| Foreword for Experienced Programmers |
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| Thread-Locals in Flask |
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| One of the design decisions in Flask was that simple tasks should be simple; |
| they should not take a lot of code and yet they should not limit you. Because |
| of that, Flask has a few design choices that some people might find |
| surprising or unorthodox. For example, Flask uses thread-local objects |
| internally so that you don’t have to pass objects around from |
| function to function within a request in order to stay threadsafe. |
| This approach is convenient, but requires a valid |
| request context for dependency injection or when attempting to reuse code which |
| uses a value pegged to the request. The Flask project is honest about |
| thread-locals, does not hide them, and calls out in the code and documentation |
| where they are used. |
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| Develop for the Web with Caution |
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| Always keep security in mind when building web applications. |
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| If you write a web application, you are probably allowing users to register |
| and leave their data on your server. The users are entrusting you with data. |
| And even if you are the only user that might leave data in your application, |
| you still want that data to be stored securely. |
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| Unfortunately, there are many ways the security of a web application can be |
| compromised. Flask protects you against one of the most common security |
| problems of modern web applications: cross-site scripting (XSS). Unless you |
| deliberately mark insecure HTML as secure, Flask and the underlying Jinja2 |
| template engine have you covered. But there are many more ways to cause |
| security problems. |
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| The documentation will warn you about aspects of web development that require |
| attention to security. Some of these security concerns are far more complex |
| than one might think, and we all sometimes underestimate the likelihood that a |
| vulnerability will be exploited - until a clever attacker figures out a way to |
| exploit our applications. And don't think that your application is not |
| important enough to attract an attacker. Depending on the kind of attack, |
| chances are that automated bots are probing for ways to fill your database with |
| spam, links to malicious software, and the like. |
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| Flask is no different from any other framework in that you the developer must |
| build with caution, watching for exploits when building to your requirements. |
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