Yes.  This is the behavior.  It was part of the original Numeric implementation.    In the code generator file:

numpy/core/code_generators/generate_umath.py

ufuncs with a registered type of 'P' have this behavior.   There is a long list of them. 

-Travis


On Feb 2, 2012, at 12:04 AM, Warren Weckesser wrote:

Bump...

On Mon, Jan 30, 2012 at 1:17 AM, Warren Weckesser <warren.weckesser@enthought.com> wrote:
In the following code, numpy.sin() calls the object's sin() function:

In [2]: class Foo(object):
   ...:     def sin(self):
   ...:         return "spam"
   ...:    

In [3]: f = Foo()

In [4]: np.sin(f)
Out[4]: 'spam'

Is this, in fact, guaranteed behavior for a ufunc?  It does not appear to be documented.

This question came up in the discussion of SciPy pull request 138 (https://github.com/scipy/scipy/pull/138), where the idea is to add numpy unary ufunc support to SciPy's sparse arrays.

(Sorry if this email shows up twice.   I sent it the first time while the Enthought servers were down, and eventually got an email back saying it had not been sent.)

Warren


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