import pytest

import pandas as pd
import pandas._testing as tm


class TestFreq:
    def test_freq_setter_deprecated(self):
        # GH#20678
        idx = pd.period_range("2018Q1", periods=4, freq="Q")

        # no warning for getter
        with tm.assert_produces_warning(None):
            idx.freq

        # PeriodIndex freq is derived from dtype and read-only
        msg = "property 'freq' of 'PeriodIndex' object has no setter"
        with pytest.raises(AttributeError, match=msg):
            idx.freq = pd.offsets.Day()