Example code for saving and retrieving a string from system.storage

[hmillington]

Is there any example code around to encode a string so it can be saved in system.storage, then retrieved later and converted back to a string?

Thanks

[dlech]

str has an encode() method and bytes has a decode() method specifically for this.

Fixed sized strings would be the easiest.

Save:

# my_str is always 3 ASCII characters so encoding is always 3 bytes.
my_str = "XYZ"
hub.system.storage(0, my_str.encode())

Restore:

my_str = hub.system.storage(0, read=3).decode()

Variable-length strings require a bit more work since you will need to add some padding to make sure it writes over any previous string that was longer with zeros.

Save:

MAX_STR_SIZE = 10

my_str = "XYZ"
encoded = my_str.encode()
hub.system.storage(0, write=encoded + "\0" * (MAX_STR_SIZE - len(encoded)))

Restore:

MAX_STR_SIZE = 10

my_str = hub.system.storage(0, read=MAX_STR_SIZE).decode().strip("\0")

[hmillington]

Perfect, thank you!

[hmillington]

Now I've tried the code, it claims that str does not have a encode() method:

AttributeError: 'str' object has no attribute 'encode'

I've just updated the hum to the latest firmware in case that was the issue but to no avail.

[laurensvalk]

Those methods don't exist (or aren't enabled). You can use:

encoded = bytes("Hello", "utf-8")

print(encoded)

decoded = str(encoded, "utf-8")

print(decoded)

[laurensvalk]

Here is an example that stores a string with arbitrary size.

from pybricks.hubs import ThisHub

hub = ThisHub()

def store_string(string):
    length = len(string).to_bytes(4, "little")
    hub.system.storage(0, write=length)
    hub.system.storage(4, write=bytes(string, "utf-8"))

def load_string():
    length = int.from_bytes(hub.system.storage(0, read=4), "little")
    return str(hub.system.storage(4, read=length), "utf-8")


# Example
store_string("Hello, world!")

result = load_string()

print(result)

[hmillington]

Thank you Laurens, worked a treat, although the to_/from_bytes methods seemed to need a byteorder parameter.

[laurensvalk]

Thanks, updated. It didn't seem to require it for some reason?