Environment Variables

CircuitPython 8.0.0 introduces support for environment variables. Environment variables are commonly used to store “secrets” such as Wi-Fi passwords and API keys. This method does not make them secure. It only separates them from the code.

CircuitPython uses a file called settings.toml at the drive root (no folder) as the environment. User code can access the values from the file using os.getenv(). It is recommended to save any values used repeatedly in a variable because os.getenv() will parse the settings.toml file contents on every access.

CircuitPython only supports a subset of the full toml specification, see below for more details. The subset is very “Python-like”, which is a key reason we selected the format.

Due to technical limitations it probably also accepts some files that are not valid TOML files; bugs of this nature are subject to change (i.e., be fixed) without the usual deprecation period for incompatible changes.

File format example:

str_key="Hello world" # with trailing comment
int_key = 7
unicode_key="œuvre"
unicode_key2="\\u0153uvre" # same as above
unicode_key3="\\U00000153uvre" # same as above
escape_codes="supported, including \\r\\n\\"\\\\"
# comment
[subtable]
subvalue="cannot retrieve this using getenv"

Details of the toml language subset

  • The content is required to be in UTF-8 encoding

  • The supported data types are string and integer

  • Only basic strings are supported, not triple-quoted strings

  • Only integers supported by strtol. (no 0o, no 0b, no underscores 1_000, 011 is 9, not 11)

  • Only bare keys are supported

  • Duplicate keys are not diagnosed.

  • Comments are supported

  • Only values from the “root table” can be retrieved

  • due to technical limitations, the content of multi-line strings can erroneously be parsed as a value.

CircuitPython behavior

CircuitPython will also read the environment to configure its behavior. Other keys are ignored by CircuitPython. Here are the keys it uses:

CIRCUITPY_BLE_NAME

Default BLE name the board advertises as, including for the BLE workflow.

CIRCUITPY_HEAP_START_SIZE

Sets the initial size of the python heap, allocated from the outer heap. Must be a multiple of 4. The default is currently 8192. The python heap will grow by doubling and redoubling this initial size until it cannot fit in the outer heap. Larger values will reserve more RAM for python use and prevent the supervisor and SDK from large allocations of their own. Smaller values will likely grow sooner than large start sizes.

CIRCUITPY_PYSTACK_SIZE

Sets the size of the python stack. Must be a multiple of 4. The default value is currently 1536. Increasing the stack reduces the size of the heap available to python code. Used to avoid “Pystack exhausted” errors when the code can’t be reworked to avoid it.

CIRCUITPY_WEB_API_PASSWORD

Password required to make modifications to the board from the Web Workflow.

CIRCUITPY_WEB_API_PORT

TCP port number used for the web HTTP API. Defaults to 80 when omitted.

CIRCUITPY_WEB_INSTANCE_NAME

Name the board advertises as for the WEB workflow. Defaults to human readable board name if omitted.

CIRCUITPY_WIFI_PASSWORD

Wi-Fi password used to auto connect to CIRCUITPY_WIFI_SSID.

CIRCUITPY_WIFI_SSID

Wi-Fi SSID to auto-connect to even if user code is not running.