Python: Decimals with trigonometric functions

Question

I'm having a little problem, take a look:

>>> import math
>>> math.sin(math.pi)
1.2246467991473532e-16

This is not what I learnt in my Calculus class (It was 0, actually)

So, now, my question:

I need to perform some heavy trigonometric calculus with Python. What library can I use to get correct values?

Can I use Decimal?

EDIT:

Sorry, What I mean is other thing.

What I want is some way to do:

>>> awesome_lib.sin(180)
0

or this:

>>> awesome_lib.sin(Decimal("180"))
0

I need a libraray that performs good trigonometric calculus. Everybody knows that sin 180° is 0, I need a library that can do that too.

Answer 1

1.2246467991473532e-16 is close to 0 -- there are 16 zeroes between the decimal point and the first significant digit -- much as 3.1415926535897931 (the value of math.pi) is close to pi. The answer is correct to sixteen decimal places!

So if you want sin(pi) to equal 0, simply round it to a reasonable number of decimal places. 15 looks good to me and should be plenty for any application:

print round(math.sin(math.pi), 15)

Answer 2

Pi is an irrational number so it can't be represented exactly using a finite number of bits. However, you can use some library for symbolic computation such as sympy.

>>> sympy.sin(sympy.pi)
0

Regarding the second part of you question, if you want to use degrees instead of radians you can define a simple conversion function

def radians(x):
    return x * sympy.pi / 180

and use it as follows:

>>> sympy.sin(radians(180))
0