Python中tuple类型只有一个元素时要写逗号

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今天同事使用scrapy遇到一个问题，报错如下： 
Traceback (most recent call last):
  File "/Library/Python/2.7/site-packages/scrapy/core/engine.py", line 111, in _next_request
    request = next(slot.start_requests)
  File "/Library/Python/2.7/site-packages/scrapy/spiders/__init__.py", line 70, in start_requests
    yield self.make_requests_from_url(url)
  File "/Library/Python/2.7/site-packages/scrapy/spiders/__init__.py", line 73, in make_requests_from_url
    return Request(url, dont_filter=True)
  File "/Library/Python/2.7/site-packages/scrapy/http/request/__init__.py", line 24, in __init__
    self._set_url(url)
  File "/Library/Python/2.7/site-packages/scrapy/http/request/__init__.py", line 59, in _set_url
    raise ValueError('Missing scheme in request url: %s' % self._url)
ValueError: Missing scheme in request url: h
看到scrapy源码中抛出异常的这个代码： 
def _set_url(self, url):
        if isinstance(url, str):
            self._url = escape_ajax(safe_url_string(url))
        elif isinstance(url, six.text_type):
            if self.encoding is None:
                raise TypeError('Cannot convert unicode url - %s has no encoding' %
                                type(self).__name__)
            self._set_url(url.encode(self.encoding))
        else:
            raise TypeError('Request url must be str or unicode, got %s:' % type(url).__name__)
        if ':' not in self._url:
            raise ValueError('Missing scheme in request url: %s' % self._url)
按说URL中是包含冒号的，怎么会报这个错，而且报错提示是 
ValueError: Missing scheme in request url: h
看到这个想起http://****，http中的h被当成了一个域名，原因是这样的：在spider文件中，start_url是个tuple，里面写的网址如果不止一个就不会出错，但是如果只有一个网址，如果你漏写了逗号，Python就不会认为它是个tuple，看下图： 1 2