File read-write mode: problem appending after reading

[Tim Peters]

[Frederic Rentsch]
>       Thanks a lot for your input. I seemed to notice that  everything
> works fine without setting the cursor as long as it stops before the end
> of the file. Is that also a coincidence that may not work?

"if you want to read following a write, or write following a read, on
the same stream, you must perform a file-positioning operation
(typically a seek) between them"