(self.move(y, x))
        elif x is not None:
            self.stream.write(self.move_x(x))
        elif y is not None:
            self.stream.write(self.move_y(y))
        try:
            yield
        finally:
            # Restore original cursor position:
            self.stream.write(self.restore)

    @contextlib.contextmanager
    def fullscreen(self):
        """
        Return a context manager that enters fullscreen mode while inside it
        and restores normal mode on leaving.

        Under the hood, this switches between the primary screen buffer and the
        secondary one. The primary one is saved on entry and restored on exit.
        Likewise, the secondary contents are also stable and are faithfully
        restored on the next entry::

            with term.fullscreen():
                main()

        .. note:: There is only one primary buffer and one secondary one. Thus, :meth:`fullscreen` calls cannot be nested: only one should
            be entered at a time.
        """
        self.stream.write(self.enter_fullscreen)
        try:
            yield
        finally:
            self.stream.write(self.exit_fullscreen)

    @contextlib.contextmanager
    def hidden_cursor(self):
        """
        Return a context manager that hides the cursor while inside it and
        makes it visible on leaving. ::

            with term.hidden_cursor():
                main()

        .. note:: :meth:`hidden_cursor` calls cannot be nested: only one
            should be entered at a time.
        """
        self.stream.write(self.hide_cursor)
        try:
            yield
        finally:
            self.stream.write(self.normal_cursor)

    @property
    def color(self):
        """
        A callable string that sets the foreground color

        :arg int num: The foreground color index. This should be within the
           bounds of :attr:`~.number_of_colors`.
        :rtype: ParameterizingString

        The capability is unparameterized until called and passed a number,
        0-15, at which point it returns another string which represents a
        specific color change. This second string can further be called to
        color a piece of text and set everything back to normal afterward.
        """
        if not self.does_styling:
            return NullCallableString()
        return ParameterizingString(self._foreground_color,
                                    self.normal, 'color')

    @property
    def on_color(self):
        """
        A capability that sets the background color

        :arg int num: The background color index.
        :rtype: ParameterizingString
        """
        if not self.does_styling:
            return NullCallableString()
        return ParameterizingString(self._background_color,
                                    self.normal, 'on_color')

    @property
    def normal(self):
        """
        A capability that resets all video attributes

        :rtype: str

        normal is an alias for ``sgr0`` or ``exit_attribute_mode``. Any
        styling attributes previously applied, such as foreground or background
        colors, reverse video, or bold are reset to defaults.
        """
        if self._normal:
            return self._normal
        self._normal = resolve_capability(self, 'normal')
        return self._normal

    @property
    def stream(self):
        """
        The stream the terminal outputs to

        This is a convenience attribute. It is used internally for implied
        writes performed by context managers :meth:`~.hidden_cursor`,
        :meth:`~.fullscreen`, :meth:`~.location`, and :meth:`~.keypad`.
        """
        return self._stream

    @property
    def number_of_colors(self):
        """
        The number of colors the terminal supports

        Common values are 0, 8, 16, 88, and 256. Most commonly, this may be
        used to test whether the terminal supports colors. Though the
        underlying capability returns -1 when there is no color support, we
        return 0. This lets you test more Pythonically::

            if term.number_of_colors:
                ...
        """
        # This is actually the only remotely useful numeric capability. We
        # don't name it after the underlying capability, because we deviate
        # slightly from its behavior, and we might someday wish to give direct
        # access to it.

        # trim value to 0, as tigetnum('colors') returns -1 if no support,
        # and -2 if no such capability.
        return max(0, self.does_styling and curses.tigetnum('colors') or -1)

    @property
    def _foreground_color(self):
        """
        Convenience capability to support :attr:`~.on_color`

        Prefers returning sequence for capability ``setaf``, "Set foreground
        color to #1, using ANSI escape". If the given terminal does not
        support such sequence, fallback to returning attribute ``setf``,
        "Set foreground color #1".
        """
        return self.setaf or self.setf

    @property
    def _background_color(self):
        """
        Convenience capability to support :attr:`~.on_color`

        Prefers returning sequence for capability ``setab``, "Set background
        color to #1, using ANSI escape". If the given terminal does not
        support such sequence, fallback to returning attribute ``setb``,
        "Set background color #1".
        """
        return self.setab or self.setb

    def ljust(self, text, width=None, fillchar=u' '):
        """
        Left-align ``text``, which may contain terminal sequences.

        :arg str text: String to be aligned
        :arg int width: Total width to fill with aligned text. If
            unspecified, the whole width of the terminal is filled.
        :arg str fillchar: String for padding the right of ``text``
        :rtype: str
        """
        # Left justification is different from left alignment, but we continue
        # the vocabulary error of the str method for polymorphism.
        if width is None:
            width = self.width
        return Sequence(text, self).ljust(width, fillchar)

    def rjust(self, text, width=None, fillchar=u' '):
        """
        Right-align ``text``, which may contain terminal sequences.

