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//lib/python2.7/site-packages/alembic/ddl/impl.py

from sqlalchemy import schema
from sqlalchemy import text
from sqlalchemy import types as sqltypes

from . import base
from .. import util
from ..util import sqla_compat
from ..util.compat import string_types
from ..util.compat import text_type
from ..util.compat import with_metaclass

class ImplMeta(type):
    def __init__(cls, classname, bases, dict_):
        newtype = type.__init__(cls, classname, bases, dict_)
        if "__dialect__" in dict_:
            _impls[dict_["__dialect__"]] = cls
        return newtype

_impls = {}

class DefaultImpl(with_metaclass(ImplMeta)):

"""Provide the entrypoint for major migration operations,
    including database-specific behavioral variances.

While individual SQL/DDL constructs already provide
    for database-specific implementations, variances here
    allow for entirely different sequences of operations
    to take place for a particular migration, such as
    SQL Server's special 'IDENTITY INSERT' step for
    bulk inserts.

"""

__dialect__ = "default"

transactional_ddl = False
    command_terminator = ";"

def __init__(
        self,
        dialect,
        connection,
        as_sql,
        transactional_ddl,
        output_buffer,
        context_opts,
    ):
        self.dialect = dialect
        self.connection = connection
        self.as_sql = as_sql
        self.literal_binds = context_opts.get("literal_binds", False)

self.output_buffer = output_buffer
        self.memo = {}
        self.context_opts = context_opts
        if transactional_ddl is not None:
            self.transactional_ddl = transactional_ddl

if self.literal_binds:
            if not self.as_sql:
                raise util.CommandError(
                    "Can't use literal_binds setting without as_sql mode"
                )

@classmethod
    def get_by_dialect(cls, dialect):
        return _impls[dialect.name]

def static_output(self, text):
        self.output_buffer.write(text_type(text + "\n\n"))
        self.output_buffer.flush()

def requires_recreate_in_batch(self, batch_op):
        """Return True if the given :class:`.BatchOperationsImpl`
        would need the table to be recreated and copied in order to
        proceed.

Normally, only returns True on SQLite when operations other
        than add_column are present.

"""
        return False

def prep_table_for_batch(self, table):
        """perform any operations needed on a table before a new
        one is created to replace it in batch mode.

the PG dialect uses this to drop constraints on the table
        before the new one uses those same names.

"""

@property
    def bind(self):
        return self.connection

def _exec(
        self,
        construct,
        execution_options=None,
        multiparams=(),
        params=util.immutabledict(),
    ):
        if isinstance(construct, string_types):
            construct = text(construct)
        if self.as_sql:
            if multiparams or params:
                # TODO: coverage
                raise Exception("Execution arguments not allowed with as_sql")

if self.literal_binds and not isinstance(
                construct, schema.DDLElement
            ):
                compile_kw = dict(compile_kwargs={"literal_binds": True})
            else:
                compile_kw = {}

self.static_output(
                text_type(
                    construct.compile(dialect=self.dialect, **compile_kw)
                )
                .replace("\t", "    ")
                .strip()
                + self.command_terminator
            )
        else:
            conn = self.connection
            if execution_options:
                conn = conn.execution_options(**execution_options)
            return conn.execute(construct, *multiparams, **params)

def execute(self, sql, execution_options=None):
        self._exec(sql, execution_options)

