# documentación de Django - 1.10.7

# Código fuente para django.db.transaction

    from django.db import (
        DEFAULT_DB_ALIAS, DatabaseError, Error, ProgrammingError, connections,
    )
    from django.utils.decorators import ContextDecorator

    class TransactionManagementError(ProgrammingError):
        """
        This exception is thrown when transaction management is used improperly.
        """
        pass

    def get_connection(using=None):
        """
        Get a database connection by name, or the default database connection
        if no name is provided. This is a private API.
        """
        if using is None:
            using = DEFAULT_DB_ALIAS
        return connections[using]

    def get_autocommit(using=None):
        """
        Get the autocommit status of the connection.
        """
        return get_connection(using).get_autocommit()

    def set_autocommit(autocommit, using=None):
        """
        Set the autocommit status of the connection.
        """
        return get_connection(using).set_autocommit(autocommit)

    def commit(using=None):
        """
        Commits a transaction.
        """
        get_connection(using).commit()

    def rollback(using=None):
        """
        Rolls back a transaction.
        """
        get_connection(using).rollback()

    def savepoint(using=None):
        """
        Creates a savepoint (if supported and required by the backend) inside the
        current transaction. Returns an identifier for the savepoint that will be
        used for the subsequent rollback or commit.
        """
        return get_connection(using).savepoint()

    def savepoint_rollback(sid, using=None):
        """
        Rolls back the most recent savepoint (if one exists). Does nothing if
        savepoints are not supported.
        """
        get_connection(using).savepoint_rollback(sid)

    def savepoint_commit(sid, using=None):
        """
        Commits the most recent savepoint (if one exists). Does nothing if
        savepoints are not supported.
        """
        get_connection(using).savepoint_commit(sid)

    def clean_savepoints(using=None):
        """
        Resets the counter used to generate unique savepoint ids in this thread.
        """
        get_connection(using).clean_savepoints()

    def get_rollback(using=None):
        """
        Gets the "needs rollback" flag -- for *advanced use* only.
        """
        return get_connection(using).get_rollback()

    def set_rollback(rollback, using=None):
        """
        Sets or unsets the "needs rollback" flag -- for *advanced use* only.

        When `rollback` is `True`, it triggers a rollback when exiting the
        innermost enclosing atomic block that has `savepoint=True` (that's the
        default). Use this to force a rollback without raising an exception.

        When `rollback` is `False`, it prevents such a rollback. Use this only
        after rolling back to a known-good state! Otherwise, you break the atomic
        block and data corruption may occur.
        """
        return get_connection(using).set_rollback(rollback)

    def on_commit(func, using=None):
        """
        Register `func` to be called when the current transaction is committed.
        If the current transaction is rolled back, `func` will not be called.
        """
        get_connection(using).on_commit(func)

    #################################
    # Decorators / context managers #
    #################################

    class Atomic(ContextDecorator):
        """
        This class guarantees the atomic execution of a given block.

        An instance can be used either as a decorator or as a context manager.

        When it's used as a decorator, __call__ wraps the execution of the
        decorated function in the instance itself, used as a context manager.

        When it's used as a context manager, __enter__ creates a transaction or a
        savepoint, depending on whether a transaction is already in progress, and
        __exit__ commits the transaction or releases the savepoint on normal exit,
        and rolls back the transaction or to the savepoint on exceptions.

        It's possible to disable the creation of savepoints if the goal is to
        ensure that some code runs within a transaction without creating overhead.

        A stack of savepoints identifiers is maintained as an attribute of the
        connection. None denotes the absence of a savepoint.

        This allows reentrancy even if the same AtomicWrapper is reused. For
        example, it's possible to define `oa = @atomic('other')` and use `@oa` or
        `with oa:` multiple times.

        Since database connections are thread-local, this is thread-safe.

