Unable to import python module from same package

Question

I'm currently writing a library in python. I have a package called Selectors as a sub-directory of the library. I am trying to implement a new module in the package, but when I try to import it I get the error:

NameError: name '_RaceSelector__ResultSelector' is not defined

My directory looks like this:

Selectors
│ ├── __init__.py
│ ├── __init__.pyc
│ ├── __pycache__
│ │ ├── SeasonSelector.cpython-38.pyc
│ │ ├── Selector.cpython-38.pyc
│ │ ├── __init__.cpython-38.pyc
│ │ ├── race_selector.cpython-38.pyc
│ │ ├── result_selector.cpython-38.pyc
│ │ └── season_selector.cpython-38.pyc
│ ├── race_selector.py
│ ├── race_selector.pyc
│ ├── result_selector.py
│ ├── result_selector.pyc
│ ├── season_selector.py
│ ├── season_selector.pyc
│ ├── selector.py
│ └── selector.pyc

I want to use the modules in race_selector.py, here is that file:

from .selector import __Selector
from .result_selector import __ResultSelector
class RaceSelector(__Selector):
 data = []
 loaded_races = []
 header = []
 result_selector = __ResultSelector()

selector.py

import os
import csv
class __Selector:
 def __init__(self, file_name):
 self.data_path = os.path.join(os.path.dirname(os.path.abspath(__file__)), '../data/' + file_name + '.csv')
 self.raw_data = self.load_data()
 self.data = self.get_data()

result_selector.py

import os
from .selector import __Selector
class __ResultSelector(__Selector):
 def __init__(self):
 super().__init__('results')

I am able to import selector just fine and works as intended, but result_selector produces the error.

Thanks

• Dschoni: Are you sure that the error comes from result_selector.py? Because it mentions _RaceSelector, which I can't spot in your code.
• Dschoni: Plus, is there any reason to not have the different selectors in one common file?

Answer 1

When you do the following:

result_selector = __ResultSelector()

Python searches for _RaceSelector__ResultSelector because there is 2 underscores.

As mentioned in PEP8:

If your class is intended to be subclassed, and you have attributes that you do not want subclasses to use, consider naming them with double leading underscores and no trailing underscores. This invokes Python's name mangling algorithm, where the name of the class is mangled into the attribute name. This helps avoid attribute name collisions should subclasses inadvertently contain attributes with the same name.

• Dschoni: Wow. Didn't know that.
• ipro_ultra: Neither did I. Very useful to know. Thank you!