2.2.1.3. Arrays / Lists#

When working with binary formats, it’s common to store multiple objects of the same type in a sequence, such as a list or array. Caterpillar simplifies this task by allowing easy access to items and defining arrays with both static and dynamic sizes.

For example, let’s consider the PLTE chunk from the PNG format, which stores a sequence of three-byte RGB objects. To define an array of RGB objects, you can use the following syntax:

>>> PLTEChunk = RGB[this.length / 3]

Tip

If you want to remove the typing issues associated with the getitem ([]) operation, add these lines to the RGB class definition:

@struct
class RGB:
    # ... previous code ...
    if typing.TYPE_CHECKING:
        def __class_getitem__(cls, length): ...

To use the faster Repeated class provided by the C-extension:

>>> from caterpillar.c import Repeated
>>> from caterpillar.shared import getstruct
>>> PLTEChunk = Repeated(getstruct(RGB), this.length / 3)

Important

The length of the array is determined dynamically. In this case, the length is calculated by dividing the chunk’s length field by 3, since each RGB object occupies 3 bytes.

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