How do I catch a numpy warning like it's an exception (not just for testing)?

divide-by-zero, exception, numpy, python, warnings

Solution

It seems that your configuration is using the `print` option for `numpy.seterr`:

>>> import numpy as np
>>> np.array([1])/0   #'warn' mode
__main__:1: RuntimeWarning: divide by zero encountered in divide
array([0])
>>> np.seterr(all='print')
{'over': 'warn', 'divide': 'warn', 'invalid': 'warn', 'under': 'ignore'}
>>> np.array([1])/0   #'print' mode
Warning: divide by zero encountered in divide
array([0])

This means that the warning you see is not a real warning, but it's just some characters printed to `stdout`(see the documentation for `seterr`). If you want to catch it you can:

- Use `numpy.seterr(all='raise')` which will directly raise the exception. This however changes the behaviour of all the operations, so it's a pretty big change in behaviour.

- Use `numpy.seterr(all='warn')`, which will transform the printed warning in a real warning and you'll be able to use the above solution to localize this change in behaviour.

Once you actually have a warning, you can use the `warnings` module to control how the warnings should be treated:

>>> import warnings
>>> 
>>> warnings.filterwarnings('error')
>>> 
>>> try:
...     warnings.warn(Warning())
... except Warning:
...     print 'Warning was raised as an exception!'
... 
Warning was raised as an exception!

Read carefully the documentation for `filterwarnings` since it allows you to filter only the warning you want and has other options. I'd also consider looking at `catch_warnings` which is a context manager which automatically resets the original `filterwarnings` function:

>>> import warnings
>>> with warnings.catch_warnings():
...     warnings.filterwarnings('error')
...     try:
...         warnings.warn(Warning())
...     except Warning: print 'Raised!'
... 
Raised!
>>> try:
...     warnings.warn(Warning())
... except Warning: print 'Not raised!'
... 
__main__:2: Warning:

Problem

I have to make a Lagrange polynomial in Python for a project I'm doing. I'm doing a barycentric style one to avoid using an explicit for-loop as opposed to a Newton's divided difference style one. The problem I have is that I need to catch a division by zero, but Python (or maybe numpy) just makes it a warning instead of a normal exception. So, what I need to know how to do is to catch this warning as if it were an exception. The related questions to this I found on this site were answered not in the way I needed. Here's my code: ``` import numpy as np import matplotlib.pyplot as plt import warnings class Lagrange: def __init__(self, xPts, yPts): self.xPts = np.array(xPts) self.yPts = np.array(yPts) self.degree = len(xPts)-1 self.weights = np.array([np.product([x_j - x_i for x_j in xPts if x_j != x_i]) for x_i in xPts]) def __call__(self, x): warnings.filterwarnings("error") try: bigNumerator = np.product(x - self.xPts) numerators = np.array([bigNumerator/(x - x_j) for x_j in self.xPts]) return sum(numerators/self.weights*self.yPts) except Exception, e: # Catch division by 0. Only possible in 'numerators' array return yPts[np.where(xPts == x)[0][0]] L = Lagrange([-1,0,1],[1,0,1]) # Creates quadratic poly L(x) = x^2 L(1) # This should catch an error, then return 1. ``` When this code is executed, the output I get is: ``` Warning: divide by zero encountered in int_scalars ``` That's the warning I want to catch. It should occur inside the list comprehension.

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