import pandas as pd
import numpy as np


class InsertContext():
    def __init__(self, tf):
        self._tf = tf
        self.__write_mode_on = False

    def __enter__(self):
        self._tf.df = None
        self._tf.set_context(self)
        self.__write_mode_on = True
        return self._tf

    def is_writable(self):
        return self.__write_mode_on

    def __exit__(self, exp, value, traceback):
        self._tf._flush()
        self._tf.unset_context()
        self.__write_mode_on = False



class YetAnotherTableFrame():

    def __init__(self, *columns):

        self.df = self.__df = pd.DataFrame(data = np.zeros((0, len(columns))), columns = columns)

        self._rows_to_be_inserted = []
        self.__insert_context = None


    def set_context(self, ic):
        self.__insert_context = ic

    def unset_context(self):
        self.__insert_context = None

    def insert(self, row):

        if not self.__insert_context or not self.__insert_context.is_writable():
            raise Exception("This method should be invoked from within a context")

        self._rows_to_be_inserted.append(row)


    def _flush(self):

        other = pd.DataFrame(self._rows_to_be_inserted)
        self.__df = self.__df.append(other)
        self.df = self.__df


if __name__ == '__main__':


    y = YetAnotherTableFrame('a', 'b', 'c')

    with InsertContext(y) as tf:
        tf.insert(dict(a = 1, b = 2, c = 3))
        tf.insert(dict(a = 4, b = 5, c = 6))
        tf.insert(dict(a = 7, b = 8, c = 9))

    print(y.df)

    # tricking like this, will throw an exception
    ic = InsertContext(tf)
    tf.set_context(ic)
    tf.insert(dict(a = 7, b = 8, c = 9))

    print(y.df)