目录不存在时的错误处理

Question

我遇到一个情况，我想把MP3文件存放在一个文件夹里。如果这个文件夹不存在，就创建它；如果创建失败，就退出程序。我听说用 os.path.exists() 检查文件夹是否存在会比 os.makedirs() 更耗费性能，所以我写了下面这段代码：

try: 
    # If directory has not yet been created
    os.makedirs('Tracks')
    with open('Tracks/' + title + '.mp3', 'w') as mp3:
        mp3.write(mp3File.content)
        print '%s has been created.' % fileName

except OSError, e:
    # If directory has already been created and is accessible
    if os.path.exists('Tracks'):
        with open('Tracks/' + title + '.mp3', 'w') as mp3:
            mp3.write(mp3File.content)
            print '%s has been created.' % fileName

    else: # Directory cannot be created because of file permissions, etc. 
        sys.exit("Error creating 'Tracks' Directory. Cannot save MP3. Check permissions.")

这样做合理吗？还是说我应该先检查文件夹是否存在，然后再创建，这样虽然代码更简洁，但可能会更耗资源？因为大部分情况下，这个文件夹都是存在的，差不多有9成的概率。

Answer 1

这个try-except块可能会更快，尽管正如@t-8ch所说，这其实并不重要。不过，它可以做得更“干净”一些：

try: 
    # try the thing you expect to work
    mp3 = open('Tracks/' + title + '.mp3', 'w')
except OSError, e:
    # exception is for the unlikely case
    os.makedirs('Tracks')
    mp3 = open('Tracks/' + title + '.mp3', 'w')

mp3.write(mp3File.content)
mp3.close()
print '%s has been created.' % fileName

如果你确实想先try创建目录，可以这样做：

try: 
    # If directory has not yet been created
    os.makedirs('Tracks')
except OSError, e:
    # If directory has already been created or is inaccessible
    if not os.path.exists('Tracks')
        sys.exit("Error creating 'Tracks' Directory. Cannot save MP3. Check permissions.")
with open('Tracks/' + title + '.mp3', 'w') as mp3:
    mp3.write(mp3File.content)
    print '%s has been created.' % fileName