Current File : //opt/alt/python37/lib/python3.7/site-packages/alembic/runtime/environment.py
from ..operations import Operations
from .migration import MigrationContext
from .. import util
class EnvironmentContext(util.ModuleClsProxy):
"""A configurational facade made available in an ``env.py`` script.
The :class:`.EnvironmentContext` acts as a *facade* to the more
nuts-and-bolts objects of :class:`.MigrationContext` as well as certain
aspects of :class:`.Config`,
within the context of the ``env.py`` script that is invoked by
most Alembic commands.
:class:`.EnvironmentContext` is normally instantiated
when a command in :mod:`alembic.command` is run. It then makes
itself available in the ``alembic.context`` module for the scope
of the command. From within an ``env.py`` script, the current
:class:`.EnvironmentContext` is available by importing this module.
:class:`.EnvironmentContext` also supports programmatic usage.
At this level, it acts as a Python context manager, that is, is
intended to be used using the
``with:`` statement. A typical use of :class:`.EnvironmentContext`::
from alembic.config import Config
from alembic.script import ScriptDirectory
config = Config()
config.set_main_option("script_location", "myapp:migrations")
script = ScriptDirectory.from_config(config)
def my_function(rev, context):
'''do something with revision "rev", which
will be the current database revision,
and "context", which is the MigrationContext
that the env.py will create'''
with EnvironmentContext(
config,
script,
fn = my_function,
as_sql = False,
starting_rev = 'base',
destination_rev = 'head',
tag = "sometag"
):
script.run_env()
The above script will invoke the ``env.py`` script
within the migration environment. If and when ``env.py``
calls :meth:`.MigrationContext.run_migrations`, the
``my_function()`` function above will be called
by the :class:`.MigrationContext`, given the context
itself as well as the current revision in the database.
.. note::
For most API usages other than full blown
invocation of migration scripts, the :class:`.MigrationContext`
and :class:`.ScriptDirectory` objects can be created and
used directly. The :class:`.EnvironmentContext` object
is *only* needed when you need to actually invoke the
``env.py`` module present in the migration environment.
"""
_migration_context = None
config = None
"""An instance of :class:`.Config` representing the
configuration file contents as well as other variables
set programmatically within it."""
script = None
"""An instance of :class:`.ScriptDirectory` which provides
programmatic access to version files within the ``versions/``
directory.
"""
def __init__(self, config, script, **kw):
"""Construct a new :class:`.EnvironmentContext`.
:param config: a :class:`.Config` instance.
:param script: a :class:`.ScriptDirectory` instance.
:param \**kw: keyword options that will be ultimately
passed along to the :class:`.MigrationContext` when
:meth:`.EnvironmentContext.configure` is called.
"""
self.config = config
self.script = script
self.context_opts = kw
def __enter__(self):
"""Establish a context which provides a
:class:`.EnvironmentContext` object to
env.py scripts.
The :class:`.EnvironmentContext` will
be made available as ``from alembic import context``.
"""
self._install_proxy()
return self
def __exit__(self, *arg, **kw):
self._remove_proxy()
def is_offline_mode(self):
"""Return True if the current migrations environment
is running in "offline mode".
This is ``True`` or ``False`` depending
on the the ``--sql`` flag passed.
This function does not require that the :class:`.MigrationContext`
has been configured.
"""
return self.context_opts.get('as_sql', False)
def is_transactional_ddl(self):
"""Return True if the context is configured to expect a
transactional DDL capable backend.
This defaults to the type of database in use, and
can be overridden by the ``transactional_ddl`` argument
to :meth:`.configure`
This function requires that a :class:`.MigrationContext`
has first been made available via :meth:`.configure`.
"""
return self.get_context().impl.transactional_ddl
def requires_connection(self):
return not self.is_offline_mode()
def get_head_revision(self):
"""Return the hex identifier of the 'head' script revision.
If the script directory has multiple heads, this
method raises a :class:`.CommandError`;
:meth:`.EnvironmentContext.get_head_revisions` should be preferred.
This function does not require that the :class:`.MigrationContext`
has been configured.
.. seealso:: :meth:`.EnvironmentContext.get_head_revisions`
"""
return self.script.as_revision_number("head")
def get_head_revisions(self):
"""Return the hex identifier of the 'heads' script revision(s).
This returns a tuple containing the version number of all
heads in the script directory.
This function does not require that the :class:`.MigrationContext`
has been configured.
