[Python-ideas] SI scale factors in Python

[Xavier Combelle]

On 25/08/2016 22:06, Ken Kundert wrote:
> Here is a fairly typical example that illustrates the usefulness of supporting 
> SI scale factors and units in Python.
>
> This is simple simulation code that is used to test a current mirror driving an 
> 100kOhm resistor ...
>
> Here is some simulation code that uses SI scale factors
>
>     for delta in [-500nA, 0, 500nA]:
>         input = 2.75uA + delta
>         wait(1us)
>         expected = 100kOhm*(2.75uA + delta)
>         tolerance = 2.2mV
>         fails = check_output(expected, tolerance)
>         print('%s: I(in)=%rA, measured V(out)=%rV, expected V(out)=%rV, diff=%rV.' % (
>             'FAIL' if fails else 'pass',
>             input, get_output(), expected, get_output() - expected
>         ))
>
> with the output being:
>
>     pass: I(in)=2.25uA, measured V(out)=226.7mV, expected V(out)=225mV, diff=1.7mV.
>     pass: I(in)=2.75uA, measured V(out)=276.8mV, expected V(out)=275mV, diff=1.8mV.
>     FAIL: I(in)=3.25uA, measured V(out)=327.4mV, expected V(out)=325mV, diff=2.4mV.
>
> And the same code in Python today ...
>
>     for delta in [-5e-7, 0, 5e-7]:
>         input = 2.75e-6 + delta
>         wait(1e-6)
>         expected = 1e5*(2.75e-6 + delta)
>         tolerance = 2.2e-3
>         fails = check_output(expected, tolerance)
>         print('%s: I(in)=%eA, measured V(out)=%eV, expected V(out)=%eV, diff=%eV.' % (
>             'FAIL' if fails else 'pass',
>             input, get_output(), expected, get_output() - expected
>         ))
>
> with the output being:
>
>     pass: I(in)=2.25e-6A, measured V(out)=226.7e-3V, expected V(out)=225e-3V, diff=1.7e-3V.
>     pass: I(in)=2.75e-6A, measured V(out)=276.8e-3V, expected V(out)=275e-3V, diff=1.8e-3V.
>     FAIL: I(in)=3.25e-6A, measured V(out)=327.4e-3V, expected V(out)=325e-3V, diff=2.4e-3V.
>
> There are two things to notice. In the first example the numbers are easier to
> read: for example, 500nA is easier to read the 5e-7. Second, there is 
> information in the units that provides provides useful information. One can 
> easily see that the input signal is a current and that the output is a voltage.  
> Furthermore, anybody can look at this code and do a simple sanity check on the 
> expressions even if they don't understand the system being simulated. They can 
> check the units on the input and output expressions to assure that they are all 
> consistent.
>
> -Ken
> _______________________________________________
>
And the same code with python today


def wait(delay):
    pass
def check_output(expected, tolerance):
    return False
def get_output():
    return 1*uA
def f():
    for delta in [-500*nA, 0, 500*nA]:
        input = 2.75*uA + delta
        wait(1*us)
        expected = 100*kOhm*(2.75*uA + delta)
        tolerance = 2.2*mV
        fails = check_output(expected, tolerance)
        print('%s: I(in)=%rA, measured V(out)=%rV, expected V(out)=%rV,
diff=%rV.' % (
            'FAIL' if fails else 'pass',
            input, get_output(), expected, get_output() - expected
        ))
f()

pass: I(in)=2.25e-05A, measured V(out)=1e-05V, expected V(out)=22.5V,
diff=-22.49999V.
pass: I(in)=2.75e-05A, measured V(out)=1e-05V, expected V(out)=27.5V,
diff=-27.49999V.
pass: I(in)=3.2500000000000004e-05A, measured V(out)=1e-05V, expected
V(out)=32.50000000000001V, diff=-32.499990000000004V.

Do you really see fundamental difference with your original code?