Скачать или смотреть Understanding Python's __str__ vs __repr__: Why Your print(obj) is Failing

Discover the reasons why Python calls `__str__` instead of `__repr__`, and learn how to fix printing issues with your custom classes.
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Understanding Python's _str_ vs __repr__: Why Your print(obj) is Failing

If you've ever run into a bug while trying to print your Python objects, you're not alone. A common issue arises from misunderstandings between the _str_ and _repr_ methods in Python classes. In this post, we'll explore why your code is failing when you attempt to print an instance of your custom class. We'll break down the problem step by step and provide clear solutions so you can avoid this pitfall in the future.

The Problem at Hand

You might encounter an error message similar to this when running your code:

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

This occurs when you're trying to print an object of a custom class where the _str_ or _repr_ methods are improperly defined. Specifically, if the methods attempt to concatenate strings and integers without converting the integers to strings first, you'll run into this error.

For instance, consider this code snippet:

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

The errors stem from mishandling the data types in your string formatting methods.

Understanding _str_ and _repr_

To resolve this issue, it's important to understand the purpose of these two special methods:

__str__: This method is supposed to return a user-friendly string representation of the object. It's called by the print() function and str() method.

__repr__: This method aims to return an "official" string representation of the object that can ideally be used to recreate the object. It's called by the repr() function and is useful for debugging.

Common Mistakes

The most critical mistake here is trying to concatenate an integer directly with a string. In Python, this results in a TypeError because you cannot add (concatenate) different types without explicit conversion.

The Solution

To fix this issue and make your printing work as expected, you can adjust the _str_ and _repr_ methods as follows:

Option 1: Explicit Conversion

You can explicitly convert integers to strings using the str() function:

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

Option 2: Using f-Strings

A more modern way to handle this in Python is to use f-strings (formatted string literals), which provide a cleaner syntax and handle type conversion automatically:

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

Using f-strings is generally recommended in Python 3.6 and later due to their readability and convenience.

Conclusion

Understanding the difference between _str_ and _repr_ and handling data types correctly is essential when creating custom classes in Python. By ensuring you convert integers to strings before concatenation, you can avoid the common pitfalls that lead to exceptions in your code.

Next time you're faced with a printing issue, remember these tips and check your method definitions. Happy coding!