.. versionadded:: 0.7.0
"""
return self.script.as_revision_number("heads")
def get_starting_revision_argument(self):
"""Return the 'starting revision' argument,
if the revision was passed using ``start:end``.
This is only meaningful in "offline" mode.
Returns ``None`` if no value is available
or was configured.
This function does not require that the :class:`.MigrationContext`
has been configured.
"""
if self._migration_context is not None:
return self.script.as_revision_number(
self.get_context()._start_from_rev)
elif 'starting_rev' in self.context_opts:
return self.script.as_revision_number(
self.context_opts['starting_rev'])
else:
# this should raise only in the case that a command
# is being run where the "starting rev" is never applicable;
# this is to catch scripts which rely upon this in
# non-sql mode or similar
raise util.CommandError(
"No starting revision argument is available.")
def get_revision_argument(self):
"""Get the 'destination' revision argument.
This is typically the argument passed to the
``upgrade`` or ``downgrade`` command.
If it was specified as ``head``, the actual
version number is returned; if specified
as ``base``, ``None`` is returned.
This function does not require that the :class:`.MigrationContext`
has been configured.
"""
return self.script.as_revision_number(
self.context_opts['destination_rev'])
def get_tag_argument(self):
"""Return the value passed for the ``--tag`` argument, if any.
The ``--tag`` argument is not used directly by Alembic,
but is available for custom ``env.py`` configurations that
wish to use it; particularly for offline generation scripts
that wish to generate tagged filenames.
This function does not require that the :class:`.MigrationContext`
has been configured.
.. seealso::
:meth:`.EnvironmentContext.get_x_argument` - a newer and more
open ended system of extending ``env.py`` scripts via the command
line.
"""
return self.context_opts.get('tag', None)
def get_x_argument(self, as_dictionary=False):
"""Return the value(s) passed for the ``-x`` argument, if any.
The ``-x`` argument is an open ended flag that allows any user-defined
value or values to be passed on the command line, then available
here for consumption by a custom ``env.py`` script.
The return value is a list, returned directly from the ``argparse``
structure. If ``as_dictionary=True`` is passed, the ``x`` arguments
are parsed using ``key=value`` format into a dictionary that is
then returned.
For example, to support passing a database URL on the command line,
the standard ``env.py`` script can be modified like this::
cmd_line_url = context.get_x_argument(
as_dictionary=True).get('dbname')
if cmd_line_url:
engine = create_engine(cmd_line_url)
else:
engine = engine_from_config(
config.get_section(config.config_ini_section),
prefix='sqlalchemy.',
poolclass=pool.NullPool)
This then takes effect by running the ``alembic`` script as::
alembic -x dbname=postgresql://user:pass@host/dbname upgrade head
This function does not require that the :class:`.MigrationContext`
has been configured.
.. versionadded:: 0.6.0
.. seealso::
:meth:`.EnvironmentContext.get_tag_argument`
:attr:`.Config.cmd_opts`
"""
if self.config.cmd_opts is not None:
value = self.config.cmd_opts.x or []
else:
value = []
if as_dictionary:
value = dict(
arg.split('=', 1) for arg in value
)
return value
def configure(self,
connection=None,
url=None,
dialect_name=None,
transactional_ddl=None,
transaction_per_migration=False,
output_buffer=None,
starting_rev=None,
tag=None,
template_args=None,
render_as_batch=False,
target_metadata=None,
include_symbol=None,
include_object=None,
include_schemas=False,
process_revision_directives=None,
compare_type=False,
compare_server_default=False,
render_item=None,
literal_binds=False,
upgrade_token="upgrades",
downgrade_token="downgrades",
alembic_module_prefix="op.",
sqlalchemy_module_prefix="sa.",
user_module_prefix=None,
**kw
):
"""Configure a :class:`.MigrationContext` within this
:class:`.EnvironmentContext` which will provide database
connectivity and other configuration to a series of
migration scripts.
Many methods on :class:`.EnvironmentContext` require that
this method has been called in order to function, as they
ultimately need to have database access or at least access
to the dialect in use. Those which do are documented as such.
The important thing needed by :meth:`.configure` is a
means to determine what kind of database dialect is in use.
An actual connection to that database is needed only if
the :class:`.MigrationContext` is to be used in
"online" mode.
If the :meth:`.is_offline_mode` function returns ``True``,
then no connection is needed here. Otherwise, the
``connection`` parameter should be present as an
instance of :class:`sqlalchemy.engine.Connection`.
