Thanks Holger

First, I am not behind any proxy - CURL to http: and https: both give the schema

Second, Your test_schema.py using http: also works for me, however is 
https:/www.loc.gov... is used then I get an error 
>python test_schema.py
Traceback (most recent call last):
  File "G:\lxml-test\test_schema.py", line 14, in <module>
    schema = etree.XMLSchema(tree)
  File "src\lxml\xmlschema.pxi", line 88, in lxml.etree.XMLSchema.__init__
lxml.etree.XMLSchemaParseError: Element 
'{http://www.w3.org/2001/XMLSchema}include': Failed to load the document 
'https://www.loc.gov/standards/xlink/xlink.xsd' for inclusion., line 8

Third, 'https://www.loc.gov/standards/xlink/xlink.xsd' does exist - CURL 
retrieves it OK
>curl --head https://www.loc.gov/standards/xlink/xlink.xsd
HTTP/2 200
date: Fri, 25 Jun 2021 13:44:27 GMT
content-type: text/xml
content-length: 3180
last-modified: Thu, 23 Aug 2007 19:02:01 GMT
etag: "119c982-c6c-867afc40"
accept-ranges: bytes
cf-cache-status: DYNAMIC
cf-request-id: 0ae5033c37000054ab1ab16000000001
expect-ct: max-age=604800, 
report-uri="https://report-uri.cloudflare.com/cdn-cgi/beacon/expect-ct";
server: cloudflare
cf-ray: 664ea1738c7e54ab-MAN

>curl --head http://www.loc.gov/standards/xlink/xlink.xsd
HTTP/1.1 200 OK
Date: Fri, 25 Jun 2021 13:45:16 GMT
Content-Type: text/xml
Content-Length: 3180
Connection: keep-alive
Last-Modified: Thu, 23 Aug 2007 19:02:01 GMT
ETag: "119c982-c6c-43862867afc40"
Accept-Ranges: bytes
X-Frame-Options: deny
Set-Cookie: HttpOnly
CF-Cache-Status: DYNAMIC
cf-request-id: 0ae503f9cd0000000a56ab5000000001
Server: cloudflare
CF-RAY: 664ea2a2ed37000a-MAN

Paul

-----Original Message-----
From: holger.jo...@lbbw.de <holger.jo...@lbbw.de> 
Sent: 25 June 2021 13:31
To: l...@lxml.de
Subject: [lxml] Re: Cannot <include> from a network location

> Just a thought: Might this be proxy- or https-related? Does it work if you 
> locally serve the xs:included schema with http?
>
> I *think* libxml2 respects http_proxy but I don’t know anything about https 
> support.

Just found that for an example adapted from 
https://bugs.launchpad.net/lxml/+bug/1234114/comments/3
(xs:include instead of xs:import):

##############

# test_schema.py
XSD = b"""<?xml version="1.0" encoding="UTF-8"?> <xs:schema
    xmlns:xs="http://www.w3.org/2001/XMLSchema";
    xmlns:xlink="http://www.w3.org/1999/xlink";
    xmlns="lxmltest"
    targetNamespace="http://www.w3.org/1999/xlink";
    elementFormDefault="qualified"
    attributeFormDefault="unqualified">
    <xs:include schemaLocation="http://www.loc.gov/standards/xlink/xlink.xsd"; 
/> </xs:schema> """
from lxml import etree
parser = etree.XMLParser(
    load_dtd=True, no_network=True, huge_tree=True, resolve_entities=True) tree 
= etree.fromstring(XSD, parser=parser) schema = etree.XMLSchema(tree)
print(schema)

##############

I can successfully run this if the xs:include location is http but not https 
(the xlink.xsd is available both with https and http URLs).

If I change it to https I get

Traceback (most recent call last):
  File "test_schema.py", line 16, in <module>
    schema = etree.XMLSchema(tree)
  File "src/lxml/xmlschema.pxi", line 88, in lxml.etree.XMLSchema.__init__
lxml.etree.XMLSchemaParseError: Element 
'{http://www.w3.org/2001/XMLSchema}include': Failed to load the document 
'https://www.loc.gov/standards/xlink/xlink.xsd' for inclusion., line 9

As I'm behind a proxy:
I also just found out that while curl happily accepts 
http_proxy=my.proxy.address.net:8080 lxml (libxml2) only works if this is set 
to http_proxy=http://my.proxy.address.net:8080 i.e. with an explicit <scheme>://

Cheers, H.








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