def alter_column(
        self,
        table_name,
        column_name,
        nullable=None,
        server_default=False,
        name=None,
        type_=None,
        schema=None,
        autoincrement=None,
        comment=False,
        existing_comment=None,
        existing_type=None,
        existing_server_default=None,
        existing_nullable=None,
        existing_autoincrement=None,
    ):
        if autoincrement is not None or existing_autoincrement is not None:
            util.warn(
                "autoincrement and existing_autoincrement "
                "only make sense for MySQL",
                stacklevel=3,
            )
        if nullable is not None:
            self._exec(
                base.ColumnNullable(
                    table_name,
                    column_name,
                    nullable,
                    schema=schema,
                    existing_type=existing_type,
                    existing_server_default=existing_server_default,
                    existing_nullable=existing_nullable,
                    existing_comment=existing_comment,
                )
            )
        if server_default is not False:
            self._exec(
                base.ColumnDefault(
                    table_name,
                    column_name,
                    server_default,
                    schema=schema,
                    existing_type=existing_type,
                    existing_server_default=existing_server_default,
                    existing_nullable=existing_nullable,
                    existing_comment=existing_comment,
                )
            )
        if type_ is not None:
            self._exec(
                base.ColumnType(
                    table_name,
                    column_name,
                    type_,
                    schema=schema,
                    existing_type=existing_type,
                    existing_server_default=existing_server_default,
                    existing_nullable=existing_nullable,
                    existing_comment=existing_comment,
                )
            )

if comment is not False:
            self._exec(
                base.ColumnComment(
                    table_name,
                    column_name,
                    comment,
                    schema=schema,
                    existing_type=existing_type,
                    existing_server_default=existing_server_default,
                    existing_nullable=existing_nullable,
                    existing_comment=existing_comment,
                )
            )

# do the new name last ;)
        if name is not None:
            self._exec(
                base.ColumnName(
                    table_name,
                    column_name,
                    name,
                    schema=schema,
                    existing_type=existing_type,
                    existing_server_default=existing_server_default,
                    existing_nullable=existing_nullable,
                )
            )

def add_column(self, table_name, column, schema=None):
        self._exec(base.AddColumn(table_name, column, schema=schema))

def drop_column(self, table_name, column, schema=None, **kw):
        self._exec(base.DropColumn(table_name, column, schema=schema))

def add_constraint(self, const):
        if const._create_rule is None or const._create_rule(self):
            self._exec(schema.AddConstraint(const))

def drop_constraint(self, const):
        self._exec(schema.DropConstraint(const))

def rename_table(self, old_table_name, new_table_name, schema=None):
        self._exec(
            base.RenameTable(old_table_name, new_table_name, schema=schema)
        )

def create_table(self, table):
        table.dispatch.before_create(
            table, self.connection, checkfirst=False, _ddl_runner=self
        )
        self._exec(schema.CreateTable(table))
        table.dispatch.after_create(
            table, self.connection, checkfirst=False, _ddl_runner=self
        )
        for index in table.indexes:
            self._exec(schema.CreateIndex(index))

with_comment = (
            sqla_compat._dialect_supports_comments(self.dialect)
            and not self.dialect.inline_comments
        )
        comment = sqla_compat._comment_attribute(table)
        if comment and with_comment:
            self.create_table_comment(table)

for column in table.columns:
            comment = sqla_compat._comment_attribute(column)
            if comment and with_comment:
                self.create_column_comment(column)

def drop_table(self, table):
        self._exec(schema.DropTable(table))

def create_index(self, index):
        self._exec(schema.CreateIndex(index))

def create_table_comment(self, table):
        self._exec(schema.SetTableComment(table))

def drop_table_comment(self, table):
        self._exec(schema.DropTableComment(table))

def create_column_comment(self, column):
        self._exec(schema.SetColumnComment(column))

def drop_index(self, index):
        self._exec(schema.DropIndex(index))

def bulk_insert(self, table, rows, multiinsert=True):
        if not isinstance(rows, list):
            raise TypeError("List expected")
        elif rows and not isinstance(rows[0], dict):
            raise TypeError("List of dictionaries expected")
        if self.as_sql:
            for row in rows:
                self._exec(
                    table.insert(inline=True).values(
                        **dict(
                            (
                                k,
                                sqla_compat._literal_bindparam(
                                    k, v, type_=table.c[k].type
                                )
                                if not isinstance(
                                    v, sqla_compat._literal_bindparam
                                )
                                else v,
                            )
                            for k, v in row.items()
                        )
                    )
                )
        else:
            # work around http://www.sqlalchemy.org/trac/ticket/2461
            if not hasattr(table, "_autoincrement_column"):
                table._autoincrement_column = None
            if rows:
                if multiinsert:
                    self._exec(table.insert(inline=True), multiparams=rows)
                else:
                    for row in rows:
                        self._exec(table.insert(inline=True).values(**row))

def compare_type(self, inspector_column, metadata_column):

conn_type = inspector_column.type
        metadata_type = metadata_column.type

metadata_impl = metadata_type.dialect_impl(self.dialect)
        if isinstance(metadata_impl, sqltypes.Variant):
            metadata_impl = metadata_impl.impl.dialect_impl(self.dialect)