This function is typically called from the ``env.py``
script within a migration environment. It can be called
multiple times for an invocation. The most recent
:class:`~sqlalchemy.engine.Connection`
for which it was called is the one that will be operated upon
by the next call to :meth:`.run_migrations`.
General parameters:
:param connection: a :class:`~sqlalchemy.engine.Connection`
to use
for SQL execution in "online" mode. When present, is also
used to determine the type of dialect in use.
:param url: a string database url, or a
:class:`sqlalchemy.engine.url.URL` object.
The type of dialect to be used will be derived from this if
``connection`` is not passed.
:param dialect_name: string name of a dialect, such as
"postgresql", "mssql", etc.
The type of dialect to be used will be derived from this if
``connection`` and ``url`` are not passed.
:param transactional_ddl: Force the usage of "transactional"
DDL on or off;
this otherwise defaults to whether or not the dialect in
use supports it.
:param transaction_per_migration: if True, nest each migration script
in a transaction rather than the full series of migrations to
run.
.. versionadded:: 0.6.5
:param output_buffer: a file-like object that will be used
for textual output
when the ``--sql`` option is used to generate SQL scripts.
Defaults to
``sys.stdout`` if not passed here and also not present on
the :class:`.Config`
object. The value here overrides that of the :class:`.Config`
object.
:param output_encoding: when using ``--sql`` to generate SQL
scripts, apply this encoding to the string output.
:param literal_binds: when using ``--sql`` to generate SQL
scripts, pass through the ``literal_binds`` flag to the compiler
so that any literal values that would ordinarily be bound
parameters are converted to plain strings.
.. warning:: Dialects can typically only handle simple datatypes
like strings and numbers for auto-literal generation. Datatypes
like dates, intervals, and others may still require manual
formatting, typically using :meth:`.Operations.inline_literal`.
.. note:: the ``literal_binds`` flag is ignored on SQLAlchemy
versions prior to 0.8 where this feature is not supported.
.. versionadded:: 0.7.6
.. seealso::
:meth:`.Operations.inline_literal`
:param starting_rev: Override the "starting revision" argument
when using ``--sql`` mode.
:param tag: a string tag for usage by custom ``env.py`` scripts.
Set via the ``--tag`` option, can be overridden here.
:param template_args: dictionary of template arguments which
will be added to the template argument environment when
running the "revision" command. Note that the script environment
is only run within the "revision" command if the --autogenerate
option is used, or if the option "revision_environment=true"
is present in the alembic.ini file.
:param version_table: The name of the Alembic version table.
The default is ``'alembic_version'``.
:param version_table_schema: Optional schema to place version
table within.
Parameters specific to the autogenerate feature, when
``alembic revision`` is run with the ``--autogenerate`` feature:
:param target_metadata: a :class:`sqlalchemy.schema.MetaData`
object that
will be consulted during autogeneration. The tables present
will be compared against
what is locally available on the target
:class:`~sqlalchemy.engine.Connection`
to produce candidate upgrade/downgrade operations.
:param compare_type: Indicates type comparison behavior during
an autogenerate
operation. Defaults to ``False`` which disables type
comparison. Set to
``True`` to turn on default type comparison, which has varied
accuracy depending on backend. See :ref:`compare_types`
for an example as well as information on other type
comparison options.
.. seealso::
:ref:`compare_types`
:paramref:`.EnvironmentContext.configure.compare_server_default`
:param compare_server_default: Indicates server default comparison
behavior during
an autogenerate operation. Defaults to ``False`` which disables
server default
comparison. Set to ``True`` to turn on server default comparison,
which has
varied accuracy depending on backend.
To customize server default comparison behavior, a callable may
be specified
which can filter server default comparisons during an
autogenerate operation.
defaults during an autogenerate operation. The format of this
callable is::
def my_compare_server_default(context, inspected_column,
metadata_column, inspected_default, metadata_default,
rendered_metadata_default):
# return True if the defaults are different,
# False if not, or None to allow the default implementation
# to compare these defaults
return None
context.configure(
# ...
compare_server_default = my_compare_server_default
)
``inspected_column`` is a dictionary structure as returned by
:meth:`sqlalchemy.engine.reflection.Inspector.get_columns`, whereas
``metadata_column`` is a :class:`sqlalchemy.schema.Column` from
the local model environment.
A return value of ``None`` indicates to allow default server default
comparison
to proceed. Note that some backends such as Postgresql actually
execute
the two defaults on the database side to compare for equivalence.
