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wsgiref.headers.Headers.__setitem__ deletes the old header before validating the new value #158225
Description
In Lib/wsgiref/headers.py, Headers.setitem deletes the existing
header before validating the new name and value:
def __setitem__(self, name, val):
"""Set the value of a header."""
del self[name]
self._headers.append(
(self._convert_string_type(name, name=True),
self._convert_string_type(val, name=False)))
If _convert_string_type raises (for example when val is not a string),
the old header has already been removed and the new one is never added,
so the header disappears entirely.
Reproduction:
from wsgiref.headers import Headers
h = Headers([("Content-Type", "text/html")])
try:
h["Content-Type"] = 123
except AssertionError:
pass
print(h.get("Content-Type"))
# Expected: 'text/html'
# Actual: None
The fix is to validate the new name and value before deleting the old
header.
Activity
pythongh-158225: Fix wsgiref Headers __setitem__ validation order
pythongh-158225: Fix wsgiref Headers __setitem__ validation order
pythongh-158225: Fix wsgiref Headers __setitem__ validation order
encukou commentedon Sep 29, 2026
This behaves as documented:
I guess it can be changed for the better, but, does this have any real-world impact?
Tony123-tech commentedon Oct 4, 2026
I opened a PR (#158226) but don't have a real-world case for the impact, so I closed it. Leaving the issue open in case someone hits it.