def _alter_field(self, model, old_field, new_field, old_type, new_type,
                     old_db_params, new_db_params, strict=False):

        # Drop any FK constraints, we'll remake them later
        fks_dropped = set()
        if old_field.remote_field and old_field.db_constraint:
            fk_names = self._constraint_names(model, [old_field.column], foreign_key=True)
            if strict and len(fk_names) != 1:
                raise ValueError("Found wrong number (%s) of foreign key constraints for %s.%s" % (
                    len(fk_names),
                    model._meta.db_table,
                    old_field.column,
                ))
            for fk_name in fk_names:
                fks_dropped.add((old_field.column,))
                self.execute(self._delete_constraint_sql(self.sql_delete_fk, model, fk_name))
        # Has unique been removed?
        if old_field.unique and (not new_field.unique or (not old_field.primary_key and new_field.primary_key)):
            # Find the unique constraint for this field
            constraint_names = self._constraint_names(model, [old_field.column], unique=True)
            if strict and len(constraint_names) != 1:
                raise ValueError("Found wrong number (%s) of unique constraints for %s.%s" % (
                    len(constraint_names),
                    model._meta.db_table,
                    old_field.column,
                ))
            for constraint_name in constraint_names:
                self.execute(self._delete_constraint_sql(self.sql_delete_unique, model, constraint_name))
        # Drop incoming FK constraints if we're a primary key and things are going
        # to change.
        if old_field.primary_key and new_field.primary_key and old_type != new_type:
            # '_meta.related_field' also contains M2M reverse fields, these
            # will be filtered out
            for _old_rel, new_rel in _related_non_m2m_objects(old_field, new_field):
                rel_fk_names = self._constraint_names(
                    new_rel.related_model, [new_rel.field.column], foreign_key=True
                )
                for fk_name in rel_fk_names:
                    self.execute(self._delete_constraint_sql(self.sql_delete_fk, new_rel.related_model, fk_name))
        # Removed an index? (no strict check, as multiple indexes are possible)
        if (old_field.db_index and not new_field.db_index and
                not old_field.unique and not
                (not new_field.unique and old_field.unique)):
            # Find the index for this field
            index_names = self._constraint_names(model, [old_field.column], index=True)
            for index_name in index_names:
                self.execute(self._delete_constraint_sql(self.sql_delete_index, model, index_name))
        # Change check constraints?
        if old_db_params['check'] != new_db_params['check'] and old_db_params['check']:
            constraint_names = self._constraint_names(model, [old_field.column], check=True)
            if strict and len(constraint_names) != 1:
                raise ValueError("Found wrong number (%s) of check constraints for %s.%s" % (
                    len(constraint_names),
                    model._meta.db_table,
                    old_field.column,
                ))
            for constraint_name in constraint_names:
                self.execute(self._delete_constraint_sql(self.sql_delete_check, model, constraint_name))
        # Have they renamed the column?
        if old_field.column != new_field.column:
            self.execute(self._rename_field_sql(model._meta.db_table, old_field, new_field, new_type))
        # Next, start accumulating actions to do
        actions = []
        null_actions = []
        post_actions = []
        # Type change?
        if old_type != new_type:
            fragment, other_actions = self._alter_column_type_sql(
                model._meta.db_table, old_field, new_field, new_type
            )
            actions.append(fragment)
            post_actions.extend(other_actions)
        # When changing a column NULL constraint to NOT NULL with a given
        # default value, we need to perform 4 steps:
        #  1. Add a default for new incoming writes
        #  2. Update existing NULL rows with new default
        #  3. Replace NULL constraint with NOT NULL
        #  4. Drop the default again.
        # Default change?
        old_default = self.effective_default(old_field)
        new_default = self.effective_default(new_field)
        needs_database_default = (
            old_default != new_default and
            new_default is not None and
            not self.skip_default(new_field)
        )
        if needs_database_default:
            if self.connection.features.requires_literal_defaults:
                # Some databases can't take defaults as a parameter (oracle)
                # If this is the case, the individual schema backend should
                # implement prepare_default
                actions.append((
                    self.sql_alter_column_default % {
                        "column": self.quote_name(new_field.column),
                        "type": new_type,
                        "default": self.prepare_default(new_default),
                    },
                    [],
                ))
            else:
                actions.append((
                    self.sql_alter_column_default % {
                        "column": self.quote_name(new_field.column),
                        "type": new_type,
                        "default": "%s",
                    },
                    [new_default],
                ))
