Скачать или смотреть Understanding Composition in Python: Importing Classes from Modules

Learn how to effectively import classes using composition in Python. This guide breaks down the process step-by-step to enhance your understanding.
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Understanding Composition in Python: Importing Classes from Modules

When working with Python, it's common to create classes that are composed of other classes, popularly known as composition. This design pattern allows for building complex types using simpler ones, promoting code reuse and better organization. However, a common question arises: How do you import such classes from a module? This guide aims to clarify that process by diving into an example and its intricacies.

The Problem: Importing Classes in Python

Let's start with a practical example of two classes defined in a module named example.py:

[[See Video to Reveal this Text or Code Snippet]]

In this example, the Company class contains an instance of the Employee class, establishing a strong relationship between the two. The question arises when we want to instantiate Company in another file called work.py. The code for work.py looks like this:

[[See Video to Reveal this Text or Code Snippet]]

At this point, you might wonder: How does Python recognize the Employee class within work.py when we only imported Company?

The Solution: How Python Handles Class Imports

Understanding Global Scope

Imports Create Global Variables: When we run from example import Company, Python creates a new global variable in work.py which refers to example.Company.

Accessing Nested Classes: Inside the Company class's initialization method (__init__), Python retains a reference to the Employee class directly from its own global scope. Thus, Company can create an instance of Employee even though it's not explicitly imported in work.py.

The Global Reference

To further clarify, you can verify that the Employee class remains accessible from within the global scope of the Company class like this:

[[See Video to Reveal this Text or Code Snippet]]

This command shows that the _globals_ attribute of the Company.__init__ method indeed contains the Employee class, enabling Company to use it without needing a direct import in work.py.

Is It Safer to Import Employee Explicitly?

While Python allows you to rely on this behavior, some programmers might feel that importing Employee directly in work.py makes the code safer and clearer. Here’s how it would look:

[[See Video to Reveal this Text or Code Snippet]]

Although this does not change the functionality, adding Employee explicitly can help improve code readability. It makes it clear to future developers that Employee is part of the structure of Company, even if it's not directly instantiated in work.py.

Conclusion

In Python, when using composition, the class references remain intact in their respective modules. While you don’t need to import every class your main class depends on, doing so can improve clarity. Understanding how Python manages scopes and globals can streamline your programming approach, making your code more manageable and understandable in the long run.

Feel free to experiment with these concepts in your own projects, and watch how they enhance your coding practice!