Cannot import certain Math modules in Jython - jython

Question

I'm trying to import some modules from the math library but only some of the modules will import. isnan and asinh for example will not import but pi and acos will. Based on the jython documentation these modules should be able to be imported.
Does anyone have a solution as to why only some of the modules are importing?

Answer 1

I'm afraid I have some bad news for you; but I may have a possible work-around.
The bad news
I recently did a pretty sizable project using Jython, and from that experience concluded that there are errors in the Jython documentation. Much of it seems to have been more or less copied directly from the Python documentation, and unfortunately there are some inconsistencies between the implementations.
If you first launch the Python interpreter, import math, and then do a dir on the module, you will find isnan:
Python 2.7.5 (default, Jun 17 2014, 18:11:42)
[GCC 4.8.2 20140120 (Red Hat 4.8.2-16)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import math
>>> dir(math)
['__doc__', '__file__', '__name__', '__package__', 'acos', 'acosh', 'asin',
'asinh', 'atan', 'atan2', 'atanh', 'ceil', 'copysign', 'cos', 'cosh', 'degrees',
'e', 'erf', 'erfc', 'exp', 'expm1', 'fabs', 'factorial', 'floor', 'fmod', 'frexp',
'fsum', 'gamma', 'hypot', 'isinf', 'isnan', 'ldexp', 'lgamma', 'log', 'log10',
'log1p', 'modf', 'pi', 'pow', 'radians', 'sin', 'sinh', 'sqrt', 'tan', 'tanh', 'trunc']
>>> print math.isnan.__doc__
isnan(x) -> bool
Check if float x is not a number (NaN).
On the other hand, if you do the same in Jython, you will find that the math module has no isnan function.
When I installed Jython, I installed the source as well. So, just to be sure, I grepped through the source and found the following:
[mdiana#tc-xdevmd modules]$ pwd
/opt/jython/jython-2.5.3/src/org/python/modules
[mdiana#tc-xdevmd modules]$ grep -Ri "isnan" *
math.java: if (Double.isNaN(v))
math.java: if (Double.isNaN(x) || Double.isInfinite(x) || x == 0.0) {
struct.java: Float.isInfinite(v) || Float.isNaN(v))) {
struct.java: (Double.isInfinite(v) || Double.isNaN(v))) {
struct.java: Float.isInfinite(v) || Float.isNaN(v))) {
struct.java: (Double.isInfinite(v) || Double.isNaN(v))) {
That's further evidence that there is no isnan anywhere in the code (no matter what the documentation might say). Jython itself is using the Java libraries for that functionality.
Suggested work-around
So, you're not going crazy, and there is nothing wrong with your installation. Working with Jython, you will run into problems like this from time to time. Sorry. What I suggest is using the Java libraries, either java.lang.Float or java.lang.Double. Both of those have a static method isNaN:
[mdiana#tc-xdevmd ~]$ jython
Jython 2.5.3 (2.5:c56500f08d34+, Aug 13 2012, 14:48:36)
[Java HotSpot(TM) 64-Bit Server VM (Oracle Corporation)] on java1.8.0_31
Type "help", "copyright", "credits" or "license" for more information.
>>> import java.lang.Float as JFloat
>>> JFloat.isNaN(4.001)
False
All in all, I like Jython; but sometimes things like the above are the best we can do. Hope that helps.