Read a repeated response header from a local message

By Batu · English technical notes

Some response headers can occur more than once, so retrieving only one field can discard information. This example parses a local, in-memory header block containing two Set-Cookie fields and one X-Request-Id field. message.get_all("Set-Cookie", []) returns both cookie field values in their header order. The assertions verify that neither cookie disappeared and that the ordinary request identifier remains available through item access.

HeaderParser is a convenient standard-library parser for a complete header block when no message body is needed. Its parsestr input ends with a blank line, which marks the end of headers. get_all returns the provided fallback when a field is absent; using [] gives callers a predictable collection in this example. These message APIs are available in Python 3.3 and later; the explicit policy.default used here is suitable for Python 3.6 and later.

This technique preserves header field strings; it does not implement HTTP response parsing, cookie storage, cookie expiration, or the browser rules for combining Set-Cookie values. In particular, do not join cookies and then treat the result as one Set-Cookie field in production. Apply an HTTP- and cookie-aware policy when the values affect requests or persistent state.

See the official email.parser documentation and email.message documentation. AI assistance disclosure: this article was drafted with AI assistance and should be checked against the application's HTTP handling requirements.

from email import policy
from email.parser import HeaderParser


raw_headers = (
    "Set-Cookie: session=abc; Path=/\n"
    "Set-Cookie: theme=dark; Path=/\n"
    "X-Request-Id: local-42\n"
    "\n"
)
message = HeaderParser(policy=policy.default).parsestr(raw_headers)
cookies = message.get_all("Set-Cookie", [])

assert cookies == ["session=abc; Path=/", "theme=dark; Path=/"]
assert message["X-Request-Id"] == "local-42"

print("cookies=" + " | ".join(cookies))
print("request_id=" + message["X-Request-Id"])

cookies=session=abc; Path=/ | theme=dark; Path=/
request_id=local-42