✏ EDIT: base_futures.py

__all__ = []

import concurrent.futures._base
import reprlib

from . import events

Error = concurrent.futures._base.Error
CancelledError = concurrent.futures.CancelledError
TimeoutError = concurrent.futures.TimeoutError

class InvalidStateError(Error):
    """The operation is not allowed in this state."""

# States for Future.
_PENDING = 'PENDING'
_CANCELLED = 'CANCELLED'
_FINISHED = 'FINISHED'

def isfuture(obj):
    """Check for a Future.

This returns True when obj is a Future instance or is advertising
    itself as duck-type compatible by setting _asyncio_future_blocking.
    See comment in Future for more details.
    """
    return (hasattr(obj.__class__, '_asyncio_future_blocking') and
            obj._asyncio_future_blocking is not None)

def _format_callbacks(cb):
    """helper function for Future.__repr__"""
    size = len(cb)
    if not size:
        cb = ''

def format_cb(callback):
        return events._format_callback_source(callback, ())

if size == 1:
        cb = format_cb(cb[0])
    elif size == 2:
        cb = '{}, {}'.format(format_cb(cb[0]), format_cb(cb[1]))
    elif size > 2:
        cb = '{}, <{} more>, {}'.format(format_cb(cb[0]),
                                        size - 2,
                                        format_cb(cb[-1]))
    return 'cb=[%s]' % cb

def _future_repr_info(future):
    # (Future) -> str
    """helper function for Future.__repr__"""
    info = [future._state.lower()]
    if future._state == _FINISHED:
        if future._exception is not None:
            info.append('exception={!r}'.format(future._exception))
        else:
            # use reprlib to limit the length of the output, especially
            # for very long strings
            result = reprlib.repr(future._result)
            info.append('result={}'.format(result))
    if future._callbacks:
        info.append(_format_callbacks(future._callbacks))
    if future._source_traceback:
        frame = future._source_traceback[-1]
        info.append('created at %s:%s' % (frame[0], frame[1]))
    return info

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