class BaseExpression

def as_sql(self, compiler, connection)

Responsible for returning a (sql, [params]) tuple to be included
in the current query.

Different backends can provide their own implementation, by
providing an `as_{vendor}` method and patching the Expression:

```
def override_as_sql(self, compiler, connection):
    # custom logic
    return super().as_sql(compiler, connection)
setattr(Expression, 'as_' + connection.vendor, override_as_sql)
```

Arguments:
 * compiler: the query compiler responsible for generating the query.
   Must have a compile method, returning a (sql, [params]) tuple.
   Calling compiler(value) will return a quoted `value`.

 * connection: the database connection used for the current query.

Return: (sql, params)
  Where `sql` is a string containing ordered sql parameters to be
  replaced with the elements of the list `params`.

    def as_sql(self, compiler, connection):
        """
        Responsible for returning a (sql, [params]) tuple to be included
        in the current query.

        Different backends can provide their own implementation, by
        providing an `as_{vendor}` method and patching the Expression:

        ```
        def override_as_sql(self, compiler, connection):
            # custom logic
            return super().as_sql(compiler, connection)
        setattr(Expression, 'as_' + connection.vendor, override_as_sql)
        ```

        Arguments:
         * compiler: the query compiler responsible for generating the query.
           Must have a compile method, returning a (sql, [params]) tuple.
           Calling compiler(value) will return a quoted `value`.

         * connection: the database connection used for the current query.

        Return: (sql, params)
          Where `sql` is a string containing ordered sql parameters to be
          replaced with the elements of the list `params`.
        """
        raise NotImplementedError("Subclasses must implement as_sql()")

def asc(self, **kwargs)

    def asc(self, **kwargs):
        return OrderBy(self, **kwargs)

def conditional(self)

def contains_aggregate(self)

def contains_column_references(self)

def contains_over_clause(self)

def contains_subquery(self)

def convert_value(self)

Expressions provide their own converters because users have the option
of manually specifying the output_field which may be a different type
from the one the database returns.

def copy(self)

    def copy(self):
        return copy.copy(self)

def desc(self, **kwargs)

    def desc(self, **kwargs):
        return OrderBy(self, descending=True, **kwargs)

def field(self)

def flatten(self)

Recursively yield this expression and all subexpressions, in
depth-first order.

    def flatten(self):
        """
        Recursively yield this expression and all subexpressions, in
        depth-first order.
        """
        yield self
        for expr in self.get_source_expressions():
            if expr:
                if hasattr(expr, "flatten"):
                    yield from expr.flatten()
                else:
                    yield expr

def get_db_converters(self, connection)

    def get_db_converters(self, connection):
        return (
            []
            if self.convert_value is self._convert_value_noop
            else [self.convert_value]
        ) + self.output_field.get_db_converters(connection)

def get_group_by_cols(self)

    def get_group_by_cols(self):
        if not self.contains_aggregate:
            return [self]
        cols = []
        for source in self.get_source_expressions():
            cols.extend(source.get_group_by_cols())
        return cols

def get_lookup(self, lookup)

    def get_lookup(self, lookup):
        return self.output_field.get_lookup(lookup)

def get_refs(self)

    def get_refs(self):
        refs = set()
        for expr in self.get_source_expressions():
            refs |= expr.get_refs()
        return refs

def get_source_expressions(self)

    def get_source_expressions(self):
        return []

def get_source_fields(self)

Return the underlying field types used by this aggregate.

    def get_source_fields(self):
        """Return the underlying field types used by this aggregate."""
        return [e._output_field_or_none for e in self.get_source_expressions()]

def get_transform(self, name)

    def get_transform(self, name):
        return self.output_field.get_transform(name)

def output_field(self)

Return the output type of this expressions.

def prefix_references(self, prefix)

    def prefix_references(self, prefix):
        clone = self.copy()
        clone.set_source_expressions(
            [
                (
                    F(f"{prefix}{expr.name}")
                    if isinstance(expr, F)
                    else expr.prefix_references(prefix)
                )
                for expr in self.get_source_expressions()
            ]
        )
        return clone

def relabeled_clone(self, change_map)

    def relabeled_clone(self, change_map):
        clone = self.copy()
        clone.set_source_expressions(
            [
                e.relabeled_clone(change_map) if e is not None else None
                for e in self.get_source_expressions()
            ]
        )
        return clone

def replace_expressions(self, replacements)

    def replace_expressions(self, replacements):
        if replacement := replacements.get(self):
            return replacement
        clone = self.copy()
        source_expressions = clone.get_source_expressions()
        clone.set_source_expressions(
            [
                expr.replace_expressions(replacements) if expr else None
                for expr in source_expressions
            ]
        )
        return clone

def resolve_expression(self, query=None, allow_joins=True, reuse=None, summarize=False, for_save=False)

Provide the chance to do any preprocessing or validation before being
added to the query.

Arguments:
 * query: the backend query implementation
 * allow_joins: boolean allowing or denying use of joins
   in this query
 * reuse: a set of reusable joins for multijoins
 * summarize: a terminal aggregate clause
 * for_save: whether this expression about to be used in a save or update

Return: an Expression to be added to the query.

    def resolve_expression(
        self, query=None, allow_joins=True, reuse=None, summarize=False, for_save=False
    ):
        """
        Provide the chance to do any preprocessing or validation before being
        added to the query.

        Arguments:
         * query: the backend query implementation
         * allow_joins: boolean allowing or denying use of joins
           in this query
         * reuse: a set of reusable joins for multijoins
         * summarize: a terminal aggregate clause
         * for_save: whether this expression about to be used in a save or update

        Return: an Expression to be added to the query.
        """
        c = self.copy()
        c.is_summary = summarize
        c.set_source_expressions(
            [
                (
                    expr.resolve_expression(query, allow_joins, reuse, summarize)
                    if expr
                    else None
                )
                for expr in c.get_source_expressions()
            ]
        )
        return c

def reverse_ordering(self)

    def reverse_ordering(self):
        return self

def select_format(self, compiler, sql, params)

Custom format for select clauses. For example, EXISTS expressions need
to be wrapped in CASE WHEN on Oracle.

    def select_format(self, compiler, sql, params):
        """
        Custom format for select clauses. For example, EXISTS expressions need
        to be wrapped in CASE WHEN on Oracle.
        """
        if hasattr(self.output_field, "select_format"):
            return self.output_field.select_format(compiler, sql, params)
        return sql, params

def set_source_expressions(self, exprs)

    def set_source_expressions(self, exprs):
        assert not exprs