.. seealso::
:paramref:`.EnvironmentContext.configure.compare_type`
:param include_object: A callable function which is given
the chance to return ``True`` or ``False`` for any object,
indicating if the given object should be considered in the
autogenerate sweep.
The function accepts the following positional arguments:
* ``object``: a :class:`~sqlalchemy.schema.SchemaItem` object such
as a :class:`~sqlalchemy.schema.Table`,
:class:`~sqlalchemy.schema.Column`,
:class:`~sqlalchemy.schema.Index`
:class:`~sqlalchemy.schema.UniqueConstraint`,
or :class:`~sqlalchemy.schema.ForeignKeyConstraint` object
* ``name``: the name of the object. This is typically available
via ``object.name``.
* ``type``: a string describing the type of object; currently
``"table"``, ``"column"``, ``"index"``, ``"unique_constraint"``,
or ``"foreign_key_constraint"``
.. versionadded:: 0.7.0 Support for indexes and unique constraints
within the
:paramref:`~.EnvironmentContext.configure.include_object` hook.
.. versionadded:: 0.7.1 Support for foreign keys within the
:paramref:`~.EnvironmentContext.configure.include_object` hook.
* ``reflected``: ``True`` if the given object was produced based on
table reflection, ``False`` if it's from a local :class:`.MetaData`
object.
* ``compare_to``: the object being compared against, if available,
else ``None``.
E.g.::
def include_object(object, name, type_, reflected, compare_to):
if (type_ == "column" and
not reflected and
object.info.get("skip_autogenerate", False)):
return False
else:
return True
context.configure(
# ...
include_object = include_object
)
:paramref:`.EnvironmentContext.configure.include_object` can also
be used to filter on specific schemas to include or omit, when
the :paramref:`.EnvironmentContext.configure.include_schemas`
flag is set to ``True``. The :attr:`.Table.schema` attribute
on each :class:`.Table` object reflected will indicate the name of the
schema from which the :class:`.Table` originates.
.. versionadded:: 0.6.0
.. seealso::
:paramref:`.EnvironmentContext.configure.include_schemas`
:param include_symbol: A callable function which, given a table name
and schema name (may be ``None``), returns ``True`` or ``False``,
indicating if the given table should be considered in the
autogenerate sweep.
.. deprecated:: 0.6.0
:paramref:`.EnvironmentContext.configure.include_symbol`
is superceded by the more generic
:paramref:`.EnvironmentContext.configure.include_object`
parameter.
E.g.::
def include_symbol(tablename, schema):
return tablename not in ("skip_table_one", "skip_table_two")
context.configure(
# ...
include_symbol = include_symbol
)
.. seealso::
:paramref:`.EnvironmentContext.configure.include_schemas`
:paramref:`.EnvironmentContext.configure.include_object`
:param render_as_batch: if True, commands which alter elements
within a table will be placed under a ``with batch_alter_table():``
directive, so that batch migrations will take place.
.. versionadded:: 0.7.0
.. seealso::
:ref:`batch_migrations`
:param include_schemas: If True, autogenerate will scan across
all schemas located by the SQLAlchemy
:meth:`~sqlalchemy.engine.reflection.Inspector.get_schema_names`
method, and include all differences in tables found across all
those schemas. When using this option, you may want to also
use the :paramref:`.EnvironmentContext.configure.include_object`
option to specify a callable which
can filter the tables/schemas that get included.
.. seealso::
:paramref:`.EnvironmentContext.configure.include_object`
:param render_item: Callable that can be used to override how
any schema item, i.e. column, constraint, type,
etc., is rendered for autogenerate. The callable receives a
string describing the type of object, the object, and
the autogen context. If it returns False, the
default rendering method will be used. If it returns None,
the item will not be rendered in the context of a Table
construct, that is, can be used to skip columns or constraints
within op.create_table()::
def my_render_column(type_, col, autogen_context):
if type_ == "column" and isinstance(col, MySpecialCol):
return repr(col)
else:
return False
context.configure(
# ...
render_item = my_render_column
)
Available values for the type string include: ``"column"``,
``"primary_key"``, ``"foreign_key"``, ``"unique"``, ``"check"``,
``"type"``, ``"server_default"``.
.. seealso::
:ref:`autogen_render_types`
:param upgrade_token: When autogenerate completes, the text of the
candidate upgrade operations will be present in this template
variable when ``script.py.mako`` is rendered. Defaults to
``upgrades``.
