How to use the pystac.Catalog.from_file function in pystac

def test_validate_label(self):
        with open(self.label_example_1_uri) as f:
            label_example_1_dict = json.load(f)
        self.validator.validate_dict(label_example_1_dict, "ITEM")

        with TemporaryDirectory() as tmp_dir:
            cat_dir = os.path.join(tmp_dir, 'catalog')
            catalog = TestCases.test_case_1()
            catalog.normalize_and_save(cat_dir, catalog_type=CatalogType.SELF_CONTAINED)

            cat_read = Catalog.from_file(os.path.join(cat_dir, 'catalog.json'))
            label_item_read = cat_read.get_item("area-2-2-labels", recursive=True)
            self.validator.validate_object(label_item_read)

def test_case_2():
        return Catalog.from_file(
            TestCases.get_path('data-files/catalogs/test-case-2/catalog.json'))

def asset_mapper(key, asset):
            if key == changed_asset:
                asset.title = 'NEW TITLE'

            return asset

        with TemporaryDirectory() as tmp_dir:
            catalog = TestCases.test_case_2()

            new_cat = catalog.map_assets(asset_mapper)

            new_cat.normalize_hrefs(os.path.join(tmp_dir, 'cat'))
            new_cat.save(catalog_type=CatalogType.ABSOLUTE_PUBLISHED)

            result_cat = Catalog.from_file(os.path.join(tmp_dir, 'cat', 'catalog.json'))

            found = False
            for item in result_cat.get_all_items():
                for key, asset in item.assets.items():
                    if key == changed_asset:
                        found = True
                        self.assertEqual(asset.title, 'NEW TITLE')
                    else:
                        self.assertNotEqual(asset.title, 'NEW TITLE')
            self.assertTrue(found)

if 'geotiff' in asset.media_type:
                asset.title = 'NEW TITLE'
                changed_assets.append(key)
                return ('{}-modified'.format(key), asset)
            else:
                return asset

        with TemporaryDirectory() as tmp_dir:
            catalog = TestCases.test_case_2()

            new_cat = catalog.map_assets(asset_mapper)

            new_cat.normalize_hrefs(os.path.join(tmp_dir, 'cat'))
            new_cat.save(catalog_type=CatalogType.ABSOLUTE_PUBLISHED)

            result_cat = Catalog.from_file(os.path.join(tmp_dir, 'cat', 'catalog.json'))

            found = False
            not_found = False
            for item in result_cat.get_all_items():
                for key, asset in item.assets.items():
                    if key.replace('-modified', '') in changed_assets:
                        found = True
                        self.assertEqual(asset.title, 'NEW TITLE')
                    else:
                        not_found = True
                        self.assertNotEqual(asset.title, 'NEW TITLE')

            self.assertTrue(found)
            self.assertTrue(not_found)

mod1.title = 'NEW TITLE 1'
                mod2 = asset.clone()
                mod2.title = 'NEW TITLE 2'
                return {'{}-mod-1'.format(key): mod1, '{}-mod-2'.format(key): mod2}
            else:
                return asset

        with TemporaryDirectory() as tmp_dir:
            catalog = TestCases.test_case_2()

            new_cat = catalog.map_assets(asset_mapper)

            new_cat.normalize_hrefs(os.path.join(tmp_dir, 'cat'))
            new_cat.save(catalog_type=CatalogType.ABSOLUTE_PUBLISHED)

            result_cat = Catalog.from_file(os.path.join(tmp_dir, 'cat', 'catalog.json'))

            found1 = False
            found2 = False
            not_found = False
            for item in result_cat.get_all_items():
                for key, asset in item.assets.items():
                    if key.replace('-mod-1', '') in changed_assets:
                        found1 = True
                        self.assertEqual(asset.title, 'NEW TITLE 1')
                    elif key.replace('-mod-2', '') in changed_assets:
                        found2 = True
                        self.assertEqual(asset.title, 'NEW TITLE 2')
                    else:
                        not_found = True
                        self.assertNotEqual(asset.title, 'NEW TITLE')

""" Test case to cover issue #88 """
        stac_uri = 'tests/data-files/catalogs/test-case-6/catalog.json'
        cat = Catalog.from_file(stac_uri)

        # Iterate over the items. This was causing failure in
        # in the later iterations as per issue #88
        for item in cat.get_all_items():
            pass

        with TemporaryDirectory() as tmp_dir:
            new_stac_uri = os.path.join(tmp_dir, 'test-case-6')
            cat.normalize_hrefs(new_stac_uri)
            cat.save(catalog_type=CatalogType.SELF_CONTAINED)

            # Open the local copy and iterate over it.
            cat2 = Catalog.from_file(os.path.join(new_stac_uri, 'catalog.json'))

            for item in cat2.get_all_items():
                # Iterate again over the items. This would fail in #88
                pass

def test_reading_iterating_and_writing_works_as_expected(self):
        """ Test case to cover issue #88 """
        stac_uri = 'tests/data-files/catalogs/test-case-6/catalog.json'
        cat = Catalog.from_file(stac_uri)

        # Iterate over the items. This was causing failure in
        # in the later iterations as per issue #88
        for item in cat.get_all_items():
            pass

        with TemporaryDirectory() as tmp_dir:
            new_stac_uri = os.path.join(tmp_dir, 'test-case-6')
            cat.normalize_hrefs(new_stac_uri)
            cat.save(catalog_type=CatalogType.SELF_CONTAINED)

            # Open the local copy and iterate over it.
            cat2 = Catalog.from_file(os.path.join(new_stac_uri, 'catalog.json'))

            for item in cat2.get_all_items():
                # Iterate again over the items. This would fail in #88

def test_create_and_read(self):
        with TemporaryDirectory() as tmp_dir:
            cat_dir = os.path.join(tmp_dir, 'catalog')
            catalog = TestCases.test_case_1()

            catalog.normalize_and_save(cat_dir, catalog_type=CatalogType.ABSOLUTE_PUBLISHED)

            read_catalog = Catalog.from_file('{}/catalog.json'.format(cat_dir))

            collections = catalog.get_children()
            self.assertEqual(len(list(collections)), 2)

            items = read_catalog.get_all_items()

            self.assertEqual(len(list(items)), 8)

def test_read_remote(self):
        # TODO: Move this URL to the main stac-spec repo once the example JSON is fixed.
        catalog_url = (
            'https://raw.githubusercontent.com/lossyrob/stac-spec/0.9.0/pystac-upgrade-fixes'
            '/extensions/label/examples/multidataset/catalog.json')
        cat = Catalog.from_file(catalog_url)

        zanzibar = cat.get_child('zanzibar-collection')

        self.assertEqual(len(list(zanzibar.get_items())), 2)