Importing package modules from C-extension

Mark English Mark.English at liffe.com
Thu Mar 17 15:20:23 CET 2005


Basic problem:
If there is a C-extension module in a package and it tries to import
another python module in the same package without using the fully
qualified path, the import fails.

Config:
Python 2.4 on Windows 2000

For example:
mypackage contains:
  cextension_example.pyd
  to_be_imported.py
  does_an_import.py

Where:
  "mypackage" is a directory with an empty "__init__.py" file
  "cextension_example.pyd" is a c-extension module
  "to_be_imported.py" is a python module used by other python modules
  "does_an_import.py" is a python module which imports
"to_be_imported.py".

In other words "does_an_import.py" looks like this:
---Start code---
import to_be_imported
print to_be_imported
---End code---

and "to_be_imported.py" looks like this:
---Start code---
print 'Importing:', __file__
---End code---

The c-extension "cextension_example.pyd" contains a line in its
initialisation routine:
---Start code---
PyObject *pToBeImported = PyImport_Import("to_be_imported");
---End code---
Which returns NULL. I'd rather it imported the module from the local
package.

If at a python prompt I do this:
>>> import mypackage.does_an_import
it works fine as I would expect from reading the documentation.
It starts by looking in the package where it is currently placed before
searching the PYTHONPATH.

If at a python prompt I do this:
>>> import mypackage.cextension_example
it fails to import "to_be_imported.py" which I do not expect.
It does not look in the package where it is currently placed.

In the past I have used the solution of a fully qualified path:
E.g.
---Start code---
PyObject *pToBeImported = PyImport_Import("mypackage.to_be_imported");
---End code---

However this time my application is building library paths at runtime
and doesn't know exactly what's going to be where. I can guarantee the
directory structure will be like that above.

So, how do I get the behaviour of "import", rather than the behaviour of
what looks more like "__import__" ?

Guesswork and speculation:
>From looking at the python source, it seems that in my C extension I
need access to the module's dictionary (passed as globals and locals
parameters to import functions), so that the parent package's __name__
entry can be used later on.

Leading to more questions:
If this is the case, how do I get at the current module and its
dictionary from a c-extension function that takes no parameters ? Do I
have to stash it away internally in the C-code on initialisation, or is
there some thread local object I can get it from (either directly, or
from the current frame) ? Is it good programming practice to use
"PyThreadState_Get()->frame" ?

Thanks for any help,
Mark



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