ValueError：名称空间...的名称...未在范围内声明

Question

调用

lxml.etree.canonicalize(node)

时引发 ValueError 异常：名称“”的命名空间“{}”未在范围内声明。

在这种特殊情况下，消息是

ValueError: Namespace "http://schemas.xmlsoap.org/soap/envelope/" of name "Header" is not declared in scope

以下代码完全符合预期：

from lxml import etree as ET

url_soap_envelope =  "http://schemas.xmlsoap.org/soap/envelope/"

# Header
nsmap_Header = {
    's': url_soap_envelope,
}
qname_s_Header = ET.QName(url_soap_envelope, "Header")
node_header = ET.Element(qname_s_Header, nsmap=nsmap_Header)

ET.canonicalize(node_header)
# '<s:Header xmlns:s="http://schemas.xmlsoap.org/soap/envelope/"></s:Header>'

但是，如果

node_header

是另一个节点的子元素，它会中断：

from lxml import etree as ET

url_wss_u = "http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-utility-1.0.xsd"
url_soap_envelope =  "http://schemas.xmlsoap.org/soap/envelope/"

# Envelope
nsmap_Envelope = {
    's': url_soap_envelope,
    'u': url_wss_u,
}
qname_s_Envelope = ET.QName(url_soap_envelope, "Envelope")
node_envelope = ET.Element(qname_s_Envelope, {}, nsmap=nsmap_Envelope)

# Envelope / Header
nsmap_Header = {
    's': url_soap_envelope,
}
qname_s_Header = ET.QName(url_soap_envelope, "Header")
node_header = ET.SubElement(node_envelope, qname_s_Header, nsmap=nsmap_Header)

ET.canonicalize(node_envelope)
# '<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/"><s:Header></s:Header></s:Envelope>'

ET.canonicalize(node_header)
# Traceback (most recent call last):
#   File "<stdin>", line 1, in <module>
#   File "src/lxml/serializer.pxi", line 918, in  lxml.etree.canonicalize
#   File "src/lxml/serializer.pxi", line 943, in lxml.etree._tree_to_target
#   File "src/lxml/serializer.pxi", line 1128, in lxml.etree.C14NWriterTarget.start
#   File "src/lxml/serializer.pxi", line 1155, in lxml.etree.C14NWriterTarget._start
#   File "src/lxml/serializer.pxi", line 1085, in lxml.etree.C14NWriterTarget._qname
# ValueError: Namespace "http://schemas.xmlsoap.org/soap/envelope/" of name "Header" is not declared in scope

调用时的预期结果

ET.canonicalize(node_header)

是

'<s:Header xmlns:s="http://schemas.xmlsoap.org/soap/envelope/"></s:Header>'