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Standardize encoding of μ in units of measurement

· 2 min read
Jan Bouwhuis
Jan Bouwhuis

One preferred encoding to represent μ in units of measurement​

The unit prefix μ is used by unit of measurement constants like μV and μS. There are however two different Unicode encodings of μ:

  1. The MICRO SIGN
  2. The Greek Small Letter Mu

Home Assistant previously mixed the two encodings, which caused issues because string comparisons between the two encodings would fail causing unit of measurement validations to fail.

We chose the "Greek Small Letter Mu" because the "MICRO SIGN" encoding is there for compatibility with old 8-bit western European character sets, and the unicode consortium recommends against using it.

With #144853 we have fixed this in the core of Home Assistant by consequently using the Greek Small Letter Mu version to encode μ. In Python literal strings this variant is encoded as "\u03bc".

Developers should check if their code and libraries have a dependency with the ambiguous MICRO SIGN (μ) "\u00b5", and migrate their code to use the Greek Small Letter Mu "\u03bc" instead to avoid issues.

The sensor and number entity platforms now include a built-in feature to automatically convert units that use the ambiguous MICRO SIGN (μ) encoding.

Find instances in Visual Studio Code​

In Visual Studio Code, enable Match Case in the Search (files) panel to find only the chosen encoding. Copy the exact μ character from above and paste it into the search box. The editor’s page search matches all variants; use the global search instead.

New linter​

To avoid issues with new or changed code, #144853 adds a linter that warns about incorrect literal assignments using the non-preferred encoding of μ.