def open(store=None, mode='a', **kwargs):
    """Convenience function to open a group or array using file-mode-like semantics.

    Parameters
    ----------
    store : MutableMapping or string, optional
        Store or path to directory in file system or name of zip file.
    mode : {'r', 'r+', 'a', 'w', 'w-'}, optional
        Persistence mode: 'r' means read only (must exist); 'r+' means
        read/write (must exist); 'a' means read/write (create if doesn't
        exist); 'w' means create (overwrite if exists); 'w-' means create
        (fail if exists).
    **kwargs
        Additional parameters are passed through to :func:`zarr.creation.open_array` or
        :func:`zarr.hierarchy.open_group`.

    Returns
    -------
    z : :class:`zarr.core.Array` or :class:`zarr.hierarchy.Group`
        Array or group, depending on what exists in the given store.

    See Also
    --------
    zarr.creation.open_array, zarr.hierarchy.open_group

    Examples
    --------

    Storing data in a directory 'data/example.zarr' on the local file system::

        >>> import zarr
        >>> store = 'data/example.zarr'
        >>> zw = zarr.open(store, mode='w', shape=100, dtype='i4')  # open new array
        >>> zw
        <zarr.core.Array (100,) int32>
        >>> za = zarr.open(store, mode='a')  # open existing array for reading and writing
        >>> za
        <zarr.core.Array (100,) int32>
        >>> zr = zarr.open(store, mode='r')  # open existing array read-only
        >>> zr
        <zarr.core.Array (100,) int32 read-only>
        >>> gw = zarr.open(store, mode='w')  # open new group, overwriting previous data
        >>> gw
        <zarr.hierarchy.Group '/'>
        >>> ga = zarr.open(store, mode='a')  # open existing group for reading and writing
        >>> ga
        <zarr.hierarchy.Group '/'>
        >>> gr = zarr.open(store, mode='r')  # open existing group read-only
        >>> gr
        <zarr.hierarchy.Group '/' read-only>

    """

    path = kwargs.get('path', None)
    # handle polymorphic store arg
    clobber = mode == 'w'
    store = normalize_store_arg(store, clobber=clobber)
    path = normalize_storage_path(path)

    if mode in {'w', 'w-', 'x'}:
        if 'shape' in kwargs:
            return open_array(store, mode=mode, **kwargs)
        else:
            return open_group(store, mode=mode, **kwargs)

    elif mode == 'a':
        if contains_array(store, path):
            return open_array(store, mode=mode, **kwargs)
        elif contains_group(store, path):
            return open_group(store, mode=mode, **kwargs)
        elif 'shape' in kwargs:
            return open_array(store, mode=mode, **kwargs)
        else:
            return open_group(store, mode=mode, **kwargs)

    else:
        if contains_array(store, path):
            return open_array(store, mode=mode, **kwargs)
        elif contains_group(store, path):
            return open_group(store, mode=mode, **kwargs)
        else:
            err_path_not_found(path)