[Python-Dev] Fix import errors to have data

[Delaney, Timothy C (Timothy)]

Delaney, Timothy C (Timothy) wrote:

> Perhaps the import machinery could keep a (weak?) mapping from module
> *object* to the ImportError it raised. If the module that would be
> returned by the import is in the mapping, raise the corresponding
> ImportError instead. The appropriate line, etc in the module would
> thus 
> be shown each time the module was imported.
> 
> I believe this would work properly for reload() as well.

Of course it won't ... (no name to reload). This would effectively
prevent ever loading that module in that session, even if it got fixed.

Instead, perhaps the following would be the right semantics:

    # import replacement

    import __builtin__
    import sys

    _builtin_import = __import__

    def __import__(name, *p):
        try:
            return _builtin_import(name, *p)
        except ImportError, e:
            module = sys.modules[name]

            for n, m in sys.modules.items():
                if m is module:
                    del sys.modules[n]

            raise e

    __builtin__.__import__ = __import__

    # a.py

    raise ImportError(__name__)

    # b.py

    try:
        import a
    except ImportError:
        import a

    print 'OK'

    >>> try:
    ...     import a
    ... except ImportError:
    ...     import a
    ...
    Traceback (most recent call last):
      File "<stdin>", line 4, in ?
      File "<stdin>", line 9, in __import__
    ImportError: a

Tim Delaney