Python subprocess Popen stdin from file alter contents as a stream

Sure, I'd be happy to help with that! The subprocess.Popen class in Python is a powerful tool for spawning new processes, and it allows you to interact with them, including providing input via stdin. In this tutorial, I'll show you how to use subprocess.Popen to read input from a file, alter its contents as a stream, and then pass it as stdin to a child process.
Let's create a simple example where we read a file, convert its contents to uppercase, and then pass it as input to a subprocess. We'll use a Python script as the child process.
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