add mixin for Database to use pandas to retrieve table/query

#!/usr/bin/env python3

# -*- coding: utf-8 -*-

"""Provide a `pandas` mixin for the Database class."""

import pandas as pd

import sqlalchemy

class Pandas_Mixin():

    """Group together all pandas logic."""

    def query_pandas(self, query=None, table=None, schema=None):

        """Return a query as a pandas table.

            Provided a query, uses the default cursor assumming a

            PostgreSQL connection.

            :param query: a query string to return as a table

            :param table: the table name to retrieve data from

            :param schema: the schema to select the table from

            :returns: a pandas.DataFframe object

        """

        def make_dataframe(query):

            """Create a pandas dataframe from a query."""

            if self.cursor is None:

                # TODO raise an appropriate error here

                self.logger.error('Cursor is not open, please create one.')

            # TODO wrap in a try except block

            self.logger.debug(f'Returning {query} as pandas dataframe.')

            data_fetch_query = self.cursor.execute(query)

            if data_fetch_query is None:  # FIXME better way to handle this?

                self.logger.error(f"Unable to fetch {query}.")

                # FIXME raise an Exception here?

            dataframe = pd.DataFrame(data_fetch_query.fetchall())

            dataframe.columns = data_fetch_query.keys()

            return dataframe

        if query is not None and not isinstance(self.connection,

                                                sqlalchemy.engine.base.Engine):

            return make_dataframe(query)

        if table is not None and schema is not None and \

                isinstance(self.connection, sqlalchemy.engine.base.Engine):

            return pd.read_from_table(table_name=table, schema=schema)

        if query is not None and \

                isinstance(self.connection, sqlalchemy.engine.base.Engine):

            return pd.read_sql_from_query(query, ur=self.connection_inf['uri'])