:param downgrade_token: When autogenerate completes, the text of the
candidate downgrade operations will be present in this
template variable when ``script.py.mako`` is rendered. Defaults to
``downgrades``.
:param alembic_module_prefix: When autogenerate refers to Alembic
:mod:`alembic.operations` constructs, this prefix will be used
(i.e. ``op.create_table``) Defaults to "``op.``".
Can be ``None`` to indicate no prefix.
:param sqlalchemy_module_prefix: When autogenerate refers to
SQLAlchemy
:class:`~sqlalchemy.schema.Column` or type classes, this prefix
will be used
(i.e. ``sa.Column("somename", sa.Integer)``) Defaults to "``sa.``".
Can be ``None`` to indicate no prefix.
Note that when dialect-specific types are rendered, autogenerate
will render them using the dialect module name, i.e. ``mssql.BIT()``,
``postgresql.UUID()``.
:param user_module_prefix: When autogenerate refers to a SQLAlchemy
type (e.g. :class:`.TypeEngine`) where the module name is not
under the ``sqlalchemy`` namespace, this prefix will be used
within autogenerate. If left at its default of
``None``, the ``__module__`` attribute of the type is used to
render the import module. It's a good practice to set this
and to have all custom types be available from a fixed module space,
in order to future-proof migration files against reorganizations
in modules.
.. versionchanged:: 0.7.0
:paramref:`.EnvironmentContext.configure.user_module_prefix`
no longer defaults to the value of
:paramref:`.EnvironmentContext.configure.sqlalchemy_module_prefix`
when left at ``None``; the ``__module__`` attribute is now used.
.. versionadded:: 0.6.3 added
:paramref:`.EnvironmentContext.configure.user_module_prefix`
.. seealso::
:ref:`autogen_module_prefix`
:param process_revision_directives: a callable function that will
be passed a structure representing the end result of an autogenerate
or plain "revision" operation, which can be manipulated to affect
how the ``alembic revision`` command ultimately outputs new
revision scripts. The structure of the callable is::
def process_revision_directives(context, revision, directives):
pass
The ``directives`` parameter is a Python list containing
a single :class:`.MigrationScript` directive, which represents
the revision file to be generated. This list as well as its
contents may be freely modified to produce any set of commands.
The section :ref:`customizing_revision` shows an example of
doing this. The ``context`` parameter is the
:class:`.MigrationContext` in use,
and ``revision`` is a tuple of revision identifiers representing the
current revision of the database.
The callable is invoked at all times when the ``--autogenerate``
option is passed to ``alembic revision``. If ``--autogenerate``
is not passed, the callable is invoked only if the
``revision_environment`` variable is set to True in the Alembic
configuration, in which case the given ``directives`` collection
will contain empty :class:`.UpgradeOps` and :class:`.DowngradeOps`
collections for ``.upgrade_ops`` and ``.downgrade_ops``. The
``--autogenerate`` option itself can be inferred by inspecting
``context.config.cmd_opts.autogenerate``.
The callable function may optionally be an instance of
a :class:`.Rewriter` object. This is a helper object that
assists in the production of autogenerate-stream rewriter functions.
.. versionadded:: 0.8.0
.. versionchanged:: 0.8.1 - The
:paramref:`.EnvironmentContext.configure.process_revision_directives`
hook can append op directives into :class:`.UpgradeOps` and
:class:`.DowngradeOps` which will be rendered in Python regardless
of whether the ``--autogenerate`` option is in use or not;
the ``revision_environment`` configuration variable should be
set to "true" in the config to enable this.
.. seealso::
:ref:`customizing_revision`
:ref:`autogen_rewriter`
Parameters specific to individual backends:
:param mssql_batch_separator: The "batch separator" which will
be placed between each statement when generating offline SQL Server
migrations. Defaults to ``GO``. Note this is in addition to the
customary semicolon ``;`` at the end of each statement; SQL Server
considers the "batch separator" to denote the end of an
individual statement execution, and cannot group certain
dependent operations in one step.
:param oracle_batch_separator: The "batch separator" which will
be placed between each statement when generating offline
Oracle migrations. Defaults to ``/``. Oracle doesn't add a
semicolon between statements like most other backends.
