Lib/_dummy_thread.py

"""Drop-in replacement for the thread module.

Meant to be used as a brain-dead substitute so that threaded code does
not need to be rewritten for when the thread module is not present.

Suggested usage is::

    try:
        import _thread
    except ImportError:
        import _dummy_thread as _thread

"""
# Exports only things specified by thread documentation;
# skipping obsolete synonyms allocate(), start_new(), exit_thread().
__all__ = ['error', 'start_new_thread', 'exit', 'get_ident', 'allocate_lock',
           'interrupt_main', 'LockType', 'RLock']

# A dummy value
TIMEOUT_MAX = 2**31

# NOTE: this module can be imported early in the extension building process,
# and so top level imports of other modules should be avoided.  Instead, all
# imports are done when needed on a function-by-function basis.  Since threads
# are disabled, the import lock should not be an issue anyway (??).

error = RuntimeError

def start_new_thread(function, args, kwargs={}):
    """Dummy implementation of _thread.start_new_thread().

    Compatibility is maintained by making sure that ``args`` is a
    tuple and ``kwargs`` is a dictionary.  If an exception is raised
    and it is SystemExit (which can be done by _thread.exit()) it is
    caught and nothing is done; all other exceptions are printed out
    by using traceback.print_exc().

    If the executed function calls interrupt_main the KeyboardInterrupt will be
    raised when the function returns.

    """
    if type(args) != type(tuple()):
        raise TypeError("2nd arg must be a tuple")
    if type(kwargs) != type(dict()):
        raise TypeError("3rd arg must be a dict")
    global _main
    _main = False
    try:
        function(*args, **kwargs)
    except SystemExit:
        pass
    except:
        import traceback
        traceback.print_exc()
    _main = True
    global _interrupt
    if _interrupt:
        _interrupt = False
        raise KeyboardInterrupt

def exit():
    """Dummy implementation of _thread.exit()."""
    raise SystemExit

def get_ident():
    """Dummy implementation of _thread.get_ident().

    Since this module should only be used when _threadmodule is not
    available, it is safe to assume that the current process is the
    only thread.  Thus a constant can be safely returned.
    """
    return 1

def allocate_lock():
    """Dummy implementation of _thread.allocate_lock()."""
    return LockType()

def stack_size(size=None):
    """Dummy implementation of _thread.stack_size()."""
    if size is not None:
        raise error("setting thread stack size not supported")
    return 0

def _set_sentinel():
    """Dummy implementation of _thread._set_sentinel()."""
    return LockType()

class LockType(object):
    """Class implementing dummy implementation of _thread.LockType.

    Compatibility is maintained by maintaining self.locked_status
    which is a boolean that stores the state of the lock.  Pickling of
    the lock, though, should not be done since if the _thread module is
    then used with an unpickled ``lock()`` from here problems could
    occur from this class not having atomic methods.

    """

    def __init__(self):
        self.locked_status = False

    def acquire(self, waitflag=None, timeout=-1):
        """Dummy implementation of acquire().

        For blocking calls, self.locked_status is automatically set to
        True and returned appropriately based on value of
        ``waitflag``.  If it is non-blocking, then the value is
        actually checked and not set if it is already acquired.  This
        is all done so that threading.Condition's assert statements
        aren't triggered and throw a little fit.

        """
        if waitflag is None or waitflag:
            self.locked_status = True
            return True
        else:
            if not self.locked_status:
                self.locked_status = True
                return True
            else:
                if timeout > 0:
                    import time
                    time.sleep(timeout)
                return False

    __enter__ = acquire

    def __exit__(self, typ, val, tb):
        self.release()

    def release(self):
        """Release the dummy lock."""
        # XXX Perhaps shouldn't actually bother to test?  Could lead
        #     to problems for complex, threaded code.
        if not self.locked_status:
            raise error
        self.locked_status = False
        return True

    def locked(self):
        return self.locked_status

    def __repr__(self):
        return "<%s %s.%s object at %s>" % (
            "locked" if self.locked_status else "unlocked",
            self.__class__.__module__,
            self.__class__.__qualname__,
            hex(id(self))
        )


class RLock(LockType):
    """Dummy implementation of threading._RLock.

    Re-entrant lock can be aquired multiple times and needs to be released
    just as many times. This dummy implemention does not check wheter the
    current thread actually owns the lock, but does accounting on the call
    counts.
    """
    def __init__(self):
        super().__init__()
        self._levels = 0

    def acquire(self, waitflag=None, timeout=-1):
        """Aquire the lock, can be called multiple times in succession.
        """
        locked = super().acquire(waitflag, timeout)
        if locked:
            self._levels += 1
        return locked

    def release(self):
        """Release needs to be called once for every call to acquire().
        """
        if self._levels == 0:
            raise error
        if self._levels == 1:
            super().release()
        self._levels -= 1

# Used to signal that interrupt_main was called in a "thread"
_interrupt = False
# True when not executing in a "thread"
_main = True

def interrupt_main():
    """Set _interrupt flag to True to have start_new_thread raise
    KeyboardInterrupt upon exiting."""
    if _main:
        raise KeyboardInterrupt
    else:
        global _interrupt
        _interrupt = True