Working with a global singleton in Flask (WSGI), do I have to worry about race conditions?

The hello world demo for Flask is:

from flask import Flask
app = Flask(__name__)

@app.route("/")
def hello():
    return "Hello World!"

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

What if I modified this like so:

from flask import Flask
app = Flask(__name__)

a = 1
b = 2
c = 3

@app.route("/")
def hello():
    a += 1
    b += a
    c += b
    return "Hello World!"

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

I understand WSGI application might have multiple threads. The hello function could be running on multiple threads at the same time, and then we'd have a race condition. Is this correct? If the above code is not thread safe, what can I do to make it thread safe?

Avoiding globals is a possible solution, but can you always avoid globals? What if I want something like a python object cache?