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90 lines
4.9 KiB
Markdown
90 lines
4.9 KiB
Markdown
# Session with FastAPI Dependency
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Before we keep adding things, let's change a bit how we get the session for each request to simplify our life later.
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## Current Sessions
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Up to now, we have been creating a session in each *path operation*, in a `with` block.
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{* ./docs_src/tutorial/fastapi/delete/tutorial001_py310.py ln[48:55] hl[50] *}
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That's perfectly fine, but in many use cases we would want to use <a href="https://fastapi.tiangolo.com/tutorial/dependencies/" class="external-link" target="_blank">FastAPI Dependencies</a>, for example to **verify** that the client is **logged in** and get the **current user** before executing any other code in the *path operation*.
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These dependencies are also very useful during **testing**, because we can **easily replace them**, and then, for example, use a new database for our tests, or put some data before the tests, etc.
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So, let's refactor these sessions to use **FastAPI Dependencies**.
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## Create a **FastAPI** Dependency
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A **FastAPI** dependency is very simple, it's just a function that returns a value.
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It could use `yield` instead of `return`, and in that case **FastAPI** will make sure it executes all the code **after** the `yield`, once it is done with the request.
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{* ./docs_src/tutorial/fastapi/session_with_dependency/tutorial001_py310.py ln[40:42] hl[40:42] *}
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## Use the Dependency
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Now let's make FastAPI execute a dependency and get its value in the *path operation*.
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We import `Depends()` from `fastapi`. Then we use it in the *path operation function* in a **parameter**, the same way we declared parameters to get JSON bodies, path parameters, etc.
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{* ./docs_src/tutorial/fastapi/session_with_dependency/tutorial001_py310.py ln[1:2,40:42,53:59] hl[1,54] *}
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/// tip
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Here's a tip about that `*,` thing in the parameters.
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Here we are passing the parameter `session` that has a "default value" of `Depends(get_session)` before the parameter `hero`, that doesn't have any default value.
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Python would normally complain about that, but we can use the initial "parameter" `*,` to mark all the rest of the parameters as "keyword only", which solves the problem.
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You can read more about it in the FastAPI documentation <a href="https://fastapi.tiangolo.com/tutorial/path-params-numeric-validations/#order-the-parameters-as-you-need-tricks" class="external-link" target="_blank">Path Parameters and Numeric Validations - Order the parameters as you need, tricks</a>
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///
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The value of a dependency will **only be used for one request**, FastAPI will call it right before calling your code and will give you the value from that dependency.
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If it had `yield`, then it will continue the rest of the execution once you are done sending the response. In the case of the **session**, it will finish the cleanup code from the `with` block, closing the session, etc.
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Then FastAPI will call it again for the **next request**.
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Because it is called **once per request**, we will still get a **single session per request** as we should, so we are still fine with that. ✅
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And because dependencies can use `yield`, FastAPI will make sure to run the code **after** the `yield` once it is done, including all the **cleanup code** at the end of the `with` block. So we are also fine with that. ✅
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## The `with` Block
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This means that in the main code of the *path operation function*, it will work equivalently to the previous version with the explicit `with` block.
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{* ./docs_src/tutorial/fastapi/session_with_dependency/tutorial001_py310.py ln[1:2,40:42,53:59] hl[55:59] *}
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In fact, you could think that all that block of code inside of the `create_hero()` function is still inside a `with` block for the **session**, because this is more or less what's happening behind the scenes.
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But now, the `with` block is not explicitly in the function, but in the dependency above:
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{* ./docs_src/tutorial/fastapi/session_with_dependency/tutorial001_py310.py ln[1:2,40:42,53:59] hl[41:42] *}
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We will see how this is very useful when testing the code later. ✅
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## Update the Path Operations to Use the Dependency
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Now we can update the rest of the *path operations* to use the new dependency.
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We just declare the dependency in the parameters of the function, with:
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```Python
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session: Session = Depends(get_session)
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```
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And then we remove the previous `with` block with the old **session**.
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{* ./docs_src/tutorial/fastapi/session_with_dependency/tutorial001_py310.py ln[1:2,40:42,53:104] hl[54,65,74,83,98] *}
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## Recap
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You just learned how to use **FastAPI dependencies** to handle the database session. This will come in handy later when testing the code.
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And you will see how much these dependencies can help the more you work with FastAPI, to handle **permissions**, **authentication**, resources like database **sessions**, etc. 🚀
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If you want to learn more about dependencies, checkout the <a href="https://fastapi.tiangolo.com/tutorial/dependencies/" class="external-link" target="_blank">FastAPI docs about Dependencies</a>.
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