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See http://tools.ietf.org/html/rfc6265#section-5.2.6
Relevant section:
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5.2.6. The HttpOnly Attribute
If the attribute-name case-insensitively matches the string HttpOnly", the user agent MUST append an attribute to the cookie-attribute-list with an attribute-name of HttpOnly and an empty attribute-value.
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If the cookie-attribute-list contains an attribute with an attribute-name of "HttpOnly", set the cookie's http-only-flag to true. Otherwise, set the cookie's http-only-flag to false.
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http.cookies creates this attribute as `httponly` not `HttpOnly`.
It is true, when interpreted by the user agent, this attribute is case insensitive, but it seems odd that Python would go out of its way to purposely use a different case then stated in the standard. When looking at other web technologies, the case used in the standard is most typical. The examples in the standard also use the `HttpOnly` style.
(Same applies to the Secure flag.)