tiangolo.fastapi/docs/python-types.md

Python 3.6+ has support for optional "type hints".

These "type hints" are a new syntax (since Python 3.6+) that allow declaring the type of a variable.

By declaring types for your variables, editors and tools can give you better support.

This is just a quick tutorial / refresher about Python type hints. It covers only the minimum necessary to use them with FastAPI... which is actually very little.

FastAPI is all based on these type hints, they give it many advantages and benefits.

But even if you never use FastAPI, you would benefit from learning a bit about them.

!!! note If you are a Python expert, and you already know everything about type hints, skip to the next chapter.

Motivation

Let's start with a simple example:

{!./src/python_types/tutorial001.py!}

Calling this program outputs:

John Doe

The function does the following:

• Takes a first_name and last_name.
• Converts the first letter of each one to upper case with title().
• Concatenates them with a space in the middle.

{!./src/python_types/tutorial001.py!}

Edit it

It's a very simple program.

But now imagine that you were writing it from scratch.

At some point you would have started the definition of the function, you had the parameters ready...

But then you have to call "that method that converts the first letter to upper case".

Was it upper? Was it uppercase? first_uppercase? capitalize?

Then, you try with the old programmer's friend, editor autocompletion.

You type the first parameter of the function, first_name, then a dot (.) and then hit Ctrl+Space to trigger the completion.

But, sadly, you get nothing useful:

Add types

Let's modify a single line from the previous version.

We will change exactly this fragment, the parameters of the function, from:

    first_name, last_name

to:

    first_name: str, last_name: str

That's it.

Those are the "type hints":

{!./src/python_types/tutorial002.py!}

That is not the same as declaring default values like would be with:

    first_name="john", last_name="doe"

It's a different thing.

We are using colons (:), not equals (=).

And adding type hints normally doesn't change what happens from what would happen without them.

But now, imagine you are again in the middle of creating that function, but with type hints.

At the same point, you try to trigger the autocomplete with Ctrl+Space and you see:

With that, you can scroll, seeing the options, until you find the one that "rings a bell":

More motivation

Check this function, it already has type hints:

{!./src/python_types/tutorial003.py!}

Because the editor knows the types of the variables, you don't only get completion, you also get error checks:

Now you know that you have to fix it, convert age to a string with str(age):

{!./src/python_types/tutorial004.py!}

Declaring types

You just saw the main place to declare type hints. As function parameters.

This is also the main place you would use them with FastAPI.

Simple types

You can declare all the standard Python types, not only str.

You can use, for example:

• int
• float
• bool
• bytes

{!./src/python_types/tutorial005.py!}

Types with subtypes

There are some data structures that can contain other values, like dict, list, set and tuple. And the internal values can have their own type too.

To declare those types and the subtypes, you can use the standard Python module typing.

It exists specifically to support these type hints.

Lists

For example, let's define a variable to be a list of str.

From typing, import List (with a capital L):

{!./src/python_types/tutorial006.py!}

Declare the variable, with the same colon (:) syntax.

As the type, put the List.

As the list is a type that takes a "subtype", you put the subtype in square brackets:

{!./src/python_types/tutorial006.py!}

That means: "the variable items is a list, and each of the items in this list is a str".

By doing that, your editor can provide support even while processing items from the list.

Without types, that's almost impossible to achieve:

Notice that the variable item is one of the elements in the list items.

And still, the editor knows it is a str, and provides support for that.

Tuples and Sets

You would do the same to declare tuples and sets:

{!./src/python_types/tutorial007.py!}

This means:

• The variable items_t is a tuple, and each of its items is an int.
• The variable items_s is a set, and each of its items is of type bytes.

Dicts

To define a dict, you pass 2 subtypes, separated by commas.

The first subtype is for the keys of the dict.

The second subtype is for the values of the dict:

{!./src/python_types/tutorial008.py!}

This means:

• The variable prices is a dict:
  • The keys of this dict are of type str (let's say, the name of each item).
  • The values of this dict are of type float (let's say, the price of each item).

Classes as types

You can also declare a class as the type of a variable.

Let's say you have a class Person, with a name:

{!./src/python_types/tutorial009.py!}

Then you can declare a variable to be of type Person:

{!./src/python_types/tutorial009.py!}

And then, again, you get all the editor support:

Pydantic models

Pydantic is a Python library to perform data validation.

You declare the "shape" of the data as classes with attributes.

And each attribute has a type.

Then you create an instance of that class with some values and it will validate the values, convert them to the appropriate type (if that's the case) and give you an object with all the data.

And you get all the editor support with that resulting object.

Taken from the official Pydantic docs:

{!./src/python_types/tutorial010.py!}

!!! info To learn more about Pydantic, check its docs.

FastAPI is all based on Pydantic.

You will see a lot more of all this in practice in the Tutorial - User Guide (the next section).

Type hints in FastAPI

FastAPI takes advantage of these type hints to do several things.

With FastAPI you declare parameters with type hints and you get:

• Editor support.
• Type checks.

...and FastAPI uses the same declarations to:

• Define requirements: from request path parameters, query parameters, headers, bodies, dependencies, etc.
• Convert data: from the request to the required type.
• Validate data: coming from each request:
  • Generating automatic errors returned to the client when the data is invalid.
• Document the API using OpenAPI:
  • which is then used by the automatic interactive documentation user interfaces.

This might all sound abstract. Don't worry. You'll see all this in action in the Tutorial - User Guide (the next section).

The important thing is that by using standard Python types, in a single place (instead of adding more classes, decorators, etc), FastAPI will do a lot of the work for you.

!!! info If you already went through all the tutorial and came back to see more about types, a good resource is the "cheat sheet" from mypy.