Update test_preparation.py

[Priyanka-43]

Added unit test.

For this assignment all pull requests going forward should contain the following (please delete the placeholder text and add your own description)

Title

• A ledgible title that briefly describes the contents of the pull request.

Linting

• Does you code lint?

Contents

• A ledgible description that indepthly describes the contents of the pull request.
• A list of files that are new / changed
• The contents that you have been assigned to submit labeled with titles that acuratly describe the assignment they are for

Labels and Milestones

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Reviewer

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This document can be changed at any time.