Source code for pysqllike.generic.column_group_operator

# -*- coding: utf-8 -*-
"""
Creates custom classes to interpret Python expression as column operations.


:githublink:`%|py|6`
"""

from .column_operator import ColumnOperator
from .others_types import NA


class ColumnGroupOperator(ColumnOperator):
    """
    Defines an operation between two columns.


    :githublink:`%|py|14`
    """

    def __init__(self):
        """
        Initiates the operator.

        :param      name:        name of the column


        :githublink:`%|py|21`
        """
        pass

    def __str__(self):
        """
        usual


        :githublink:`%|py|27`
        """
        raise NotImplementedError()

    def __call__(self, columns):
        """
        returns the results of this operation between a list of columns


        :githublink:`%|py|33`
        """
        raise NotImplementedError()


class OperatorGroupLen(ColumnGroupOperator):

    """
    defines the group function ``len``


    :githublink:`%|py|41`
    """

    def __str__(self):
        """
        usual


        :githublink:`%|py|46`
        """
        return "len"

    def __call__(self, columns):
        """
        returns the results of this operation between a list of columns


        :githublink:`%|py|52`
        """
        if not hasattr(columns, '__iter__'):
            raise TypeError(
                "we expect an iterator here not " + str(type(columns)))
        return len(columns)


class OperatorGroupAvg(ColumnGroupOperator):

    """
    defines the group function ``avg``, the default value when the set is empty is None


    :githublink:`%|py|63`
    """

    def __str__(self):
        """
        usual


        :githublink:`%|py|68`
        """
        return "avg"

    def __call__(self, columns):
        """
        returns the results of this operation between a list of columns,
        it returns :class:`NA <pysqllike.generic.others_types.NA>` for a null set


        :githublink:`%|py|75`
        """
        if not hasattr(columns, '__iter__'):
            raise TypeError(
                "we expect an iterator here not " + str(type(columns)))

        # we walk through the set only once
        nb = 0
        for val in columns:
            if nb == 0:
                s = val
            else:
                s += val
            nb += 1

        if nb == 0:
            return NA
        else:
            return s / nb