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Learn how to dynamically set an environment variable in pytest based on a shell command without using external parameters.
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How to Set an Environment Variable in pytest Using a Bash Expression

When writing tests using pytest, you might encounter scenarios where you need to set environment variables based on dynamic values derived from shell commands. This is particularly useful for configurations that depend on external data, such as timezone settings or file paths. If you've ever tried to assign a variable from a shell command directly in your pytest.ini file, you may have run into an issue where the command is evaluated literally instead of executing it. This guide will walk you through how to set environment variables dynamically in pytest without resorting to command-line manipulation.

The Challenge

You may have a configuration file like pytest.ini with a section that looks like this:

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

Your .test_env file might contain something like:

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

However, when running pytest, this command is not executed. Instead, it is taken literally, meaning your TZ variable is set to the string “ls -la /etc/localtime | cut -d/ -f8-9” rather than the result of that command, which is not what you want.

The Solution: Using a pytest Fixture

To dynamically set environment variables like TZ based on a bash expression within the pytest environment, you can use a session-scoped fixture in the conftest.py file. This approach allows you to execute shell commands and assign their outputs to environment variables during the test session.

Steps to Implement

Create a conftest.py File: This file is a special configuration file for pytest where you can define fixtures and hooks.

Define the Fixture:

Use the subprocess module in Python to run your shell command.

Capture the output and set it to the desired environment variable.

Here’s how you can define this fixture in your conftest.py:

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

Example Test Case

Next, in your test file (let's call it test_foo.py), you can simply access the environment variable TZ and use it as needed in your tests:

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

Running Your Tests

When you run your tests using the command:

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

You should see the dynamically set value being printed out successfully in the output. For example, it may print New York or whatever the output of your shell command is, confirming that the environment variable was set correctly.

Conclusion

Setting environment variables dynamically within pytest can be easily achieved by leveraging Python's subprocess and environment manipulation features. This solution allows you to maintain flexibility in your testing setup without relying on external command-line parameters, streamlining your test environments efficiently.

By following the steps outlined above, you can ensure your tests are configured with the necessary dynamic environment settings to reflect your application's requirements accurately. Happy testing!