See: https://github.com/jcupitt/pyvips/issues/39

readable.py

#!/usr/bin/python

from pyvips import ffi, vips_lib, Image, Error, _to_bytes


def has_loader(file):
    name = vips_lib.vips_foreign_find_load(_to_bytes(file))

    if name == ffi.NULL:
        return False

    return True


# tmp file content = https://www.welt.de/favicon.ico
tmp_file = 'x'

loader = has_loader(tmp_file)

if loader:
    print('.ico file has loader')
    print('try to load it with new_from_file')
    try:
        image = Image.new_from_file(tmp_file)
        print('Successfully load .ico with new_from_file. Loader: {}'.format(image.get('vips-loader')))
    except Error:
        print('Failed to load .ico with new_from_file')

    print('try to load it with thumbnail')
    try:
        thumb = Image.thumbnail(tmp_file, 200)
        print('Successfully load .ico with thumbnail. Loader: {}'.format(thumb.get('vips-loader')))
    except Error:
        print('Failed to load .ico with thumbnail')
else:
    print('no loader for this image format')