[Python-Dev] Re: [Python-checkins] python/dist/src/Pythonmarshal.c, 1.79, 1.80

Tim Peters tim.peters at gmail.com
Tue Dec 21 23:21:29 CET 2004

[Armin Rigo]
> Some code in the 'py' lib used to use marshal to send simple objects between
> the main process and a subprocess.  We ran into trouble when we extended the
> idea to a subprocess that would actually run via ssh on a remote machine, and
> the remote machine's Python version didn't match the local one.  The obvious
> quick fix was to set the 'version' argument to 0 and pretend that it would be
> fine forever.

Yes, I believed you had *some* use for it <wink>.

> Now that we essentially can't use this trick any more because of the 2.4.0
> bug,

Well, you can still use 2.3.4 -- or wait for 2.4.1 -- or use your
patched 2.4.  Or use the stock 2.4, but set up a "marshal server"
running 2.3.4 <heh>.

> we reverted to repr/eval, which is quite slower (and actually not guaranteed to
> work across Python versions either: string escapes sometimes change).

Really?  The precise rules str's __repr__ uses for which escapes to
produce certainly change, but I don't recall any case outside
Unicodeland where a new string escape was ever introduced.  So, e.g.,
current Python str.__repr__() produces '\n' for a newline, while
long-ago Pythons produced '\012', but all versions of Python *accept*
either form of escape.  The biggest change of this kind was moving
from octal escapes to hex escapes in Python 2.1, but hex escapes have
always been accepted -- repr() just didn't produce them before 2.1.

>  We avoid to use cPickle because we want to be sure that only "simple
> enough" objects are sent over this way -- essentially nothing that depends on
> a precise Python module to be installed and identical on both machines.

It's possible (but irksome) to subclass pickle.py's Pickler class, and
override its save_global() and save_inst() methods.  For example,
replace them with one-liners that just raise an exception.  Then any
attempt to pickle an object requiring a specific module will raise
that exception.  But if you're worried about speed, going thru
pickle.py is significantly slower than going thru repr().

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