        :arg str text: String to be aligned
        :arg int width: Total width to fill with aligned text. If
            unspecified, the whole width of the terminal is used.
        :arg str fillchar: String for padding the left of ``text``
        :rtype: str
        """
        if width is None:
            width = self.width
        return Sequence(text, self).rjust(width, fillchar)

    def center(self, text, width=None, fillchar=u' '):
        """
        Center ``text``, which may contain terminal sequences.

        :arg str text: String to be centered
        :arg int width: Total width in which to center text. If
            unspecified, the whole width of the terminal is used.
        :arg str fillchar: String for padding the left and right of ``text``
        :rtype: str
        """
        if width is None:
            width = self.width
        return Sequence(text, self).center(width, fillchar)

    def length(self, text):
        u"""
        Return printable length of a string containing sequences.

        :arg str text: String to measure. May contain terminal sequences.
        :rtype: int
        :returns: The number of terminal character cells the string will occupy
            when printed

        Wide characters that consume 2 character cells are supported:

        >>> term = Terminal()
        >>> term.length(term.clear + term.red(u'コンニチハ'))
        10

        .. note:: Sequences such as 'clear', which is considered as a
            "movement sequence" because it would move the cursor to
            (y, x)(0, 0), are evaluated as a printable length of
            *0*.
        """
        return Sequence(text, self).length()

    def strip(self, text, chars=None):
        r"""
        Return ``text`` without sequences and leading or trailing whitespace.

        :rtype: str

        >>> term = blessings.Terminal()
        >>> term.strip(u' \x1b[0;3m XXX ')
        u'XXX'
        """
        return Sequence(text, self).strip(chars)

    def rstrip(self, text, chars=None):
        r"""
        Return ``text`` without terminal sequences or trailing whitespace.

        :rtype: str

        >>> term = blessings.Terminal()
        >>> term.rstrip(u' \x1b[0;3m XXX ')
        u'  XXX'
        """
        return Sequence(text, self).rstrip(chars)

    def lstrip(self, text, chars=None):
        r"""
        Return ``text`` without terminal sequences or leading whitespace.

        :rtype: str

        >>> term = blessings.Terminal()
        >>> term.lstrip(u' \x1b[0;3m XXX ')
        u'XXX '
        """
        return Sequence(text, self).lstrip(chars)

    def strip_seqs(self, text):
        r"""
        Return ``text`` stripped of only its terminal sequences

        :rtype: str

        >>> term = blessings.Terminal()
        >>> term.strip_seqs(u'\x1b[0;3mXXX')
        u'XXX'
        """
        return Sequence(text, self).strip_seqs()

    def wrap(self, text, width=None, **kwargs):
        """
        Text-wrap a string, returning a list of wrapped lines.

        :arg str text: Unlike :func:`textwrap.wrap`, ``text`` may contain
            terminal sequences, such as colors, bold, or underline. By
            default, tabs in ``text`` are expanded by
            :func:`string.expandtabs`.
        :arg int width: Unlike :func:`textwrap.wrap`, ``width`` will
            default to the width of the attached terminal.
        :rtype: list

        See :class:`textwrap.TextWrapper` for keyword arguments that can
        customize wrapping behaviour.
        """
        width = self.width if width is None else width
        lines = []
        for line in text.splitlines():
            lines.extend(
                (_linewrap for _linewrap in SequenceTextWrapper(
                    width=width, term=self, **kwargs).wrap(text))
                if line.strip() else (u'',))

        return lines

    def _next_char(self):
        """
        Read, decode, and return the next byte from the keyboard stream.

        :rtype: unicode
        :returns: a single unicode character, or ``u''`` if a multi-byte
            sequence has not yet been fully received.

        This method supports :meth:`keystroke`, reading only one byte from
        the keyboard string at a time. This method should always return
        without blocking if called when :meth:`_char_is_ready` returns
        True.

        Implementors of alternate input stream methods should override
        this method.
        """
        assert self._keyboard_fd is not None
        byte = os.read(self._keyboard_fd, 1)
        return self._keyboard_decoder.decode(byte, final=False)

    def _char_is_ready(self, timeout=None):
        """
        Return whether a keypress has been detected on the keyboard

        This method is used by method :meth:`keystroke` to determine if
        a byte may be read using method :meth:`_next_char` without blocking.

