Most of the library should only work with strings. Being able to pass bytes to most functions is an artifact of only supporting Python 2, then also supporting 3, then dropping support for 2. WSGI on Python 3 only deals with ISO-8859-1 characters. Modern WHATWG HTML and URL standards require UTF-8. HTTP headers must be ISO-8859-1 (new headers should be ASCII), using quoting or other encoding schemes to first convert UTF-8 to valid characters before encoding to bytes.
As #2406 points out, we spend too much time doing instance checks and encoding. This is often redundant because we currently support bytes or strings being passed to any function, and one function calls others that do the same. Very few places should be allowing bytes and strings, we should be dealing with strings for most data, and bytes only where binary data makes sense.
Most of the library should only work with strings. Being able to pass bytes to most functions is an artifact of only supporting Python 2, then also supporting 3, then dropping support for 2. WSGI on Python 3 only deals with ISO-8859-1 characters. Modern WHATWG HTML and URL standards require UTF-8. HTTP headers must be ISO-8859-1 (new headers should be ASCII), using quoting or other encoding schemes to first convert UTF-8 to valid characters before encoding to bytes.
As #2406 points out, we spend too much time doing instance checks and encoding. This is often redundant because we currently support bytes or strings being passed to any function, and one function calls others that do the same. Very few places should be allowing bytes and strings, we should be dealing with strings for most data, and bytes only where binary data makes sense.