Absolultely confused...

Tim Peters tim.peters at gmail.com
Thu Oct 6 18:09:47 CEST 2005

[Jeremy Moles]
> ...
> I may be missing something critical here, but I don't exactly grok what
> you're saying; how is it even possible to have two instances of
> PyType_vector3d? It is (like all the examples show and all the extension
> modules I've done in the past) a static structure declared and assigned
> to all at once, only once.

The most common cause is inconsistent import statements, with one
place using package-relative import but another place explicitly
specifying the package.  Here's a simple pure Python example, with
this directory structure:


__init__.py is empty; it just serves to make `package_dir` a package.

source.py defines a single type, named Type:

class Type(object):

module.py imports source.Type in two different ways, then prints various stuff:

from source import Type as one_way # package-relative import
from package_dir.source import Type as another_way

print one_way is another_way
print one_way.__name__, another_way.__name__
print repr(one_way), repr(another_way)

import sys
for k, v in sys.modules.items():
     if "source" in k:
         print k, v

Now _from_ playground, run module.py:

    python playground/module.py

This is the output, with annotations; I ran this on Windows, so expect
to see backslashes <wink>:


That is, one_way is not the same object as another_way:  there are two
distinct instances of the Type object.

    Type Type

Although they're distinct, they have the same __name__, "Type".

    <class 'source.Type'> <class 'package_dir.source.Type'>

Their reprs differ, though, showing the path via which they were imported.

    source <module 'source' from 'C:\playground\package_dir\source.pyc'>
    package_dir.source <module 'package_dir.source'
                                    from 'C:\playground\package_dir\source.pyc'>

That's two lines of output from crawling over sys.modules:  there are
two distinct instances of the entire `source` module.  That's the real
cause of the multiple `Type` instances.

package-relative import is rarely a good idea.  Are you doing that anywhere?

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