# Code Structure and Multiple Files
Let's stop for a second to think about how to structure the code, particularly in **large projects** with multiple files.
## Circular Imports
The class `Hero` has a reference to the class `Team` internally.
But the class `Team` also has a reference to the class `Hero`.
So, if those two classes were in separate files and you tried to import the classes in each other's file directly, it would result in a **circular import**. π
And Python will not be able to handle it and will throw an error. π¨
But we actually want to *mean* that **circular reference**, because in our code, we would be able to do crazy things like:
```Python
hero.team.heroes[0].team.heroes[1].team.heroes[2].name
```
And that circular reference is what we are expressing with these **relationship attributes**, that:
* A hero can have a team
* That team can have a list of heroes
* Each of those heroes can have a team
* ...and so on.
Let's see different strategies to **structure the code** accounting for this.
## Single Module for Models
This is the simplest way. β¨
In this solution we are still using **multiple files**, for the `models`, for the `database`, and for the `app`.
And we could have any **other files** necessary.
But in this first case, all the models would live in a **single file**.
The file structure of the project could be:
```
.
βββ project
βββ __init__.py
βββ app.py
βββ database.py
βββ models.py
```
We have 3 **Python modules** (or files):
* `app`
* `database`
* `models`
And we also have an empty `__init__.py` file to make this project a "**Python package**" (a collection of Python modules). This way we can use **relative imports** in the `app.py` file/module, like:
```Python
from .models import Hero, Team
from .database import engine
```
We can use these relative imports because, for example, in the file `app.py` (the `app` module) Python knows that it is **part of our Python package** because it is in the same directory as the file `__init__.py`. And all the Python files on the same directory are part of the same Python package too.
### Models File
You could put all the database Models in a single Python module (a single Python file), for example `models.py`:
```Python
{!./docs_src/tutorial/code_structure/tutorial001/models.py!}
```
This way, you wouldn't have to deal with circular imports for other models.
And then you could import the models from this file/module in any other file/module in your application.
### Database File
Then you could put the code creating the **engine** and the function to create all the tables (if you are not using migrations) in another file `database.py`:
```Python
{!./docs_src/tutorial/code_structure/tutorial001/database.py!}
```
This file would also be imported by your application code, to use the shared **engine** and to get and call the function `create_db_and_tables()`.
### Application File
Finally, you could put the code to create the **app** in another file `app.py`:
```Python hl_lines="3-4"
{!./docs_src/tutorial/code_structure/tutorial001/app.py!}
```
Here we import the models, the engine, and the function to create all the tables and then we can use them all internally.
### Order Matters
Remember that [Order Matters](create-db-and-table.md#sqlmodel-metadata-order-matters){.internal-link target=_blank} when calling `SQLModel.metadata.create_all()`?
The point of that section in the docs is that you have to import the module that has the models **before** calling `SQLModel.metadata.create_all()`.
We are doing that here, we import the models in `app.py` and **after** that we create the database and tables, so we are fine and everything works correctly. π
### Run It in the Command Line
Because now this is a larger project with a **Python package** and not a single Python file, we **cannot** call it just passing a single file name as we did before with:
```console
$ python app.py
```
Now we have to tell Python that we want it to execute a *module* that is part of a package:
```console
$ python -m project.app
```
The `-m` is to tell Python to call a *module*. And the next thing we pass is a string with `project.app`, that is the same format we would use in an **import**:
```Python
import project.app
```
Then Python will execute that module *inside* of that package, and because Python is executing it directly, the same trick with the **main block** that we have in `app.py` will still work:
```Python
if __name__ == '__main__':
main()
```
So, the output would be:
```console
$ python -m project.app
Created hero: id=1 secret_name='Dive Wilson' team_id=1 name='Deadpond' age=None
Hero's team: name='Z-Force' headquarters='Sister Margaretβs Bar' id=1
```
## Make Circular Imports Work
Let's say that for some reason you hate the idea of having all the database models together in a single file, and you really want to have **separate files** a `hero_model.py` file and a `team_model.py` file.
You can also do it. π There's a couple of things to keep in mind. π€
!!! warning
This is a bit more advanced.
If the solution above already worked for you, that might be enough for you, and you can continue in the next chapter. π€
Let's assume that now the file structure is:
```
.
βββ project
βββ __init__.py
βββ app.py
βββ database.py
βββ hero_model.py
βββ team_model.py
```
### Circular Imports and Type Annotations
The problem with circular imports is that Python can't resolve them at *runtime*.
But when using Python **type annotations** it's very common to need to declare the type of some variables with classes imported from other files.
And the files with those classes might **also need to import** more things from the first files.
And this ends up *requiring* the same **circular imports** that are not supported in Python at *runtime*.
### Type Annotations and Runtime
But these **type annotations** we want to declare are not needed at *runtime*.
In fact, remember that we used `List["Hero"]`, with a `"Hero"` in a string?
For Python, at runtime, that is **just a string**.
So, if we could add the type annotations we need using the **string versions**, Python wouldn't have a problem.
But if we just put strings in the type annotations, without importing anything, the editor wouldn't know what we mean, and wouldn't be able to help us with **autocompletion** and **inline errors**.
So, if there was a way to "import" some things that act as "imported" only while editing the code but not at *runtime*, that would solve it... And it exists! Exactly that. π
### Import Only While Editing with `TYPE_CHECKING`
To solve it, there's a special trick with a special variable `TYPE_CHECKING` in the `typing` module.
It has a value of `True` for editors and tools that analyze the code with the type annotations.
But when Python is executing, its value is `False`.
So, we can use it in an `if` block and import things inside the `if` block. And they will be "imported" only for editors, but not at runtime.
### Hero Model File
Using that trick of `TYPE_CHECKING` we can "import" the `Team` in `hero_model.py`:
```Python hl_lines="1 5-6 16"
{!./docs_src/tutorial/code_structure/tutorial002/hero_model.py!}
```
Have in mind that now we *have* to put the annotation of `Team` as a string: `"Team"`, so that Python doesn't have errors at runtime.
### Team Model File
We use the same trick in the `team_model.py` file:
```Python hl_lines="1 5-6 14"
{!./docs_src/tutorial/code_structure/tutorial002/team_model.py!}
```
Now we get editor support, autocompletion, inline errors, and **SQLModel** keeps working. π
### App File
Now, just for completeness, the `app.py` file would import the models from both modules:
```Python hl_lines="4-5 10 12-14"
{!./docs_src/tutorial/code_structure/tutorial002/app.py!}
```
And of course, all the tricks with `TYPE_CHECKING` and type annotations in strings are **only needed in the files with circular imports**.
As there are no circular imports with `app.py`, we can just use normal imports and use the classes as normally here.
And running that achieves the same result as before:
```console
$ python -m project.app
Created hero: id=1 age=None name='Deadpond' secret_name='Dive Wilson' team_id=1
Hero's team: id=1 name='Z-Force' headquarters='Sister Margaretβs Bar'
```
## Recap
For the **simplest cases** (for most of the cases) you can just keep all the models in a single file, and structure the rest of the application (including setting up the **engine**) in as many files as you want.
And for the **complex cases** that really need separating all the models in different files, you can use the `TYPE_CHECKING` to make it all work and still have the best developer experience with the best editor support. β¨