## 18.4.2 `wsgiref.headers` - WSGI response header tools

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

class `Headers`( | headers)

  Create a mapping-like object wrapping headers, which must be a list of header name/value tuples as described in PEP 333. Any changes made to the new `Headers` object will directly update the headers list it was created with.

  `Headers` objects support typical mapping operations including `__getitem__`, `get`, `__setitem__`, `setdefault`, `__delitem__`, `__contains__` and `has_key`. 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 `str()` on a `Headers` object returns a formatted string 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 string 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:

  `get_all`( | name)

    Return a list of all the values for the named header.

    The returned list will be sorted in the order they appeared in the original header list or were added to this instance, and may contain duplicates. Any fields deleted and re-inserted are always appended to the header list. If no fields exist with the given name, returns an empty list.

  `add_header`( | name, value, **_params)

    Add a (possibly multi-valued) header, with optional MIME parameters specified via keyword arguments.

    name is the header field to add. Keyword arguments can be used to set MIME parameters for the header field. Each parameter must be a string or `None`. Underscores in parameter names are converted to dashes, since dashes are illegal in Python identifiers, but many MIME parameter names include dashes. If the parameter value is a string, it is added to the header value parameters in the form `name="value"`. If it is `None`, only the parameter name is added. (This is used for MIME parameters without a value.) Example usage:

        h.add_header('content-disposition', 'attachment', filename='bud.gif')

    The above will add a header that looks like this:

        Content-Disposition: attachment; filename="bud.gif"