Function _new_to_legacy_freq

xarray/coding/cftime_offsets.py:1463–1502

(freq)

def _new_to_legacy_freq(freq):
    # xarray will now always return "ME" and "QE" for MonthEnd and QuarterEnd
    # frequencies, but older versions of pandas do not support these as
    # frequency strings.  Until xarray's minimum pandas version is 2.2 or above,
    # we add logic to continue using the deprecated "M" and "Q" frequency
    # strings in these circumstances.

    # NOTE: other conversions ("h" -> "H", ..., "ns" -> "N") not required

    # TODO: remove once requiring pandas >= 2.2
    if not freq or Version(pd.__version__) >= Version("2.2"):
        return freq

    try:
        freq_as_offset = to_offset(freq)
    except ValueError:
        # freq may be valid in pandas but not in xarray
        return freq

    if isinstance(freq_as_offset, MonthEnd) and "ME" in freq:
        freq = freq.replace("ME", "M")
    elif isinstance(freq_as_offset, QuarterEnd) and "QE" in freq:
        freq = freq.replace("QE", "Q")
    elif isinstance(freq_as_offset, YearBegin) and "YS" in freq:
        freq = freq.replace("YS", "AS")
    elif isinstance(freq_as_offset, YearEnd):
        # testing for "Y" is required as this was valid in xarray 2023.11 - 2024.01
        if "Y-" in freq:
            # Check for and replace "Y-" instead of just "Y" to prevent
            # corrupting anchored offsets that contain "Y" in the month
            # abbreviation, e.g. "Y-MAY" -> "A-MAY".
            freq = freq.replace("Y-", "A-")
        elif "YE-" in freq:
            freq = freq.replace("YE-", "A-")
        elif "A-" not in freq and freq.endswith("Y"):
            freq = freq.replace("Y", "A")
        elif freq.endswith("YE"):
            freq = freq.replace("YE", "A")

    return freq


def _legacy_to_new_freq(freq: T_FreqStr) -> T_FreqStr: