python - 将 Python ElementTree 写入文件会抛出 TypeError

Question

我正在尝试使用 Python 的 ElementTree 包编写 XML 文件。基本上，我创建一个名为 allDepts 的根元素，然后在 for 循环的每次迭代中，我调用一个函数，该函数返回一个 deptElement ，其中包含有关大学系的一堆信息。我将每个 deptElement 添加到 allDepts，从 allDepts 中创建一个 ElementTree，然后尝试将其写入文件.

def crawl(year, season, campus):
  departments = getAllDepartments(year, season, campus)
  allDepts = ET.Element('depts')

  for dept in departments:
    deptElement = getDeptElement(allDepts, dept, year, season, campus)
    print ET.tostring(deptElement)    #Prints fine here!
    ET.SubElement(allDepts, deptElement)

    if deptElement == None:
        print "ERROR: " + dept

  with open(str(year) + season + "_" + campus + "_courses.xml", 'w') as f:
    tree = ET.ElementTree(allDepts)
    tree.write(f)

由于某种原因，在 tree.write(f) 行，我收到此错误:“TypeError:无法连接 'str' 和 'instance' 对象”。每个 deptElement 在 for 循环中都可以正常打印，这让我认为 getDeptElement() 工作正常。我从来没有打印出“错误”消息。有谁知道我做错了什么？

编辑:这是完整的堆栈跟踪:

File "./CourseInfoCrawl.py", line 210, in <module>
crawl("2013", "S", "UBC")
File "./CourseInfoCrawl.py", line 207, in crawl
tree.write(f)
File "/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/xml/etree/ElementTree.py", line 663, in write
self._write(file, self._root, encoding, {})
File "/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/xml/etree/ElementTree.py", line 707, in _write
self._write(file, n, encoding, namespaces)
File "/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/xml/etree/ElementTree.py", line 681, in _write
file.write("<" + _encode(tag, encoding))

Answer 1

似乎以下行是原因。

    print "ERROR: " + dept

更改如下并重试:

    print "ERROR: ", dept

或者

    print "ERROR: " + str(dept)

添加

ET.SubElement 的第二个参数应该是 str。 deptElement 是 str 吗？ 如果 deptElement 是 Element，则使用 allDepts.append(deptElement)。

http://docs.python.org/2/library/xml.etree.elementtree.html#xml.etree.ElementTree.SubElement http://docs.python.org/2/library/xml.etree.elementtree.html#xml.etree.ElementTree.Element.append

添加 2 重现错误(Python 2.6):

>>> from xml.etree import ElementTree as ET
>>> allDepts = ET.Element('depts')
>>> ET.SubElement(allDepts, ET.Element('a'))
<Element <Element a at b727b96c> at b727b22c>
>>> with open('a', 'wb') as f:
...     tree = ET.ElementTree(allDepts)
...     tree.write(f)
...
Traceback (most recent call last):
  File "<stdin>", line 3, in <module>
  File "/home/falsetru/t/Python-2.6/Lib/xml/etree/ElementTree.py", line 663, in write
    self._write(file, self._root, encoding, {})
  File "/home/falsetru/t/Python-2.6/Lib/xml/etree/ElementTree.py", line 707, in _write
    self._write(file, n, encoding, namespaces)
  File "/home/falsetru/t/Python-2.6/Lib/xml/etree/ElementTree.py", line 681, in _write
    file.write("<" + _encode(tag, encoding))
TypeError: cannot concatenate 'str' and 'instance' objects

重现错误(Python 2.7，不同的错误消息):

>>> from xml.etree import ElementTree as ET
>>> allDepts = ET.Element('depts')
>>> ET.SubElement(allDepts, ET.Element('a'))
<Element <Element 'a' at 0xb745a8ec> at 0xb74601ac>
>>> with open('a', 'wb') as f:
...     tree = ET.ElementTree(allDepts)
...     tree.write(f)
...
Traceback (most recent call last):
  File "<stdin>", line 3, in <module>
  File "/usr/lib/python2.7/xml/etree/ElementTree.py", line 817, in write
    self._root, encoding, default_namespace
  File "/usr/lib/python2.7/xml/etree/ElementTree.py", line 886, in _namespaces
    _raise_serialization_error(tag)
  File "/usr/lib/python2.7/xml/etree/ElementTree.py", line 1052, in _raise_serialization_error
    "cannot serialize %r (type %s)" % (text, type(text).__name__)
TypeError: cannot serialize <Element 'a' at 0xb745a8ec> (type Element)