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I would at least like the documentation modified to make it
clearer that certain characters are not permitted for
persistent ID's. I think the text which indicates the
requirement of printable ASCII characters is too subtle -
there should be something which makes the requirement more
obvious, the use of a "must" or "should" would help get the
point across (as would some text after the statement
indicating that characters such as '\b', '\n', '\r' are not
permitted).

Perhaps it would be an idea for save_pers to do some
argument checking before storing the persistent ID, perhaps
using an assertion statement to verify that the ID doesn't
contain non-permitted characters (or at least, checking for
the presence of a '\n' character embedded in the string).

I think it is preferable to have safeguards implemented in
Pickler to prevent potentially dodgy data being stored - I
would rather have an error raised when I'm trying to pickle
something than have the data stored and corrupted, only to
notice it when it is unpickled (when it is too late).

Confusingly, the code in save_pers in the pickle module
seems to indicate that it would happily accept non-String
based persistent ID's:

----
else:
 self.write(PERSID + str(pid) + '\n')
----

I don't understand why we are using the str function if we
are expecting pid to be a string in the first place. I would
rather that this method would raise an error if it tried to
perform string concatenation on something which isn't a string.

I agree with SMST, I would like the restriction removed over
what persistent ID's we can use, it seems somewhat arbitary
- there's no reason, for example, why we can't use any
simple data type which can be marshalled as an ID.

Apart from the reason that it wouldn't be backwardly
compatible, which is probably a good enough reason. :)