        This is a private API.
        """

        def __init__(self, using, savepoint):
            self.using = using
            self.savepoint = savepoint

        def __enter__(self):
            connection = get_connection(self.using)

            if not connection.in_atomic_block:
                # Reset state when entering an outermost atomic block.
                connection.commit_on_exit = True
                connection.needs_rollback = False
                if not connection.get_autocommit():
                    # Some database adapters (namely sqlite3) don't handle
                    # transactions and savepoints properly when autocommit is off.
                    # Turning autocommit back on isn't an option; it would trigger
                    # a premature commit. Give up if that happens.
                    if connection.features.autocommits_when_autocommit_is_off:
                        raise TransactionManagementError(
                            "Your database backend doesn't behave properly when "
                            "autocommit is off. Turn it on before using 'atomic'.")
                    # Pretend we're already in an atomic block to bypass the code
                    # that disables autocommit to enter a transaction, and make a
                    # note to deal with this case in __exit__.
                    connection.in_atomic_block = True
                    connection.commit_on_exit = False

            if connection.in_atomic_block:
                # We're already in a transaction; create a savepoint, unless we
                # were told not to or we're already waiting for a rollback. The
                # second condition avoids creating useless savepoints and prevents
                # overwriting needs_rollback until the rollback is performed.
                if self.savepoint and not connection.needs_rollback:
                    sid = connection.savepoint()
                    connection.savepoint_ids.append(sid)
                else:
                    connection.savepoint_ids.append(None)
            else:
                connection.set_autocommit(False, force_begin_transaction_with_broken_autocommit=True)
                connection.in_atomic_block = True

        def __exit__(self, exc_type, exc_value, traceback):
            connection = get_connection(self.using)

            if connection.savepoint_ids:
                sid = connection.savepoint_ids.pop()
            else:
                # Prematurely unset this flag to allow using commit or rollback.
                connection.in_atomic_block = False

            try:
                if connection.closed_in_transaction:
                    # The database will perform a rollback by itself.
                    # Wait until we exit the outermost block.
                    pass

                elif exc_type is None and not connection.needs_rollback:
                    if connection.in_atomic_block:
                        # Release savepoint if there is one
                        if sid is not None:
                            try:
                                connection.savepoint_commit(sid)
                            except DatabaseError:
                                try:
                                    connection.savepoint_rollback(sid)
                                    # The savepoint won't be reused. Release it to
                                    # minimize overhead for the database server.
                                    connection.savepoint_commit(sid)
                                except Error:
                                    # If rolling back to a savepoint fails, mark for
                                    # rollback at a higher level and avoid shadowing
                                    # the original exception.
                                    connection.needs_rollback = True
                                raise
                    else:
                        # Commit transaction
                        try:
                            connection.commit()
                        except DatabaseError:
                            try:
                                connection.rollback()
                            except Error:
                                # An error during rollback means that something
                                # went wrong with the connection. Drop it.
                                connection.close()
                            raise
                else:
                    # This flag will be set to True again if there isn't a savepoint
                    # allowing to perform the rollback at this level.
                    connection.needs_rollback = False
                    if connection.in_atomic_block:
                        # Roll back to savepoint if there is one, mark for rollback
                        # otherwise.
                        if sid is None:
                            connection.needs_rollback = True
                        else:
                            try:
                                connection.savepoint_rollback(sid)
                                # The savepoint won't be reused. Release it to
                                # minimize overhead for the database server.
                                connection.savepoint_commit(sid)
                            except Error:
                                # If rolling back to a savepoint fails, mark for
                                # rollback at a higher level and avoid shadowing
                                # the original exception.
                                connection.needs_rollback = True
                    else:
                        # Roll back transaction
                        try:
                            connection.rollback()
                        except Error:
                            # An error during rollback means that something
                            # went wrong with the connection. Drop it.
                            connection.close()

            finally:
                # Outermost block exit when autocommit was enabled.
                if not connection.in_atomic_block:
                    if connection.closed_in_transaction:
                        connection.connection = None
                    else:
                        connection.set_autocommit(True)
                # Outermost block exit when autocommit was disabled.
                elif not connection.savepoint_ids and not connection.commit_on_exit:
                    if connection.closed_in_transaction:
                        connection.connection = None
                    else:
                        connection.in_atomic_block = False

    def atomic(using=None, savepoint=True):
        # Bare decorator: @atomic -- although the first argument is called
        # `using`, it's actually the function being decorated.
        if callable(using):
            return Atomic(DEFAULT_DB_ALIAS, savepoint)(using)
        # Decorator: @atomic(...) or context manager: with atomic(...): ...
        else:
            return Atomic(using, savepoint)

    def _non_atomic_requests(view, using):
        try:
            view._non_atomic_requests.add(using)
        except AttributeError:
            view._non_atomic_requests = {using}
        return view

    def non_atomic_requests(using=None):
        if callable(using):
            return _non_atomic_requests(using, DEFAULT_DB_ALIAS)
        else:
            if using is None:
                using = DEFAULT_DB_ALIAS
            return lambda view: _non_atomic_requests(view, using)