---
title: Data Types
description: Data Types in CocoIndex
toc_max_heading_level: 4
---

# Data Types in CocoIndex

In CocoIndex, all data processed by the flow have a type determined when the flow is defined, before any actual data is processed at runtime.

This makes schema of data processed by CocoIndex clear, and easily determine the schema of your index.

## Data Types

As an engine written in Rust, designed to be used in different languages and data are always serializable, CocoIndex defines a type system independent of any specific programming language.

CocoIndex automatically infers data types of the output created by CocoIndex sources and functions.
You don't need to spell out any data type explicitly when you define the flow.
All you need to do is to make sure the data passed to functions and targets are compatible with them.

Each type in CocoIndex type system is mapped to one or multiple types in Python.
When you define a [custom function](/docs/custom_ops/custom_functions), you need to annotate the data types of arguments and return values.

*   When you pass a Python value to the engine (e.g. return values of a custom function), a specific type annotation is required.
    The type annotation needs to be specific in describing the target data type, as it provides the ground truth of the data type in the flow.

*   When you use a Python variable to bind to an engine value (e.g. arguments of a custom function),
    the engine already knows the specific data type, so we don't require a specific type annotation, e.g. type annotations can be omitted, or you can use `Any` at any level.
    When a specific type annotation is provided, it's still used as a guidance to construct the Python value with compatible type.
    Otherwise, we will bind to a default Python type.

### Basic Types

#### Primitive Types

Primitive types are basic types that are not composed of other types.
This is the list of all primitive types supported by CocoIndex:

| CocoIndex Type | Python Types | Convertible to | Explanation |
|------|-------------|--------------|----------------|
| *Bytes* | `bytes` | | |
| *Str* | `str` | | |
| *Bool* | `bool` | | |
| *Int64* | `cocoindex.Int64`, `int`, `numpy.int64` | | |
| *Float32* | `cocoindex.Float32`, `numpy.float32` | *Float64* | |
| *Float64* |  `cocoindex.Float64`, `float`, `numpy.float64` | | |
| *Range* | `cocoindex.Range` | | |
| *Uuid* | `uuid.UUId` | | |
| *Date* | `datetime.date` | | |
| *Time* | `datetime.time` | | |
| *LocalDatetime* | `cocoindex.LocalDateTime` | *OffsetDatetime* | without timezone |
| *OffsetDatetime* | `cocoindex.OffsetDateTime`, `datetime.datetime` | | with timezone |
| *TimeDelta* | `datetime.timedelta` | | |

Notes:

*   For some CocoIndex types, we support multiple Python types. You can annotate with any of these Python types.
    The first one is the default type, i.e. CocoIndex will create a value with this type when a specific type annotation is not provided (e.g. for arguments of a custom function).

*   All Python types starting with `cocoindex.` are type aliases exported by CocoIndex. They're annotated types based on certain Python types:

    *   `cocoindex.Int64`: `int`
    *   `cocoindex.Float64`: `float`
    *   `cocoindex.Float32`: `float`
    *   `cocoindex.Range`: `tuple[int, int]`, i.e. a start offset (inclusive) and an end offset (exclusive)
    *   `cocoindex.OffsetDateTime`: `datetime.datetime`
    *   `cocoindex.LocalDateTime`: `datetime.datetime`

    These aliases provide a non-ambiguous way to represent a specific type in CocoIndex, given their base Python types can represent a superset of possible values.

*   When we say a CocoIndex type is *convertible to* another type, it means Python types for the second type can be also used to bind to a value of the first type.
    *   For example, *Float32* is convertible to *Float64*, so you can bind a value of *Float32* to a Python value of `float` or `np.float64` types.
    *   For *LocalDatetime*, when you use `cocoindex.OffsetDateTime` or `datetime.datetime` as the annotation to bind its value, the timezone will be set to UTC.


#### Json Type

*Json* type can hold any data convertible to JSON by `json` package.
In Python, it's represented by `cocoindex.Json`.
It's useful to hold data without fixed schema known at flow definition time.


#### Vector Types

A vector type is a collection of elements of the same basic type.
Optionally, it can have a fixed dimension. Noted as *Vector[Type]* or *Vector[Type, Dim]*, e.g. *Vector[Float32]* or *Vector[Float32, 384]*.

It supports the following Python types:

*   `cocoindex.Vector[T]` or `cocoindex.Vector[T, typing.Literal[Dim]]`, e.g. `cocoindex.Vector[cocoindex.Float32]` or `cocoindex.Vector[cocoindex.Float32, typing.Literal[384]]`
    *   The underlying Python type is `numpy.typing.NDArray[T]` where `T` is a numpy numeric type (`numpy.int64`, `numpy.float32` or `numpy.float64`) or array type (`numpy.typing.NDArray[T]`), or `list[T]` otherwise
*   `numpy.typing.NDArray[T]` where `T` is a numpy numeric type or array type
*   `list[T]`


#### Union Types

A union type is a type that can represent values in one of multiple basic types.
Noted as *Type1* | *Type2* | ..., e.g. *Int64* | *Float32* | *Float64*.

The Python type is `T1 | T2 | ...`, e.g. `cocoindex.Int64 | cocoindex.Float32 | cocoindex.Float64`, `int | float` (equivalent to `cocoindex.Int64 | cocoindex.Float64`)


### Struct Types

A *Struct* has a bunch of fields, each with a name and a type.

