Pack value to bytes: pack

The pack template function converts a value into a bytes object using a Python struct format string. This is the same as Python’s struct.pack function, allowing you to convert numbers and other values into their binary representations according to a specified format.

This function is primarily useful for IoT and low-level device communication. Many IoT devicesA device is a model representing a physical or logical unit that contains entities. communicate using binary protocols where you need to send data in a specific byte format. For example, you might need to send a temperature value as a big-endian 16-bit integer to a Bluetooth device, or pack a command byte sequence for an MQTT-connected microcontroller. The format strings follow Python’s struct module syntax.

Usage

Here’s how to use this template function. Copy any example and adjust it to your setup.

{{ pack(1234, ">H") }}

b''\x04\xd2''

{{ 1234 | pack(">H") }}

b''\x04\xd2''

Function signature

The signature is a technical summary of this template function. It shows the name of the function, the values (called parameters) it accepts, and what type of data each parameter expects (for example, a piece of text or a number).

Function parameters that have a = with a value after them are optional. If you leave them out, the default value shown is used automatically.

Function parameters without a = are usually required. A few optional parameters have no default value, so the signature shows them without a = as well. To check whether a parameter is required, look at the parameter list below the signature. Each parameter is marked as Required or Optional.

The most common optional parameter without a default value is default. The default parameter sets the value a function returns when the function can’t produce a result, for example, when float gets text that isn’t a number. If you leave out default, there is no fallback value, so most functions raise an error and the template fails. The parameter list of each function says what happens. If you set default, the function returns your value instead.

round(
    value: Any,
    precision: int = 0,
    method: str = "common",
    default: Any,
) -> float | int | Any

pack(
    value: Any,
    format_string: str,
) -> bytes | None

Function parameters

The following parameters can be provided to this function.

Common format strings

Here are the format strings you will reach for most often. The > prefix means big-endian (most significant byte first) and < means little-endian (least significant byte first).

• >B: big-endian unsigned byte (1 byte)
• >H: big-endian unsigned short (2 bytes)
• >I: big-endian unsigned int (4 bytes)
• >f: big-endian float (4 bytes)
• <H: little-endian unsigned short (2 bytes)
• <i: little-endian signed int (4 bytes)

Good to know

• The value type must match the format specifier, or packing fails. >H expects an integer between 0 and 65535.
• Endianness matters. > is big-endian (network order) and < is little-endian.
• Returns raw bytes. For transmission over text channels, combine with base64_encode.

Try it yourself

Ready to test this? Open Settings > Tools > Template, paste the example into the Template editor, and watch the result update on the right. Edit the values to see how the function adapts to your own entitiesAn entity represents a sensor, actor, or function in Home Assistant. Entities are used to monitor physical properties or to control other entities. An entity is usually part of a device or a service. [Learn more].

More examples

Real scenarios where this function comes up in automations and templates. Copy any example and adapt it to your setup.

Pack a temperature for a BLE device

Pack a temperature reading as a big-endian 16-bit integer for transmission to a Bluetooth Low Energy device.

{{
  states("sensor.temperature") | float | round(0) | int
  | pack(">H") | base64_encode
}}

ABU=

Pack a float value

Pack a floating-point number into its binary representation.

{{ 3.14 | pack(">f") }}

b'@H\xf5\xc3'

Still stuck?

The Home Assistant community is quick to help: join Discord for real-time chat, post on the community forum with your template and expected result, or share on our subreddit /r/homeassistant.

Related template functions

These functions work well alongside this one:

• Unpack bytes to value: unpack - Unpacks bytes into a value using a Python struct format string.

• Decode hex string to bytes: from_hex - Decodes a hexadecimal string into bytes.

• Encode to base64: base64_encode - Encodes a string or bytes to a base64 string.