# wsgiref --- WSGI Utilities and Reference Implementation — wsgiref.headers -- WSGI response header tools

> synopsis: WSGI response header tools. This module provides a single class, Headers, for convenient manipulation of WSGI response headers using a mapping-like interface. Create a mapping-like object wrapping headers, which must be a list of header name/value tuples as described in 3333. The default v

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## Knowledge

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synopsis: WSGI response header tools.

This module provides a single class, Headers, for convenient manipulation of WSGI response headers using a mapping-like interface.

Create a mapping-like object wrapping headers, which must be a list of header name/value tuples as described in 3333. The default value of headers is an empty list.

Headers objects support typical mapping operations including ~object.getitem, ~dict.get, ~object.setitem, ~dict.setdefault, ~object.delitem and ~object.contains. For each of these methods, the key is the header name (treated case-insensitively), and the value is the first value associated with that header name. Setting a header deletes any existing values for that header, then adds a new value at the end of the wrapped header list. Headers' existing order is generally maintained, with new headers added to the end of the wrapped list.

Unlike a dictionary, Headers objects do not raise an error when you try to get or delete a key that isn't in the wrapped header list. Getting a nonexistent header just returns None, and deleting a nonexistent header does nothing.

Headers objects also support keys, values, and items methods. The lists returned by keys and items can include the same key more than once if there is a multi-valued header. The len() of a Headers object is the same as the length of its items, which is the same as the length of the wrapped header list. In fact, the items method just returns a copy of the wrapped header list.

Calling bytes() on a Headers object returns a formatted bytestring suitable for transmission as HTTP response headers. Each header is placed on a line with its value, separated by a colon and a space. Each line is terminated by a carriage return and line feed, and the bytestring is terminated with a blank line.

In addition to their mapping interface and formatting features, Headers objects also have the following methods for querying and adding multi-valued headers, and for adding headers with MIME parameters

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