lxml在写入文件时验证xml，并进行回读，但不验证从中生成文件的DOM树

Question

我使用lxml生成xml并对其进行验证。我发现，当我尝试使用XSD验证器验证DOM树时，验证器会抱怨它无法识别根元素；相比之下，当该树转换为文本、写入文件、读回并解析到DOM树时，新DOM树的根元素被识别。希望下面能说明我的意思。在

我的问题是：为什么会这样？除了呈现为字符串并重新分析之外，有没有一种方法可以让原始DOM树进行验证？在

>>> from lxml import etree
>>> import io
>>> validator = etree.XMLSchema(etree.parse(io.open('mainapp/xsd/forms/CompanyIncorporation-v2-6.xsd')))
>>> c = CompanyUK.objects.all()[5]
>>> cotree = xml.Company.generate_company_incorporation_tree(c)
>>> cotree
<Element CompanyIncorporation at 0x38f2750>
>>> validator.assertValid(cotree)
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "lxml.etree.pyx", line 3006, in lxml.etree._Validator.assertValid (src/lxml/lxml.etree.c:125415)
DocumentInvalid: Element 'CompanyIncorporation': No matching global declaration available for the validation root.
>>> cott = etree.ElementTree(cotree)
>>> validator.assertValid(cott)
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "lxml.etree.pyx", line 3006, in lxml.etree._Validator.assertValid (src/lxml/lxml.etree.c:125415)
DocumentInvalid: Element 'CompanyIncorporation': No matching global declaration available for the validation root.
>>> xmlfile = open("xmlfile", "w")
>>> xmlfile.write(etree.tostring(cott))
>>> xmlfile.flush()
>>> xmlfile.close()
>>> xmlfile = open("xmlfile", "r")
>>> ptree = etree.parse(xmlfile)
>>> validator.assertValid(ptree)
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "lxml.etree.pyx", line 3006, in lxml.etree._Validator.assertValid (src/lxml/lxml.etree.c:125415)
DocumentInvalid: Element '{http://xmlgw.companieshouse.gov.uk}Country': [facet 'enumeration'] The value 'EW' is not an element of the set {'GB-ENG', 'GB-WLS', 'GB-SCT', 'GB-NIR', 'GBR', 'UNDEF'}., line 1
>>> etree.tostring(ptree) == etree.tostring(cott)
True
>>> validator.assertValid(etree.parse(io.StringIO(etree.tounicode(ptree))))
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "lxml.etree.pyx", line 3006, in lxml.etree._Validator.assertValid (src/lxml/lxml.etree.c:125415)
DocumentInvalid: Element '{http://xmlgw.companieshouse.gov.uk}Country': [facet 'enumeration'] The value 'EW' is not an element of the set {'GB-ENG', 'GB-WLS', '
GB-SCT', 'GB-NIR', 'GBR', 'UNDEF'}., line 1
>>> validator.assertValid(etree.parse(io.StringIO(etree.tounicode(cot))))
Traceback (most recent call last):
  File "<console>", line 1, in <module>
NameError: name 'cot' is not defined
>>> validator.assertValid(etree.parse(io.StringIO(etree.tounicode(cott))))
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "lxml.etree.pyx", line 3006, in lxml.etree._Validator.assertValid (src/lxml/lxml.etree.c:125415)
DocumentInvalid: Element '{http://xmlgw.companieshouse.gov.uk}Country': [facet 'enumeration'] The value 'EW' is not an element of the set {'GB-ENG', 'GB-WLS', '
GB-SCT', 'GB-NIR', 'GBR', 'UNDEF'}., line 1
>>> validator.assertValid(etree.parse(io.BytesIO(etree.tostring(cott))))
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "lxml.etree.pyx", line 3006, in lxml.etree._Validator.assertValid (src/lxml/lxml.etree.c:125415)
DocumentInvalid: Element '{http://xmlgw.companieshouse.gov.uk}Country': [facet 'enumeration'] The value 'EW' is not an element of the set {'GB-ENG', 'GB-WLS', '
GB-SCT', 'GB-NIR', 'GBR', 'UNDEF'}., line 1
>>>

正如您在上面看到的，从磁盘解析的版本，验证得到了进一步的发展，即使它们呈现为相同的字符串（同样，验证在往返字符串后超出了根元素，并重新解析为DOM树）。那么，是什么原因造成的呢？在