Code length overflow

[1848]

I am getting a DataOverflowError: Code length overflow. Data size (132) > size available (128)
when using the following code.

qr = qrcode.QRCode(version=2, error_correction=qrcode.ERROR_CORRECT_H, box_size=10, border=4)
# 15 chars
qr.add_data("sdfdsfnfffffnsd")
qr.make(fit=False)

The capacity of version 2 with H should be 128bits. I am trying to add 15 chars (15*8 => 120 bits).
I guess I am missing something?

Using qrcode v8 with python 3.10.

[ddawson]

The capacities in this library are for the raw bitstream and include non-payload things. First, there is a mode indicator (4 bits). The string you used will be encoded in bytes mode (8 bits per character), and you selected version 2, so there will be an 8-bit character count. After the payload may be a 4-bit termination marker and padding, but these will be cut off if there is not enough space left. In any case, the required overhead indeed brings the size up to 132 bits. There is no bug here, just some confusion.

Unfortunately, just how many characters you can use with given parameters isn't always straightforward, due to aforementioned overhead, as well as other things. For example, multiple sections can exist, each in its own mode, with corresponding limitations, bits per character, and overhead. But what we do always know is how many bits are available for the whole bitstream.

Usually, you wouldn't worry about which version to use and just let the library select the smallest one. But if you're constrained, then I think your choices are either to know how this all works, or to use trial and error.