# work around SQLAlchemy bug "stale value for type affinity"
        # fixed in 0.7.4
        metadata_impl.__dict__.pop("_type_affinity", None)

if hasattr(metadata_impl, "compare_against_backend"):
            comparison = metadata_impl.compare_against_backend(
                self.dialect, conn_type
            )
            if comparison is not None:
                return not comparison

if conn_type._compare_type_affinity(metadata_impl):
            comparator = _type_comparators.get(conn_type._type_affinity, None)

return comparator and comparator(metadata_impl, conn_type)
        else:
            return True

def compare_server_default(
        self,
        inspector_column,
        metadata_column,
        rendered_metadata_default,
        rendered_inspector_default,
    ):
        return rendered_inspector_default != rendered_metadata_default

def correct_for_autogen_constraints(
        self,
        conn_uniques,
        conn_indexes,
        metadata_unique_constraints,
        metadata_indexes,
    ):
        pass

def render_ddl_sql_expr(self, expr, is_server_default=False, **kw):
        """Render a SQL expression that is typically a server default,
        index expression, etc.

.. versionadded:: 1.0.11

"""

compile_kw = dict(
            compile_kwargs={"literal_binds": True, "include_table": False}
        )
        return text_type(expr.compile(dialect=self.dialect, **compile_kw))

def _compat_autogen_column_reflect(self, inspector):
        return self.autogen_column_reflect

def correct_for_autogen_foreignkeys(self, conn_fks, metadata_fks):
        pass

def autogen_column_reflect(self, inspector, table, column_info):
        """A hook that is attached to the 'column_reflect' event for when
        a Table is reflected from the database during the autogenerate
        process.

Dialects can elect to modify the information gathered here.

"""

def start_migrations(self):
        """A hook called when :meth:`.EnvironmentContext.run_migrations`
        is called.

Implementations can set up per-migration-run state here.

"""

def emit_begin(self):
        """Emit the string ``BEGIN``, or the backend-specific
        equivalent, on the current connection context.

This is used in offline mode and typically
        via :meth:`.EnvironmentContext.begin_transaction`.

"""
        self.static_output("BEGIN" + self.command_terminator)

def emit_commit(self):
        """Emit the string ``COMMIT``, or the backend-specific
        equivalent, on the current connection context.

This is used in offline mode and typically
        via :meth:`.EnvironmentContext.begin_transaction`.

"""
        self.static_output("COMMIT" + self.command_terminator)

def render_type(self, type_obj, autogen_context):
        return False

def _string_compare(t1, t2):
    return t1.length is not None and t1.length != t2.length

def _numeric_compare(t1, t2):
    return (t1.precision is not None and t1.precision != t2.precision) or (
        t1.precision is not None
        and t1.scale is not None
        and t1.scale != t2.scale
    )

def _integer_compare(t1, t2):
    t1_small_or_big = (
        "S"
        if isinstance(t1, sqltypes.SmallInteger)
        else "B"
        if isinstance(t1, sqltypes.BigInteger)
        else "I"
    )
    t2_small_or_big = (
        "S"
        if isinstance(t2, sqltypes.SmallInteger)
        else "B"
        if isinstance(t2, sqltypes.BigInteger)
        else "I"
    )
    return t1_small_or_big != t2_small_or_big

def _datetime_compare(t1, t2):
    return t1.timezone != t2.timezone

_type_comparators = {
    sqltypes.String: _string_compare,
    sqltypes.Numeric: _numeric_compare,
    sqltypes.Integer: _integer_compare,
    sqltypes.DateTime: _datetime_compare,
}