Function "round" in python3 not working

CAP 33

Python 3.6.0 (default, Mar 27 2018, 01:18:40)
[GCC 5.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import AdcExp
>>> adc_1 = AdcExp.AdcExp(address=0x48)
>>> accel_volt_x = adc_1.read_voltage(0)
>>> g_x = (accel_volt_x - 1.51) / 0.35
>>> g_x
0.34267857142857155
>>> round(g_x, 1)
0.29999999999999999

I want to round the number to tenths, but 'round' does not work correctly.

crispyoz

@CAP-33 python rounding is a pain point for many users for quite some time, using your value for g_x try this:

print (round(g_x))

result is 0

Now try this: print (round (g_x,2))

result is 0.34000000000000002

and now this: print (round (g_x,2) * 2)

result is 0.68000000000000005

Why is the final 5 not a 4 which is 2 x 2?

Now try this:

print("% 0.2f" %(g_x))

the result is 0.34 which is what you would expect.

So we can better understand how rounding in python performs, set g_x to
g_x = 0.34567999999999

So a round up to 2 decimal places would be 0.35 while a truncation would be 0.34.

round (g_x,2) produces a result of 0.34999999999999998 which if you rounded to 2 decimal places would be 0.35. You can see it is modifying the storage in order to produce a correctly rounded result.

Now try this:
print("% 0.2f" %(g_x)) outputs 0.35 which is what you would expect.

Another reason why I consider python to be junk. junk. junk.

But this is not a question for the Onion forums, it's really a question for a Python forum because it's a quirk/junk in python

CAP 33

@crispyoz
Many thanks. I like python, but it, like other languages, has its drawbacks.

crispyoz

@CAP-33 You are correct this is a "drawback" of python. I find python has a lot of drawbacks That's why I develop my apps using C.