wsgiref.headers.Headers.__setitem__ deletes the old header before validating the new value

[Tony123-tech]

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.

Linked PRs

• gh-158225: Fix wsgiref Headers __setitem__ validation order #158226