Perhaps you should make a table of some potential body object types, and figure out what the behaviour should be for request() with and without relevant headers, and endheaders() and send() with and without encode_chunked=True:

* Add/don’t add Content-Length/Transfer-Encoding
* Send with/without chunked encoding
* Raise exception
* Not supported or undefined behaviour

Potential body types:

* None with GET/POST request
* bytes()
* Latin-1/non Latin-1 str()
* BytesIO/StringIO
* Ordinary binary/Latin-1/other file object
* File object reading a special file like a pipe (st_size == 0)
* File object wrapping a pipe or similar that does not support fileno() (ESPIPE)
* Custom file object not supporting fileno() nor seeking
* File object at non-zero offset
* GzipFile object, where fileno() corresponds to the compressed size
* GzipFile not supporting fileno(), where seeking is possible but expensive
* Iterator yielding bytes() and/or strings
* Generator
* File object considered as an iterable of lines
* List/tuple of bytes/text
* Other sequences of bytes/text
* Other iterables of bytes/text, e.g. set(), OrderedDict.values()

This list could go on and on. I would rather have a couple of simple rules, or explicit APIs for the various modes so you don’t have to guess which mode your particular body type will trigger.