MultipartPart() and __str__()

[defnull]

MultipartPart() instances should be usable as strings but throw an exception if they are to big to fit into memory.

[wobsta]

MultipartPart is not aware of a mem_limit. It sure has a memfile_limit, but that's about it. (The disk_limit and mem_limit is enforced by MultipartParser.) MultipartPart has a value function (!), not a value property (anymore -- I changed this recently).