python - Best way to detect floating round off error in python

Question

So I have a list of floating numbers, some of them have round off errors and appears in the form 0.3599999. It is trivial to detect by convert it to string and see if there is a bunch of 999 following. I wonder how a python hacker will do for this or if there is a mathematical way to do this.

Thanks

Answer 1

Consider using Python's decimal module

>>> from decimal import Decimal
>>> Decimal(0.35)
Decimal('0.34999999999999997779553950749686919152736663818359375')

Answer 2

Also have a look at Numpy's assert_approx_equal() function:

>>> np.testing.assert_approx_equal(0.12345677777777e-20, 0.1234567e-20)
>>> np.testing.assert_approx_equal(0.12345670e-20, 0.12345671e-20,
                               significant=8)
>>> np.testing.assert_approx_equal(0.12345670e-20, 0.12345672e-20,
                               significant=8)
...
<type 'exceptions.AssertionError'>:
Items are not equal to 8 significant digits:
 ACTUAL: 1.234567e-021
 DESIRED: 1.2345672000000001e-021