Question about math.pi is mutable

[Steven D'Aprano]

On Sun, 8 Nov 2015 02:01 am, Bartc wrote:

> Neither have the simplicity of concept of Pascal's 'const', which is
> just a named value. Not a variable that won't change once initialised,
> not a parameter that won't be changed nor any addressable location.)

Unfortunately the concept of "named value" doesn't match well with Python's
design. That implies a separate compilation step which doesn't fit well
with Python's runtime semantics. Very little happens at compile-time in
Python that *must* happen at compile-time.

I'm also not sure what difference you think there is between "named value"
and "variable that won't change once initialised".

Python has a convention for "constants" -- all UPPERCASE names. The fact
that the convention exists is enough to prove that the concept
of "constant" is a useful one. The difference between Python's
pseudo-constants and (say) Pascal's actual constants is that in Python, the
burden of ensuring that neither you, nor any of the libraries you call,
modifies the "constant" falls on you, the user, whereas in Pascal the
compiler or interpreter performs that checking for you.

Besides, the Python convention is honoured more in theory than in practice.
We have math.pi, not math.PI.



-- 
Steven