Flask hangs when sending a post request to itself

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Prior to 1.0, Flask’s development server was single-threaded by default. In that mode, it can only handle one request at a time. Making a request blocks until it receives the response. Your Flask code makes a request in the one thread, and then waits. There are no other threads to handle this second request. So the request never completes, and the original request waits forever.

Enable threads in the dev server to avoid the deadlock and fix the immediate problem.

app.run(threaded=True)

However, making a full HTTP request to the app from within the app should never be necessary and indicates a deeper design issue. For example, observe that the internal request will not have access to the session on the client’s browser. Extract the common code and call it internally, rather than making a new request.

def common_login(data):
    ...

@app.route("/login")
def login():
    ...
    common_login(data)
    ...

@app.route("/api/login")
def api_login():
    ...
    common_login(data)
    ...

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