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gh-159044: Restore string conversion for HTTP tunnel header values #159216

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ZeroIntensity merged 1 commit into from
Oct 13, 2026
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gh-159044: Restore string conversion for HTTP tunnel header values #159216

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3 changes: 2 additions & 1 deletion Doc/library/http.client.rst
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Expand Up @@ -370,7 +370,8 @@ HTTPConnection Objects
proxy server).

The *headers* argument should be a mapping of extra HTTP headers to send with
the CONNECT request.
the CONNECT request. Non-string header values are converted to strings via
:func:`str`.

As HTTP/1.1 is used for HTTP CONNECT tunnelling request, `as per the RFC
<https://datatracker.ietf.org/doc/html/rfc7231#section-4.3.6>`_, a HTTP ``Host:``
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2 changes: 1 addition & 1 deletion Lib/http/client.py
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Expand Up @@ -1015,7 +1015,7 @@ def _tunnel(self):
headers = [connect]
for header, value in self._tunnel_headers.items():
header_bytes = header.encode("latin-1")
value_bytes = value.encode("latin-1")
value_bytes = str(value).encode("latin-1")
if not _is_legal_header_name(header_bytes):
raise ValueError('Invalid header name %r' % (header_bytes,))
if _is_illegal_header_value(value_bytes):
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17 changes: 17 additions & 0 deletions Lib/test/test_httplib.py
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Expand Up @@ -2793,6 +2793,23 @@ def _create_connection(address, timeout=None, source_address=None):
self.assertIsNotNone(exc)
self.assertTrue(sock.file_closed)

def test_tunnel_nonstr_headers(self):
class CustomObject:
def __str__(self):
return 'custom-str'

self.conn.set_tunnel('destination.com', headers={

@edvilme edvilme Oct 12, 2026 •
edited
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Suggested change
self.conn.set_tunnel('destination.com', headers={
self.conn.set_tunnel('example.com', headers={

If you are using an example domain maybe use that one instead (destination.com is a real non-example domain)?

@bielpieroli bielpieroli Oct 12, 2026

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Thanks for the suggestion, @edvilme! I used destination.com to stay consistent with the other tests in TunnelTests. The file also uses example.com, but in this class destination.com is the most common. So I'd lean towards keeping it, but I'm happy to switch to example.com if you think it reads better.

'None-Value': None,
'Integer-Value': 9,
'Custom-Value': CustomObject(),

@edvilme edvilme Oct 12, 2026

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You are converting header values and keys to strings. Maybe the tests can also include this to avoid future regressions?

@bielpieroli bielpieroli Oct 12, 2026

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Good idea, I think it makes sense.

However, I tried adding a non-string key to the test, but it fails earlier: set_tunnel() calls header.lower() on every key to check for a Host header, so non-string keys raise AttributeError there, before _tunnel() is reached.

That check predates this issue, so supporting non-string keys would probably be new behavior.

I think it'd make sense to update the docs to only mention values. Do you think that's the right approach?

@bielpieroli bielpieroli Oct 12, 2026

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ERROR: test_tunnel_nonstr_headers (test.test_httplib.TunnelTests.test_tunnel_nonstr_headers)
----------------------------------------------------------------------
Traceback (most recent call last):
  File ".../cpython/Lib/test/test_httplib.py", line 2801, in test_tunnel_nonstr_headers
    self.conn.set_tunnel('destination.com', headers={
    ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        'None-Value': None,
        ^^^^^^^^^^^^^^^^^^^
    ...<4 lines>...
        None: 'none-key',
        ^^^^^^^^^^^^^^^^^
    })
    ^^
  File ".../cpython/Lib/http/client.py", line 974, in set_tunnel
    if not any(header.lower() == "host" for header in self._tunnel_headers):
           ^^^
  File ".../cpython/Lib/http/client.py", line 974, in <genexpr>
    if not any(header.lower() == "host" for header in self._tunnel_headers):
               ^^^^^^^^^^^^
AttributeError: 'int' object has no attribute 'lower'

----------------------------------------------------------------------

@ZeroIntensity ZeroIntensity Oct 12, 2026

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Yes, let's just limit the PR to values only if keys didn't work anyway.

@bielpieroli bielpieroli Oct 12, 2026 •
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Thanks for the review, @ZeroIntensity! I reverted str() on keys (since non-string keys already fail earlier in set_tunnel() at the header.lower() check) and updated the docs to mention only values.

-            header_bytes = str(header).encode("latin-1")
+            header_bytes = header.encode("latin-1")
             value_bytes = str(value).encode("latin-1")

And thanks a lot, @edvilme, for the discussion here and in the issue. Your review comments helped us find this gap in the set_tunnel() key check, which we wouldn't have noticed otherwise :D

@edvilme edvilme Oct 12, 2026

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Thank YOU, glad you found it :)

})

self.conn.request('HEAD', '/', '')

self.assertIn(b'None-Value: None\r\n', self.conn.sock.data)
self.assertIn(b'Integer-Value: 9\r\n', self.conn.sock.data)
self.assertIn(b'Custom-Value: custom-str\r\n', self.conn.sock.data)


if __name__ == '__main__':
unittest.main(verbosity=2)
4 changes: 4 additions & 0 deletions Misc/NEWS.d/next/Library/2026-10-12-00-00-00.gh-issue-159044.Tr8kXv.rst
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@@ -0,0 +1,4 @@
Restore the previous behaviour of :meth:`http.client.HTTPConnection.set_tunnel`
where header values that are not strings (such as :const:`None`, :class:`int`
or other objects) are converted with :class:`str` instead of raising
:exc:`AttributeError` when the request is sent.
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