        :arg float timeout: When ``timeout`` is 0, this call is
            non-blocking, otherwise blocking indefinitely until keypress
            is detected when None (default). When ``timeout`` is a
            positive number, returns after ``timeout`` seconds have
            elapsed (float).
        :rtype: bool
        :returns: True if a keypress is awaiting to be read on the keyboard
            attached to this terminal. If input is not a terminal, False is
            always returned.
        """
        stime = time.time()
        ready_r = [None, ]
        check_r = [self._keyboard_fd] if self._keyboard_fd is not None else []

        while HAS_TTY and True:
            try:
                ready_r, _, _ = select.select(check_r, [], [], timeout)
            except InterruptedError:
                # Beginning with python3.5, IntrruptError is no longer thrown
                # https://www.python.org/dev/peps/pep-0475/
                #
                # For previous versions of python, we take special care to
                # retry select on InterruptedError exception, namely to handle
                # a custom SIGWINCH handler. When installed, it would cause
                # select() to be interrupted with errno 4 (EAGAIN).
                #
                # Just as in python3.5, it is ignored, and a new timeout value
                # is derived from the previous unless timeout becomes negative.
                # because the signal handler has blocked beyond timeout, then
                # False is returned. Otherwise, when timeout is None, we
                # continue to block indefinitely (default).
                if timeout is not None:
                    # subtract time already elapsed,
                    timeout -= time.time() - stime
                    if timeout > 0:
                        continue
                    # no time remains after handling exception (rare)
                    ready_r = []
                    break
            else:
                break

        return False if self._keyboard_fd is None else check_r == ready_r

    @contextlib.contextmanager
    def keystroke_input(self, raw=False):
        """
        Return a context manager that enables key-at-a-time input.

        Normally, characters received from the keyboard cannot be read by
        Python until the Return key is pressed. This is called
        "cooked" or "canonical input" mode, and it allows the tty driver to provide
        line-editing shuttling the input to your program. It is usually the
        default mode set by a Unix shell before executing a program.

        This context manager activates 'rare' mode, the opposite of 'cooked'
        mode: On entry, :func:`tty.setcbreak` mode is activated, disabling
        line-buffering of keyboard input and turning off automatic echoing of
        input. This allows each keystroke to be read immediately after it is
        pressed.

        :arg bool raw: When True, enter :func:`tty.setraw` mode instead.
           Raw mode differs in that the interrupt, quit, suspend, and
           flow-control characters are all passed through as their raw
           character values instead of generating signals.

        Technically, this context manager sets the :mod:`termios` attributes of
        the terminal attached to :obj:`sys.__stdin__`.

        .. note:: You must explicitly print any input you would like displayed.
            If you provide any kind of editing, you must handle backspace and
            other line-editing control characters.

        .. note:: :func:`tty.setcbreak` sets ``VMIN = 1`` and ``VTIME = 0``,
            see http://www.unixwiz.net/techtips/termios-vmin-vtime.html
        """
        if HAS_TTY and self._keyboard_fd is not None:
            # Save current terminal mode:
            save_mode = termios.tcgetattr(self._keyboard_fd)
            mode_setter = tty.setraw if raw else tty.setcbreak
            mode_setter(self._keyboard_fd, termios.TCSANOW)
            try:
                yield
            finally:
                # Restore prior mode:
                termios.tcsetattr(self._keyboard_fd,
                                  termios.TCSAFLUSH,
                                  save_mode)
        else:
            yield

    @contextlib.contextmanager
    def keypad(self):
        r"""
        Return a context manager that enables directional keypad input.

        On entrying, this puts the terminal into "keyboard_transmit" mode by
        emitting the keypad_xmit (smkx) capability. On exit, it emits
        keypad_local (rmkx).

        On an IBM-PC keyboard with numeric keypad of terminal-type *xterm*,
        with numlock off, the lower-left diagonal key transmits sequence
        ``\\x1b[F``, translated to :class:`~.Terminal` attribute
        ``KEY_END``.

        However, upon entering :meth:`keypad`, ``\\x1b[OF`` is transmitted,
        translating to ``KEY_LL`` (lower-left key), allowing you to determine
        diagonal direction keys.
        """
        try:
            self.stream.write(self.smkx)
            yield
        finally:
            self.stream.write(self.rmkx)

    def keystroke(self, timeout=None, esc_delay=0.35):
        """
        Read and return the next keystroke within a given timeout.

        Generally, this should be used inside the :meth:`keystroke_input`
        context manager.

