Unpickling python objects with a changed module path

Question

I'm trying to integrate a project `Project A` built by a colleague into another python project. Now this colleague has not used relative imports in his code but instead done ``` from packageA.moduleA import ClassA from packageA.moduleA import ClassB ``` and consequently pickled the classes with `cPickle`. For neatness I'd like to hide the package that his (`Project A`) built inside my project. This however changes the path of the classes defined in `packageA`. No problem, I'll just redefine the import using ``` from ..packageA.moduleA import ClassA from ..packageA.moduleA import ClassB ``` but now the un pickling the classes fails with the following message ``` with open(fname) as infile: self.clzA = cPickle.load(infile) ImportError: No module named packageA.moduleA ``` So why doesn't `cPickle` apparently see the module defs. Do I need to add the root of `packageA` to system path? Is this the correct way to solve the problem? The `cPickled` file looks something like ``` ccopy_reg _rec…

Accepted Answer

You'll need to create an alias for the pickle import to work; the following to the `__init__.py` file of the `WrapperPackage` package:

```
from .packageA import * # Ensures that all the modules have been loaded in their new locations *first*.
from . import packageA  # imports WrapperPackage/packageA
import sys
sys.modules['packageA'] = packageA  # creates a packageA entry in sys.modules
```

It may be that you'll need to create additional entries though:

```
sys.modules['packageA.moduleA'] = moduleA
# etc.
```

Now cPickle will find `packageA.moduleA` and `packageA.moduleB` again at their old locations.

You may want to re-write the pickle file afterwards, the new module location will be used at that time. The additional aliases created above should ensure that the modules in question have the new location name for `cPickle` to pick up when writing the classes again.

Unpickling python objects with a changed module path

Solution

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