Q: I watched a simple tutorial on Flask, but there is a question I don’t understand. Will the global variables of the app in Flask not go wrong when multiple clients access it?
For example, will the tasks list in the code below not go wrong when multiple clients access it at the same time? When I used two clients to perform the deletion operation, I found no problem, but will there be any problems when the concurrency is higher? I don't understand, and I'd like to ask for an explanation.

PS: The following code is in python3.0, you need to put
task = filter(lambda t: t['id'] == task_id, tasks)

if len(task) == 0:

Change to
task = filter(lambda t: t['id'] == task_id, tasks)
task = list(task)

if len(task) == 0:

#!flask/bin/python
from flask import Flask, jsonify, abort, request, make_response, url_for
from flask.ext.httpauth import HTTPBasicAuth

app = Flask(__name__, static_url_path = "")
auth = HTTPBasicAuth()

@auth.get_password
def get_password(username):
    if username == 'miguel':
        return 'python'
    return None

@auth.error_handler
def unauthorized():
    return make_response(jsonify( { 'error': 'Unauthorized access' } ), 403)
    # return 403 instead of 401 to prevent browsers from displaying the default auth dialog
    
@app.errorhandler(400)
def not_found(error):
    return make_response(jsonify( { 'error': 'Bad request' } ), 400)

@app.errorhandler(404)
def not_found(error):
    return make_response(jsonify( { 'error': 'Not found' } ), 404)

tasks = [
    {
        'id': 1,
        'title': u'Buy groceries',
        'description': u'Milk, Cheese, Pizza, Fruit, Tylenol', 
        'done': False
    },
    {
        'id': 2,
        'title': u'Learn Python',
        'description': u'Need to find a good Python tutorial on the web', 
        'done': False
    }
]

def make_public_task(task):
    new_task = {}
    for field in task:
        if field == 'id':
            new_task['uri'] = url_for('get_task', task_id = task['id'], _external = True)
        else:
            new_task[field] = task[field]
    return new_task
    
@app.route('/todo/api/v1.0/tasks', methods = ['GET'])
@auth.login_required
def get_tasks():
    return jsonify( { 'tasks': map(make_public_task, tasks) } )

@app.route('/todo/api/v1.0/tasks/<int:task_id>', methods = ['GET'])
@auth.login_required
def get_task(task_id):
    task = filter(lambda t: t['id'] == task_id, tasks)
    if len(task) == 0:
        abort(404)
    return jsonify( { 'task': make_public_task(task[0]) } )

@app.route('/todo/api/v1.0/tasks', methods = ['POST'])
@auth.login_required
def create_task():
    if not request.json or not 'title' in request.json:
        abort(400)
    task = {
        'id': tasks[-1]['id'] + 1,
        'title': request.json['title'],
        'description': request.json.get('description', ""),
        'done': False
    }
    tasks.append(task)
    return jsonify( { 'task': make_public_task(task) } ), 201

@app.route('/todo/api/v1.0/tasks/<int:task_id>', methods = ['PUT'])
@auth.login_required
def update_task(task_id):
    task = filter(lambda t: t['id'] == task_id, tasks)
    if len(task) == 0:
        abort(404)
    if not request.json:
        abort(400)
    if 'title' in request.json and type(request.json['title']) != unicode:
        abort(400)
    if 'description' in request.json and type(request.json['description']) is not unicode:
        abort(400)
    if 'done' in request.json and type(request.json['done']) is not bool:
        abort(400)
    task[0]['title'] = request.json.get('title', task[0]['title'])
    task[0]['description'] = request.json.get('description', task[0]['description'])
    task[0]['done'] = request.json.get('done', task[0]['done'])
    return jsonify( { 'task': make_public_task(task[0]) } )
    
@app.route('/todo/api/v1.0/tasks/<int:task_id>', methods = ['DELETE'])
@auth.login_required
def delete_task(task_id):
    task = filter(lambda t: t['id'] == task_id, tasks)
    if len(task) == 0:
        abort(404)
    tasks.remove(task[0])
    return jsonify( { 'result': True } )
    
if __name__ == '__main__':
    app.run(debug = True)

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