Solving AssertionError in Pytest: Proper Parameter Formatting Explained

Описание: Learn how to format input and output parameters correctly in Pytest to avoid `AssertionError`. This guide covers practical solutions with code examples.
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This video is based on the question https://stackoverflow.com/q/63713911/ asked by the user 'Slowat_Kela' ( https://stackoverflow.com/u/8407951/ ) and on the answer https://stackoverflow.com/a/63714345/ provided by the user 'Alex Solovyov' ( https://stackoverflow.com/u/8436060/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Solving AssertionError in Pytest: Proper Parameter Formatting Explained

When working with unit tests in Pytest, it's common to come across AssertionError messages, especially when you're dealing with lists and their ordering. In this guide, we'll explore a specific issue that arises from unordered data types and how to resolve it. We'll break down the problem using a practical example of an autocomplete feature implemented in Python.

The Problem: Understanding the AssertionError

Suppose you have the following code that implements three functions for autocompleting strings based on the start of given inputs against a list of names. However, when you run your tests, you encounter the following assertion errors:

Error Example:

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The challenge stems from the fact that the output lists may not match the expected order, leading to AssertionError during the tests.

Why the Error Occurs

The core issue lies in how Python's set data type works. Sets are unordered collections, which means the order of elements is not guaranteed. When you convert a list to a set and back again, the order may change, leading to failed assertions if your tests expect outputs in a specific order.

The Solution: Ensuring Consistent Output with Ordered Collections

To solve this problem effectively, you need to ensure that the output of your functions maintains a predictable order. Here are a couple of strategies to achieve that.

Method 1: Sort the Resulting List

One straightforward way to handle this issue is to sort your results after obtaining them from the set. For example:

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By applying sorted(), you ensure that the output will always be in the same order, making your test assertions reliable.

Method 2: Use OrderedDict from the collections Module

Another effective method is to utilize OrderedDict, which maintains the order of elements as they are added. Here is how it can be implemented:

Import OrderedDict:

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Modify the Autocomplete Functions:
Update your functions to use OrderedDict for maintaining the order of elements based on their first occurrence in the original list:

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Full Working Code

Here's how the complete working solution looks after implementing the discussed methods:

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Conclusion

In summary, when testing functions that return lists in Python, especially in the context of Pytest, it's crucial to handle unordered data types thoughtfully. You can either sort the results or use OrderedDict to maintain the desired order and avoid AssertionError. This ensures your tests run smoothly and accurately reflect expected outcomes.

By following these guidelines, you can streamline your testing process and improve the reliability of your code. Happy coding!