I have subtle import issue that creates variables with references to objects different to what I expect, and it all depends on whether the module I’m writing is imported “as is” or as part of a package.

Let’s say I have a simple module that defines a Config class and a function to set it up:

class Config:
    def __init__(self, **kwargs):
        if kwargs.get("print_keys", None):
            self.print_keys = kwargs.pop("print_keys")
        else:
            self.print_keys = ["centre", "shortName"]


rcParams = Config()


def set_config(config=None):
    global rcParams
    print("Before change:", id(rcParams))
    if config is None:
        config = Config()
    rcParams = config
    print("After change:", id(rcParams))

If I save this in a module named simple.py, then I can successfully test it with this module:

# import gribtool as gt # note this line is uncommented later
import simple as gt     # note this line is commented later


def test_set_config():
    config = gt.Config(print_keys=["centre"])
    print("Initial rcParams id:", id(gt.rcParams))
    gt.set_config(config=config)
    print("Updated rcParams id:", id(gt.rcParams))
    assert gt.rcParams.print_keys == ["centre"]


test_set_config()

The output is:

Initial rcParams id: 128205347716160
Before change: 128205347716160
After change: 128205347716832
Updated rcParams id: 128205347716832

So far so good. Now, the problem is that I want this to be part of a package. I move this module to a package (a directory) named gribtool, and then I create the __init__.py file with the expected content:

from .simple import *

Then, I uncomment the line with import gribtool as gt in the test module above, and then it fails with this output:

Initial rcParams id: 134251413862864
Before change: 134251413862864
After change: 134251413864064
Updated rcParams id: 134251413862864
Traceback (most recent call last):
  File "/home/navarro/AEMET/GRIB_TOOL/test_config.py", line 13, in <module>
    test_set_config()
  File "/home/navarro/AEMET/GRIB_TOOL/test_config.py", line 10, in test_set_config
    assert gt.rcParams.print_keys == ["centre"]
AssertionError

As you can see, the problem is that the rcParams that is being modified within the set_config function is different from the one in the simple module despite the global instruction.

How can I make sure I’m referring to the right objects?