Writing Python ElementTree to file throws TypeError

Question

I'm trying to write an XML file using Python's ElementTree package. Basically I make a root element called allDepts, and then in each iteration of my for loop I call a function that returns a deptElement containing a bunch of information about a university department. I add every deptElement to allDepts, make an ElementTree out of allDepts, and try to write it to a file.

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)

For some reason, at the tree.write(f) line, I get this error: "TypeError: cannot concatenate 'str' and 'instance' objects". Each deptElement prints out fine in the for loop, making me think that getDeptElement() is working fine. I never get my "ERROR" message printed out. Does anyone know what I'm doing wrong?

EDIT: Here's the full stack trace:

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))

falsetru · Accepted Answer · 2013-06-14 19:00:19Z

2

Seem following line is cause.

    print "ERROR: " + dept

Change as follow and retry:

    print "ERROR: ", dept

OR

    print "ERROR: " + str(dept)

ADD

Second argument to ET.SubElement should be str. Is deptElement is str? If deptElement is Element, use 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

ADD 2 To reproduce error (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

To reproduce error (Python 2.7, different error message):

>>> 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)

edited Jun 14, 2013 at 19:00

answered Jun 14, 2013 at 18:24

falsetru

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3 Comments

FrancesKR Over a year ago

Tried that and it didn't help - dept is already a string and this line is never executed anyway. I've added the stack trace in my question, which shows that it's the tree.write(f) line that's causing the error.

falsetru Over a year ago

@FrancesKR, added another possible causes.

FrancesKR Over a year ago

Thank you, it was append() rather than SubElement()!

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