        :arg float timeout: Number of seconds to wait for a keystroke before
            returning. When None (default), this method blocks indefinitely.
        :arg float esc_delay: To distinguish between ``KEY_ESCAPE`` and
           sequences beginning with escape, the parameter ``esc_delay``
           specifies the amount of time after receiving the escape character
           (``chr(27)``) to seek for the completion of an application key
           before returning a :class:`~.Keystroke` for ``KEY_ESCAPE``.
        :rtype: :class:`~.Keystroke`.
        :raises NoKeyboard: The :attr:`stream` is not a terminal, and
            ``timeout`` is None, which would cause the program to hang
            forever.
        :returns: :class:`~.Keystroke`, which may be empty (``u''``) if
           ``timeout`` is specified and keystroke is not received.

        .. note:: When used without the context manager
            :meth:`keystroke_input`, :obj:`sys.__stdin__` remains
            line-buffered, and this function will block until the return key
            is pressed.
        """
        if timeout is None and self._keyboard_fd is None:
            raise NoKeyboard(
                'Waiting for a keystroke on a terminal with no keyboard '
                'attached and no timeout would hang forever. Add a timeout, '
                'and revise your program logic.')

        def time_left(stime, timeout):
            """
            Return time remaining since ``stime`` before given ``timeout``.

            This function assists determining the value of ``timeout`` for
            class method :meth:`_char_is_ready`.

            :arg float stime: starting time for measurement
            :arg float timeout: timeout period, may be set to None to
               indicate no timeout (where 0 is always returned).
            :rtype: float or int
            :returns: time remaining as float. If no time is remaining,
               then the integer ``0`` is returned.
            """
            if timeout is not None:
                if timeout == 0:
                    return 0
                return max(0, timeout - (time.time() - stime))

        resolve = functools.partial(resolve_sequence,
                                    mapper=self._keymap,
                                    codes=self._keycodes)

        stime = time.time()

        # re-buffer previously received keystrokes,
        ucs = u''
        while self._keyboard_buf:
            ucs += self._keyboard_buf.pop()

        # receive all immediately available bytes
        while self._char_is_ready(timeout=0):
            ucs += self._next_char()

        # decode keystroke, if any
        keystroke = resolve(text=ucs)

        # so long as the most immediately received or buffered keystroke is
        # incomplete, (which may be a multibyte encoding), block until until
        # one is received.
        while (not keystroke and
               self._char_is_ready(timeout=time_left(stime, timeout))):
            ucs += self._next_char()
            keystroke = resolve(text=ucs)

        # handle escape key (KEY_ESCAPE) vs. escape sequence (which begins
        # with KEY_ESCAPE, \x1b[, \x1bO, or \x1b?), up to esc_delay when
        # received. This is not optimal, but causes least delay when
        # (currently unhandled, and rare) "meta sends escape" is used,
        # or when an unsupported sequence is sent.
        if keystroke.code == self.KEY_ESCAPE:
            esctime = time.time()
            while (keystroke.code == self.KEY_ESCAPE and
                   self._char_is_ready(timeout=time_left(esctime, esc_delay))):
                ucs += self._next_char()
                keystroke = resolve(text=ucs)

        # buffer any remaining text received
        self._keyboard_buf.extendleft(ucs[len(keystroke):])
        return keystroke


class NoKeyboard(Exception):
    """An error raised when an Illegal operation requiring a keyboard without
    one attached."""


class WINSZ(collections.namedtuple('WINSZ', (
        'ws_row', 'ws_col', 'ws_xpixel', 'ws_ypixel'))):
    """
    Structure represents return value of :const:`termios.TIOCGWINSZ`.

    .. py:attribute:: ws_row

        rows, in characters

    .. py:attribute:: ws_col

        columns, in characters

    .. py:attribute:: ws_xpixel

        horizontal size, pixels

    .. py:attribute:: ws_ypixel

        vertical size, pixels
    """
    #: format of termios structure
    _FMT = 'hhhh'
    #: buffer of termios structure appropriate for ioctl argument
    _BUF = '\x00' * struct.calcsize(_FMT)


#: From libcurses/doc/ncurses-intro.html (ESR, Thomas Dickey, et. al)::
#:
#:   "After the call to setupterm(), the global variable cur_term is set to
#:    point to the current structure of terminal capabilities. By calling
#:    setupterm() for each terminal, and saving and restoring cur_term, it
#:    is possible for a program to use two or more terminals at once."
#:
#: However, if you study Python's ``./Modules/_cursesmodule.c``, you'll find::
#:
#:   if (!initialised_setupterm && setupterm(termstr,fd,&err) == ERR) {
#:
#: Python - perhaps wrongly - will not allow for re-initialisation of new
#: terminals through :func:`curses.setupterm`, so the value of cur_term cannot
#: be changed once set: subsequent calls to :func:`setupterm` have no effect.
#:
#: Therefore, the :attr:`Terminal.kind` of each :class:`Terminal` is
#: essentially a singleton. This global variable reflects that, and a warning
#: is emitted if somebody expects otherwise.
_CUR_TERM = None