**FastAPI** will make sure that the "exit code" in each dependency with `yield` is run in the correct order.

For example, `dependency_c` can have a dependency on `dependency_b`, and `dependency_b` on `dependency_a`:

{* ../../docs_src/dependencies/tutorial008_an_py39.py hl[6,14,22] *}

And all of them can use `yield`.

In this case `dependency_c`, to execute its exit code, needs the value from `dependency_b` (here named `dep_b`) to still be available.

		</dependency>
		<dependency>
			<groupId>org.apache.logging.log4j</groupId>
			<artifactId>log4j-api</artifactId>
			<version>${log4j.version}</version>
		</dependency>
		<dependency>
			<groupId>org.apache.logging.log4j</groupId>
			<artifactId>log4j-core</artifactId>
			<version>${log4j.version}</version>
		</dependency>
		<dependency>
			<groupId>org.apache.logging.log4j</groupId>

We can do better...

## What makes a dependency { #what-makes-a-dependency }

Up to now you have seen dependencies declared as functions.

But that's not the only way to declare dependencies (although it would probably be the more common).

The key factor is that a dependency should be a "callable".

A "**callable**" in Python is anything that Python can "call" like a function.

    </dependency>
    <dependency>
      <groupId>org.jline</groupId>
      <artifactId>jline-terminal-jni</artifactId>
    </dependency>
    <dependency>
      <groupId>org.jline</groupId>
      <artifactId>jline-terminal-ffm</artifactId>
    </dependency>

    <!-- (legacy) DI annotations -->
    <dependency>
      <groupId>javax.inject</groupId>
      <artifactId>javax.inject</artifactId>
    </dependency>

///

## Dependencies errors and return values { #dependencies-errors-and-return-values }

You can use the same dependency *functions* you use normally.

### Dependency requirements { #dependency-requirements }

They can declare request requirements (like headers) or other sub-dependencies:

{* ../../docs_src/dependencies/tutorial006_an_py39.py hl[8,13] *}

This is quite simple (not very useful), but will help us focus on how the sub-dependencies work.

## Second dependency, "dependable" and "dependant" { #second-dependency-dependable-and-dependant }

Then you can create another dependency function (a "dependable") that at the same time declares a dependency of its own (so it is a "dependant" too):

{* ../../docs_src/dependencies/tutorial005_an_py310.py hl[13] *}

Let's focus on the parameters declared:

			<version>1.7.36</version>
		</dependency>
		<dependency>
			<groupId>org.slf4j</groupId>
			<artifactId>slf4j-reload4j</artifactId>
			<version>1.7.36</version>
			<scope>test</scope>
		</dependency>
		<dependency>
			<groupId>org.bouncycastle</groupId>
			<artifactId>bcprov-jdk18on</artifactId>
			<version>1.80</version>
		</dependency>
		<dependency>
			<groupId>org.junit.jupiter</groupId>

    <dependencies>
      <dependency>
        <groupId>com.google.guava</groupId>
        <artifactId>guava</artifactId>
        <version>${project.version}</version>
      </dependency>
      <dependency>
        <groupId>com.google.guava</groupId>
        <artifactId>guava-gwt</artifactId>
        <version>${project.version}</version>
      </dependency>
      <dependency>
        <groupId>com.google.guava</groupId>

It is designed to be very simple to use, and to make it very easy for any developer to integrate other components with **FastAPI**.

## What is "Dependency Injection" { #what-is-dependency-injection }

And then **FastAPI** will call that override instead of the original dependency.

{* ../../docs_src/dependency_testing/tutorial001_an_py310.py hl[26:27,30] *}

/// tip

You can set a dependency override for a dependency used anywhere in your **FastAPI** application.