from typing import Union


def crs_to_str(crs: Union[str, int]) -> str:
    """
    Return a string representation of a crs
    Parameters
    ----------
    crs: an epsg number, a string or a class that support __str__() (e.g. pyproj.CRS)

    Returns
    -------
    a string representation of the crs
    """
    if isinstance(crs, int):
        return f"epsg:{crs}"
    return str(crs)