def alter_column(self, table_name, column_name,
nullable=None,
server_default=False,
name=None,
type_=None,
autoincrement=None,
**kw
):
existing = self.columns[column_name]
existing_transfer = self.column_transfers[column_name]
if name is not None and name != column_name:
# note that we don't change '.key' - we keep referring
# to the renamed column by its old key in _create(). neat!
existing.name = name
existing_transfer["name"] = name
if type_ is not None:
type_ = sqltypes.to_instance(type_)
# old type is being discarded so turn off eventing
# rules. Alternatively we can
# erase the events set up by this type, but this is simpler.
# we also ignore the drop_constraint that will come here from
# Operations.implementation_for(alter_column)
if isinstance(existing.type, SchemaEventTarget):
existing.type._create_events = \
existing.type.create_constraint = False
existing.type = type_
# we *dont* however set events for the new type, because
# alter_column is invoked from
# Operations.implementation_for(alter_column) which already
# will emit an add_constraint()
existing_transfer["expr"] = cast(existing_transfer["expr"], type_)
if nullable is not None:
existing.nullable = nullable
if server_default is not False:
sql_schema.DefaultClause(server_default)._set_parent(existing)
if autoincrement is not None:
existing.autoincrement = bool(autoincrement)
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