Python - 从元组返回一个值

Question

我正在使用pyacoustid，但我不明白为什么这段代码有效（艺术家实际上是艺术家等等..）：

first = True
        for score, rid, title, artist in self.fpresults:
            if first:
                first = False
            else:
                print
            print '%s - %s' % (artist, title)
            print 'http://musicbrainz.org/recording/%s' % rid
            print 'Score: %i%%' % (int(score * 100))

虽然这个块没有（当我打印时它似乎是空的）：

def getFingerprintArtist(self):
        """
        Returns tuples with possible artists fetched from the MusicBrainz DB
        """
        return  [artist for score, rid, title, artist in self.fpresults]

这是整个班级（欢迎提出建议！）：

class SongFP:
    """
    Song with FINGERPRINTS
    """
    fpresults = None

    def __init__(self, path = None):
        """
        :param path: the path of the song
        """
        self.path = path
        try:
            self.fpresults = acoustid.match(api_key, path)
        except acoustid.NoBackendError:
            logger(paths['log'], "ERROR: chromaprint library/tool not found")
        except acoustid.FingerprintGenerationError:
            logger(paths['log'], "ERROR: fingerprint could not be calculated")
        except acoustid.WebServiceError, exc:
            logger(paths['log'], ("ERROR: web service request failed: %s" % exc.message))

    def setPath(self, path):
        self.path = path

    def printResults(self):
        first = True
        for score, rid, title, artist in self.fpresults:
            if first:
                first = False
            else:
                print
            print '%s - %s' % (artist, title)
            print 'http://musicbrainz.org/recording/%s' % rid
            print 'Score: %i%%' % (int(score * 100))

    def setFPResults(self):
        self.fpresults = acoustid.match(api_key, self.path)

    def getFingerprintArtist(self):
        """
        Returns tuples with possible artists fetched from the MusicBrainz DB
        """
        return [artist for score, rid, title, artist in self.fpresults]

    def getFingerprintTitle(self):
        """
        Returns tuples with possible titles fetched from the MusicBrainz DB
        """
        return [title for score, rid, title, artist in self.fpresults]

    def getFingerPrintID(self):
        """
        Returns tuples with IDs fetched from the MusicBrainz DB
        """
        return [rid for score, rid, title, artist in self.fpresults]

    def getFingerPrintScore(self):
        """
        Returns tuples with scores fetched from the MusicBrainz DB
        """
        return [score for score, rid, title, artist in self.fpresults]

注意： acoustid.match(api_key, path) 返回元组！

编辑：

这个小例子

songfp = SongFP(sys.argv[1])
songfp.printResults()

SongFP 在哪里

class SongFP:
    """
    Song with FINGERPRINTS
    """
    fpresults = None

def __init__(self, path = None):
    """
    :param path: the path of the song
    """
    self.path = path
    try:
        self.fpresults = acoustid.match(api_key, path)
    except acoustid.NoBackendError:
        logger(paths['log'], "ERROR: chromaprint library/tool not found")
    except acoustid.FingerprintGenerationError:
        logger(paths['log'], "ERROR: fingerprint could not be calculated")
    except acoustid.WebServiceError, exc:
        logger(paths['log'], ("ERROR: web service request failed: %s" % exc.message))

def getFingerprintArtist(self):
        """
        Returns tuples with possible artists fetched from the MusicBrainz DB
        """
        return [artist for _, _, _, artist in self.fpresults]

def getFingerprintTitle(self):
    """
    Returns tuples with possible titles fetched from the MusicBrainz DB
    """
    return [title for _, _, title, _ in self.fpresults]

def getFingerprintID(self):
    """
    Returns tuples with IDs fetched from the MusicBrainz DB
    """
    return [rid for _, rid, _, _ in self.fpresults]

def getFingerprintScore(self):
    """
    Returns tuples with scores fetched from the MusicBrainz DB
    """
    return [score for score, _, _, _ in self.fpresults]

def printResults(self):
        print("Titles: %s" % self.getFingerprintTitle())
        print("Artists: %s" % self.getFingerprintArtist())
        print("IDs: %s" % self.getFingerprintID())
        print("Scores: %s" % self.getFingerprintScore())

当被称为./app song.mp3 时，只输出某个字段（如果一个字段为空，则其他字段也应为空，反之亦然，因为它会获取在线 MP3 元数据）

Titles: [u'Our Day Will Come', u'Our Day Will Come', u'Our Day Will Come', u'Our Day Will Come', u'Our Day Will Come']
Artists: []
IDs: []
Scores: []

日志中没有例外！

Answer 1

这很难诊断，但通常更常见的是这样分配你不使用的变量：

def getFingerprintArtist(self):
    """
    Returns (list of)* possible artists fetched from the MusicBrainz DB
    """
    return  [artist for _, _, _, artist in self.fpresults]

您能否将其重写为一个可重现的最小示例，以便我们提供进一步的指导？

*这不是返回一个元组，它只是返回一个（语义上）艺术家姓名的列表！

---

编辑分析

我认为这里发生的事情是您正在耗尽发电机。

self.fpresults

在对象的实例化中填充一次，而不是在您的 __init__ 中执行以下操作：

try:
    self.fpresults = list(acoustid.match(api_key, path))

它将生成器生成的信息作为属性保存在内存中，直到不再引用list 或SongFP 对象，然后进行垃圾回收。

Answer 2

如果它们只是元组，你可以这样做：

def get_value_at_index(tuple_list, index):
    """Returns a list of the values at a given index."""
    return [tup[index] for tup in tuple_list]

Answer 3

与

[artist for score, rid, title, artist in self.fpresults]

您只是商店艺术家。你需要这样的东西：

[(artist, score, rid, title) for artist, score, rid, title in self.fpresults]