def get_field_definition(schema: 'GraphQLSchema', parent_type:
    'GraphQLObjectType', field_name: str) ->'GraphQLField':
    """
    Returns the field corresponding to the parent type and field name.
    :param schema: the GraphQLSchema instance linked to the engine
    :param parent_type: GraphQLObjectType of the field's parent
    :param field_name: field name to retrieve
    :type schema: GraphQLSchema
    :type parent_type: GraphQLObjectType
    :type field_name: str
    :return: the GraphQLField instance
    :rtype: GraphQLField
    """
    try:
        return schema.get_field_by_name(f'{parent_type}.{field_name}')
    except Exception:
        pass
    return None
