Now that we know how to import our modules, we might want to restrict what is exposed. In this tutorial, we’ll look at how we can hide some of our module’s contents from being imported by other modules and scripts.

What Are Module Entities?

When we see module entities, we need to see variables, functions, and classes. A module entity is anything we provide with a name in our module. As we’ve seen, these things are importable by name when we used from <module> import <name>.

If we want to prevent someone from importing an entity from our module, there aren’t very many options. There are only two reasonable things we can do to restrict what is imported if someone uses from <module> import *.

The first is by setting the __all__ variable in our module. Let’s test this out by setting __all__ to a list including only extract_upper to see what happens in main.py.

__all__:

Let’s create a python module and write a few functions that can manipulate strings.

~/using_modules/helpers.py
__all__ = ["extract_upper"]

def extract_upper(phrase):
    return list(filter(str.isupper, phrase))

def extract_lower(phrase):
    return list(filter(str.islower, phrase))

if __name__ == "__main__":
    print("HELLO FROM HELPERS")

In main.py, we had been using both of these functions after loading them from helpers import *. Here’s another look at what main.py currently looks like.

~/using_modules/main.py
from helpers import *

print(f"Lowercase letters: {extract_lower('Chandra Shekhar')}")
print(f"Uppercase letters: {extract_upper('Chandra Shekhar')}")

With __all__ set in helpers, let’s run main.py to see what happens.

Terminal
Traceback (most recent call last):
  File "main.py", line 4, in 
    print(f"Lowercase letters: {extract_lower('Chandra Shekhar')}")
NameError: name 'extract_lower' is not defined

Although name exists within helpers.py, it is not available in other modules via helpers import *. This does not mean that we can’t explicitly import extract_lower though. Let’s modify main.py to import extract_lower by name.

~/using_modules/main.py
from helpers import *
from helpers import extract_lower

print(f"Lowercase letters: {extract_lower('Chandra Shekhar')}")
print(f"Uppercase letters: {extract_upper('Chandra Shekhar')}")

Let’s run this one more time.

Terminal
Lowercase letters: ['h', 'a', 'n', 'd', 'r', 'a', 'h', 'e', 'k', 'h', 'a', 'r']
Uppercase letters: ['C', 'S']

While it doesn’t allow us to prevent an entity from ever being imported, using __all__ does provide a way of sometimes restricting what is imported by modules and scripts consuming our modules and packages.

Using Underscored (_) Entities:

The other way we can prevent an entity from being exported automatically when someone uses from import * is by making the first character an underscore (_).

If we removed __all__ from helpers.py and created a variable called _hidden_var = “test”, we would not have access to _hidden_var after running from helpers import *.

References:

Happy Learning 🙂

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