"""
opts = self.context_opts
if transactional_ddl is not None:
opts["transactional_ddl"] = transactional_ddl
if output_buffer is not None:
opts["output_buffer"] = output_buffer
elif self.config.output_buffer is not None:
opts["output_buffer"] = self.config.output_buffer
if starting_rev:
opts['starting_rev'] = starting_rev
if tag:
opts['tag'] = tag
if template_args and 'template_args' in opts:
opts['template_args'].update(template_args)
opts["transaction_per_migration"] = transaction_per_migration
opts['target_metadata'] = target_metadata
opts['include_symbol'] = include_symbol
opts['include_object'] = include_object
opts['include_schemas'] = include_schemas
opts['render_as_batch'] = render_as_batch
opts['upgrade_token'] = upgrade_token
opts['downgrade_token'] = downgrade_token
opts['sqlalchemy_module_prefix'] = sqlalchemy_module_prefix
opts['alembic_module_prefix'] = alembic_module_prefix
opts['user_module_prefix'] = user_module_prefix
opts['literal_binds'] = literal_binds
opts['process_revision_directives'] = process_revision_directives
if render_item is not None:
opts['render_item'] = render_item
if compare_type is not None:
opts['compare_type'] = compare_type
if compare_server_default is not None:
opts['compare_server_default'] = compare_server_default
opts['script'] = self.script
opts.update(kw)
self._migration_context = MigrationContext.configure(
connection=connection,
url=url,
dialect_name=dialect_name,
environment_context=self,
opts=opts
)
def run_migrations(self, **kw):
"""Run migrations as determined by the current command line
configuration
as well as versioning information present (or not) in the current
database connection (if one is present).
The function accepts optional ``**kw`` arguments. If these are
passed, they are sent directly to the ``upgrade()`` and
``downgrade()``
functions within each target revision file. By modifying the
``script.py.mako`` file so that the ``upgrade()`` and ``downgrade()``
functions accept arguments, parameters can be passed here so that
contextual information, usually information to identify a particular
database in use, can be passed from a custom ``env.py`` script
to the migration functions.
This function requires that a :class:`.MigrationContext` has
first been made available via :meth:`.configure`.
"""
with Operations.context(self._migration_context):
self.get_context().run_migrations(**kw)
def execute(self, sql, execution_options=None):
"""Execute the given SQL using the current change context.
The behavior of :meth:`.execute` is the same
as that of :meth:`.Operations.execute`. Please see that
function's documentation for full detail including
caveats and limitations.
This function requires that a :class:`.MigrationContext` has
first been made available via :meth:`.configure`.
"""
self.get_context().execute(sql,
execution_options=execution_options)
def static_output(self, text):
"""Emit text directly to the "offline" SQL stream.
Typically this is for emitting comments that
start with --. The statement is not treated
as a SQL execution, no ; or batch separator
is added, etc.
"""
self.get_context().impl.static_output(text)
def begin_transaction(self):
"""Return a context manager that will
enclose an operation within a "transaction",
as defined by the environment's offline
and transactional DDL settings.
e.g.::
with context.begin_transaction():
context.run_migrations()
:meth:`.begin_transaction` is intended to
"do the right thing" regardless of
calling context:
* If :meth:`.is_transactional_ddl` is ``False``,
returns a "do nothing" context manager
which otherwise produces no transactional
state or directives.
* If :meth:`.is_offline_mode` is ``True``,
returns a context manager that will
invoke the :meth:`.DefaultImpl.emit_begin`
and :meth:`.DefaultImpl.emit_commit`
methods, which will produce the string
directives ``BEGIN`` and ``COMMIT`` on
the output stream, as rendered by the
target backend (e.g. SQL Server would
emit ``BEGIN TRANSACTION``).
* Otherwise, calls :meth:`sqlalchemy.engine.Connection.begin`
on the current online connection, which
returns a :class:`sqlalchemy.engine.Transaction`
object. This object demarcates a real
transaction and is itself a context manager,
which will roll back if an exception
is raised.
Note that a custom ``env.py`` script which
has more specific transactional needs can of course
manipulate the :class:`~sqlalchemy.engine.Connection`
directly to produce transactional state in "online"
mode.
"""
return self.get_context().begin_transaction()
def get_context(self):
"""Return the current :class:`.MigrationContext` object.
If :meth:`.EnvironmentContext.configure` has not been
called yet, raises an exception.
"""
if self._migration_context is None:
raise Exception("No context has been configured yet.")
return self._migration_context
def get_bind(self):
"""Return the current 'bind'.
In "online" mode, this is the
:class:`sqlalchemy.engine.Connection` currently being used
to emit SQL to the database.
This function requires that a :class:`.MigrationContext`
has first been made available via :meth:`.configure`.
"""
return self.get_context().bind
def get_impl(self):
return self.get_context().impl
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