Well, technically it's not necessary. It can be OK as it is now. I personally hate to represent code expressions inside strings.

But, why are `length`, `mass` and `time` associated with MKS? What about creating a new class, e.g. `DimensionSymbol`, and make those basic dimensions instances of it.

For example:
```
mass = DimensionSymbol('mass')
length = DimensionSymbol('length')
... ...
```
So the MKS could be accessed by:
```
length, mass, time = mks[length], mks[mass], mks[time]
```
or still in some other way.