如果string可以为None，则获取嵌套字典中len(string)的最大值

def get_max_string_length(dictionary):
    max_length = 0

    for value in dictionary.values():
        if isinstance(value, dict):
            length = get_max_string_length(value)
            max_length = max(max_length, length)
        elif value is not None and isinstance(value, str):
            max_length = max(max_length, len(value))

    return max_length

dictionary = {
    'key1': 'abc',
    'key2': {
        'nested_key1': 'def',
        'nested_key2': None,
        'nested_key3': 'ghi'
    },
    'key3': {
        'nested_key4': {
            'nested_key5': 'jkl',
            'nested_key6': 'mno'
        },
        'nested_key7': 'pqr'
    }
}

max_length = get_max_string_length(dictionary)
print(max_length)