Why reading a large text file in Flask does not occupy memory?

I write a simple Flask app to test its memory usage.

Here is the code.

from flask import Flask
app = Flask(__name__)

@app.route("/")
def hello():
        with open("file.txt", "r") as fd:
                text = fd.read()
        return "Hello World!"

if __name__ == "__main__":
        app.run(host="0.0.0.0")

file.txt is created by this command and its size is 50MB.

base64 /dev/urandom | head -c 50000000 > file.txt

Run it with this command:

python3 test.app

If running without reading file.txt, the memory usage is 18448KB.

If running and reading file.txt, the memory usage is 18988KB.

This file is 50MB, but the memory usage only increases 540KB. I don't understand how Python works.