def get_collection(db, collection):
    """Get a collection if exists on MongoDB system.

    Keyword arguments:
    db -- Database from MongoDB
    collection -- Name to search inside MongoDB.

    Return -> collection
    """
    if collection in db.list_collection_names():
        print("This collection exists.")
        return db[f'{collection}']
    if collection not in db.list_collection_names():
        print("This collection doesn't exists.")
        return None