python3: why writing to socket require bytes type while writing to a file require str ?

[Frank Millman]

"Yubin Ruan"  wrote in message 
news:47f3acf9-8da2-4aad-a6f0-7a9efbdfe4c3 at googlegroups.com...

>   In my understanding, writing to a file would requires that everything be 
> written byte by byte. So writing objects of type 'bytes' to socket makes 
> sense. But, how could python write to file using unicode(type 'str')? Does 
> python encode 'str'(Unicode) automatically before writing things to file? 
> If it's true, then why can't it do that automatic encoding when I trying 
> to write a 'str' to socket ?
>

If you open a file for writing in text mode, using 'w', it expects a string.

If you open it in binary mode, using 'wb', it expects bytes.

HTH

Frank Millman