In Python, a *Struct* type is represented by either a [dataclass](https://docs.python.org/3/library/dataclasses.html)
or a [NamedTuple](https://docs.python.org/3/library/typing.html#typing.NamedTuple), with all fields annotated with a specific type.
Both options define a structured type with named fields, but they differ slightly:

- **Dataclass**: A flexible class-based structure, mutable by default, defined using the `@dataclass` decorator.
- **NamedTuple**: An immutable tuple-based structure, defined using `typing.NamedTuple`.

For example:

```python
from dataclasses import dataclass
from typing import NamedTuple
import datetime

# Using dataclass
@dataclass
class Person:
    first_name: str
    last_name: str
    dob: datetime.date

# Using NamedTuple
class PersonTuple(NamedTuple):
    first_name: str
    last_name: str
    dob: datetime.date
```

Both `Person` and `PersonTuple` are valid Struct types in CocoIndex, with identical schemas (three fields: `first_name` (Str), `last_name` (Str), `dob` (Date)).
Choose `dataclass` for mutable objects or when you need additional methods, and `NamedTuple` for immutable, lightweight structures.

Besides, for arguments of custom functions, CocoIndex also supports using dictionaries (`dict[str, Any]`) to represent a *Struct* type.
It's the default Python type if you don't annotate the function argument with a specific type.

### Table Types

A *Table* type models a collection of rows, each with multiple columns.
Each column of a table has a specific type.

We have two specific types of *Table* types: *KTable* and *LTable*.

#### KTable

*KTable* is a *Table* type whose one or more columns together serve as the key.
The row order of a *KTable* is not preserved.
Each key column must be a [key type](#key-types). When multiple key columns are present, they form a composite key.

In Python, a *KTable* type is represented by `dict[K, V]`.
`K` represents the key and `V` represents the value for each row:

- `K` can be a Struct type (either a frozen dataclass or a `NamedTuple`) that contains all key parts as fields. This is the general way to model multi-part keys.
- When there is only a single key part and it is a basic type (e.g. `str`, `int`), you may use that basic type directly as the dictionary key instead of wrapping it in a Struct.
- `V` should be the type bound to a *Struct* representing the non-key value fields of each row.

When a specific type annotation is not provided:
- For composite keys (multiple key parts), the key binds to a Python tuple of the key parts, e.g. `tuple[str, str]`.
- For a single basic key part, the key binds to that basic Python type.
- The value binds to `dict[str, Any]`.


For example, you can use `dict[str, Person]` or `dict[str, PersonTuple]` to represent a *KTable*, with 4 columns: key (*Str*), `first_name` (*Str*), `last_name` (*Str*), `dob` (*Date*).
It's bound to `dict[str, dict[str, Any]]` if you don't annotate the function argument with a specific type.

Note that when using a Struct as the key, it must be immutable in Python. For a dataclass, annotate it with `@dataclass(frozen=True)`. For `NamedTuple`, immutability is built-in. For example:

```python
@dataclass(frozen=True)
class PersonKey:
    id_kind: str
    id: str

class PersonKeyTuple(NamedTuple):
    id_kind: str
    id: str
```

Then you can use `dict[PersonKey, Person]` or `dict[PersonKeyTuple, PersonTuple]` to represent a KTable keyed by both `id_kind` and `id`.
If you don't annotate the function argument with a specific type, it's bound to `dict[tuple[str, str], dict[str, Any]]`.


#### LTable

*LTable* is a *Table* type whose row order is preserved. *LTable* has no key column.

In Python, a *LTable* type is represented by `list[R]`, where `R` is the type binding to the *Struct* type representing the value fields of each row.
For example, you can use `list[Person]` to represent a *LTable* with 3 columns: `first_name` (*Str*), `last_name` (*Str*), `dob` (*Date*).
It's bound to `list[dict[str, Any]]` if you don't annotate the function argument with a specific type.

## Key Types

Currently, the following types are key types

- *Bytes*
- *Str*
- *Bool*
- *Int64*
- *Range*
- *Uuid*
- *Date*
- *Struct* with all fields being key types (using `@dataclass(frozen=True)` or `NamedTuple`)

## *None* Values

CocoIndex supports *None* values. A *None* value represents the absence of data or an unknown value, distinct from empty strings, zero numbers, or false boolean values.

### Optional Type

For any data (e.g. a field of a *Struct*, an argument or return value of a CocoIndex function), if it is optional, it means its value can be *None*.
We use *Optional[T]* to indicate an optional type, e.g. *Optional[Str]*, *Optional[Person]*.

In Python, *None* is represented as `None`, so an optional type can be represented by `T | None` or `typing.Optional[T]`.

### *None* propagating on CocoIndex functions

A function may specify whether each input argument is optional or not.
Non-optional argument means the function needs a known value for the argument to work.
However, it doesn't forbid the argument to be *None* at runtime.
When a non-optional argument receives a *None* value, the function execution is skipped and the result is *None*.

For example, for [`SplitRecursively` function](/docs/ops/functions#splitrecursively), the `text` and `chunk_size` arguments are not optional. If the input value of either of them is *None*, the function will return *None*.
