[Python-Dev] serious bug in new import X as Y code (original) (raw)

Thomas Wouters thomas@xs4all.net
Sun, 20 Aug 2000 15:51:17 +0200

On Sun, Aug 20, 2000 at 01:26:05PM +0200, Sjoerd Mullender wrote:

Here's another pretty serious bug. Can you verify that this time it isn't my configurations?

It isn't your config, this is a genuine bug. I'll be checking in a quick fix in a few minutes, and start thinking about a test case that would've caught this.

Try this: from encodings import cp1006, cp1026

I get the error ImportError: cannot import name cp1026 but if I try to import the two modules separately I get no error.

Yes. 'find_from_args' wasn't trying hard enough to find out what the argument to an import were. Previously, all it had to do was scan the bytecodes immediately following an 'IMPORT_NAME' for IMPORT_FROM statements, and record its names. However, now that IMPORT_FROM generates a STORE, it stops looking after the first IMPORT_FROM. This worked fine for normal object-retrieval imports, which don't use the list generated by find_from_args, but not for dynamic loading tricks such as 'encodings' uses.

The fix I made was to unconditionally jump over 5 bytes, after an IMPORT_FROM, rather than 2 (2 for the oparg, 1 for the next instruction (a STORE) and two more for the oparg for the STORE)

This does present a problem for the proposed change in semantics for the 'as' clause, though. If we allow all expressions that yield valid l-values in import-as and from-import-as, we can't easily find out what the import arguments are by examining the future bytecode stream. (It might be possible, if we changed the POP_TOP to, say, END_IMPORT, that pops the module from the stack and can be used to end the search for import arguments.

However, I find this hackery a bit appalling :) Why are we constructing a list of import arguments at runtime, from compile-time information, if all that information is available at compile time too ? And more easily so ? What would break if I made IMPORT_NAME retrieve the from-arguments from a list, which is built on the stack by com_import_stmt ? Or is there a more convenient way of passing a variable list of strings to a bytecode ? It won't really affect performance, since find_from_args is called for all imports anyway.

-- Thomas Wouters <thomas@xs4all.net>

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