</p>

<pre>
math.Atan2(x, y)  // function call
var pt *Point
pt.Scale(3.5)     // method call with receiver pt
</pre>

<p>
In a function call, the function value and arguments are evaluated in
<a href="#Order_of_evaluation">the usual order</a>.
After they are evaluated, the parameters of the call are passed by value to the function
and the called function begins execution.

import static java.util.concurrent.TimeUnit.SECONDS;

import com.google.common.annotations.GwtCompatible;
import com.google.common.annotations.GwtIncompatible;
import com.google.common.base.Function;
import com.google.common.base.Functions;
import com.google.common.base.Joiner;
import com.google.common.base.Predicate;
import com.google.common.base.Predicates;
import com.google.common.collect.testing.IteratorFeature;

</p>

<p>
Each function also needs annotations giving the location of
live pointers in its arguments, results, and local stack frame.
For an assembly function with no pointer results and
either no local stack frame or no function calls,
the only requirement is to define a Go prototype for the function
in a Go source file in the same package. The name of the assembly

In this case, the task function will write to a file (simulating sending an email).

And as the write operation doesn't use `async` and `await`, we define the function with normal `def`:

{* ../../docs_src/background_tasks/tutorial001.py hl[6:9] *}

## Add the background task

    return {"fresh_value": fresh_value}
```

////

## Recap

Apart from all the fancy words used here, the **Dependency Injection** system is quite simple.

Just functions that look the same as the *path operation functions*.

But still, it is very powerful, and allows you to declare arbitrarily deeply nested dependency "graphs" (trees).

/// tip

All this might not seem as useful with these simple examples.

# Dependencies in path operation decorators

In some cases you don't really need the return value of a dependency inside your *path operation function*.

Or the dependency doesn't return a value.

But you still need it to be executed/solved.

For those cases, instead of declaring a *path operation function* parameter with `Depends`, you can add a `list` of `dependencies` to the *path operation decorator*.

// aggregation.
//
// Aggregation functions - An expression that involves aggregation of
// rows in some manner. Requires all input rows to be processed,
// before a result is returned.
//
// Row function - An expression that depends on a value in the
// row. They have an output for each input row.
//
// Some types of a queries are not valid. For example, an aggregation

```

////

But then we get a `dict` in the parameter `commons` of the *path operation function*.

And we know that editors can't provide a lot of support (like completion) for `dict`s, because they can't know their keys and value types.

We can do better...

## What makes a dependency

Up to now you have seen dependencies declared as functions.

# Get Current User

In the previous chapter the security system (which is based on the dependency injection system) was giving the *path operation function* a `token` as a `str`:

//// tab | Python 3.9+

```Python hl_lines="12"
{!> ../../docs_src/security/tutorial001_an_py39.py!}
```

////

//// tab | Python 3.8+

```Python hl_lines="11"
{!> ../../docs_src/security/tutorial001_an.py!}
```

////

   * exception (typically, but not necessarily, {@link IllegalArgumentException}).
   *
   * <p>The returned converter is serializable if both provided functions are.
   *
   * @since 17.0
   */
  public static <A, B> Converter<A, B> from(
      Function<? super A, ? extends B> forwardFunction,
      Function<? super B, ? extends A> backwardFunction) {