        # Nullability change?
        if old_field.null != new_field.null:
            if (self.connection.features.interprets_empty_strings_as_nulls and
                    new_field.get_internal_type() in ("CharField", "TextField")):
                # The field is nullable in the database anyway, leave it alone
                pass
            elif new_field.null:
                null_actions.append((
                    self.sql_alter_column_null % {
                        "column": self.quote_name(new_field.column),
                        "type": new_type,
                    },
                    [],
                ))
            else:
                null_actions.append((
                    self.sql_alter_column_not_null % {
                        "column": self.quote_name(new_field.column),
                        "type": new_type,
                    },
                    [],
                ))
        # Only if we have a default and there is a change from NULL to NOT NULL
        four_way_default_alteration = (
            new_field.has_default() and
            (old_field.null and not new_field.null)
        )
        if actions or null_actions:
            if not four_way_default_alteration:
                # If we don't have to do a 4-way default alteration we can
                # directly run a (NOT) NULL alteration
                actions = actions + null_actions
            # Combine actions together if we can (e.g. postgres)
            if self.connection.features.supports_combined_alters and actions:
                sql, params = tuple(zip(*actions))
                actions = [(", ".join(sql), sum(params, []))]
            # Apply those actions
            for sql, params in actions:
                self.execute(
                    self.sql_alter_column % {
                        "table": self.quote_name(model._meta.db_table),
                        "changes": sql,
                    },
                    params,
                )
            if four_way_default_alteration:
                # Update existing rows with default value
                self.execute(
                    self.sql_update_with_default % {
                        "table": self.quote_name(model._meta.db_table),
                        "column": self.quote_name(new_field.column),
                        "default": "%s",
                    },
                    [new_default],
                )
                # Since we didn't run a NOT NULL change before we need to do it
                # now
                for sql, params in null_actions:
                    self.execute(
                        self.sql_alter_column % {
                            "table": self.quote_name(model._meta.db_table),
                            "changes": sql,
                        },
                        params,
                    )
        if post_actions:
            for sql, params in post_actions:
                self.execute(sql, params)
        # Added a unique?
        if (not old_field.unique and new_field.unique) or (
            old_field.primary_key and not new_field.primary_key and new_field.unique
        ):
            self.execute(self._create_unique_sql(model, [new_field.column]))
        # Added an index?
        if (not old_field.db_index and new_field.db_index and
                not new_field.unique and not
                (not old_field.unique and new_field.unique)):
            self.execute(self._create_index_sql(model, [new_field], suffix="_uniq"))
        # Type alteration on primary key? Then we need to alter the column
        # referring to us.
        rels_to_update = []
        if old_field.primary_key and new_field.primary_key and old_type != new_type:
            rels_to_update.extend(_related_non_m2m_objects(old_field, new_field))
        # Changed to become primary key?
        # Note that we don't detect unsetting of a PK, as we assume another field
        # will always come along and replace it.
        if not old_field.primary_key and new_field.primary_key:
            # First, drop the old PK
            constraint_names = self._constraint_names(model, primary_key=True)
            if strict and len(constraint_names) != 1:
                raise ValueError("Found wrong number (%s) of PK constraints for %s" % (
                    len(constraint_names),
                    model._meta.db_table,
                ))
            for constraint_name in constraint_names:
                self.execute(self._delete_constraint_sql(self.sql_delete_pk, model, constraint_name))
            # Make the new one
            self.execute(
                self.sql_create_pk % {
                    "table": self.quote_name(model._meta.db_table),
                    "name": self.quote_name(self._create_index_name(model, [new_field.column], suffix="_pk")),
                    "columns": self.quote_name(new_field.column),
                }
            )
            # Update all referencing columns
            rels_to_update.extend(_related_non_m2m_objects(old_field, new_field))
        # Handle our type alters on the other end of rels from the PK stuff above
        for old_rel, new_rel in rels_to_update:
            rel_db_params = new_rel.field.db_parameters(connection=self.connection)
            rel_type = rel_db_params['type']
            fragment, other_actions = self._alter_column_type_sql(
                new_rel.related_model._meta.db_table, old_rel.field, new_rel.field, rel_type
            )
            self.execute(
                self.sql_alter_column % {
                    "table": self.quote_name(new_rel.related_model._meta.db_table),
                    "changes": fragment[0],
                },
                fragment[1],
            )
            for sql, params in other_actions:
                self.execute(sql, params)
        # Does it have a foreign key?
        if (new_field.remote_field and
                (fks_dropped or not old_field.remote_field or not old_field.db_constraint) and
                new_field.db_constraint):
            self.execute(self._create_fk_sql(model, new_field, "_fk_%(to_table)s_%(to_column)s"))
        # Rebuild FKs that pointed to us if we previously had to drop them
        if old_field.primary_key and new_field.primary_key and old_type != new_type:
            for rel in new_field.model._meta.related_objects:
                if not rel.many_to_many:
                    self.execute(self._create_fk_sql(rel.related_model, rel.field, "_fk"))
        # Does it have check constraints we need to add?
        if old_db_params['check'] != new_db_params['check'] and new_db_params['check']:
            self.execute(
                self.sql_create_check % {
                    "table": self.quote_name(model._meta.db_table),
                    "name": self.quote_name(self._create_index_name(model, [new_field.column], suffix="_check")),
                    "column": self.quote_name(new_field.column),
                    "check": new_db_params['check'],
                }
            )
        # Drop the default if we need to
        # (Django usually does not use in-database defaults)
        if needs_database_default:
            sql = self.sql_alter_column % {
                "table": self.quote_name(model._meta.db_table),
                "changes": self.sql_alter_column_no_default % {
                    "column": self.quote_name(new_field.column),
                    "type": new_type,
                }
            }
            self.execute(sql)
        # Reset connection if required
        if self.connection.features.connection_persists_old_columns:
            self.connection.close()
Actually perform a "physical" (non-ManyToMany) field update.
dataset/ETHPy150Open django/django/django/db/backends/base/schema.py/BaseDatabaseSchemaEditor._alter_field