Scrapy仅在循环中返回第一个结果

Question

I have a loop (as shown below) that executes twice (indexes 1->3), but Scrapy only returns the first trackname in both results. But the print item line shows different values for str_selector so I know the loop works but Scrapy isn't seeing the changing value of x .

Any idea what mistake I have made?

items = []
item = scrapyItem()

for x in range (1,3):
    str_selector = '//tr[@name="tracks-grid-browse_track_{0}"]/td[contains(@class,"secondColumn")]/a/text()'.format(x)
    item['trackname'] = hxs.select(str_selector).extract()
    print item
    items.append(item)
return items

Answer 1

It's just that you should build a new item for each iteration, instead of keeping the same: you add in items the same object, which is mutable (as for all user-defined classes by default in python) and so when you update item['trackname'] , all items contained are updated !

Here is some code to illustrate:

>>> class C(object):
        # Basic user-defined class
    def __init__(self):
        self.test = None


>>> c = C()
>>> items = []
>>> for x in range (1,3):
    c.test = x
    print c, c.test
    items.append(c)


<__main__.C object at 0x01CEB130> 1
<__main__.C object at 0x01CEB130> 2
>>> items # All objects contained are the same !!!
[<__main__.C object at 0x01CEB130>, <__main__.C object at 0x01CEB130>]
>>> for c in items:
    print c.test


2
2

Now create a new object each time:

>>> items = []
>>> for x in range (1,3):
    c = C()
    c.test = x
    print c, c.test
    items.append(c)


<__main__.C object at 0x01CEB110> 1
<__main__.C object at 0x011F2270> 2

Objects are now different !

>>> for c in items:
    print c.test


1
2

Answer 2

what actually you are doing right now is creating an item object and changing its value in loop, you need to create item in loop.

items = []
#item = scrapyItem()

for x in range (1,3):
    item = scrapyItem()
    str_selector = '//tr[@name="tracks-grid-browse_track_{0}"]/td[contains(@class,"secondColumn")]/a/text()'.format(x)
    item['trackname'] = hxs.select(str_selector).extract()
    print item
    items.append(item)
return items