The functions in the math module have the (pleasantly) surprising and apparently undocumented property that they'll accept not just floats, but any Python object having a float method: >>> class test1(object): ... def __float__(self): ... return 3. ... >>> from math import sqrt >>> sqrt(test1()) 1.7320508075688772 Based on this, one might expect the functions in the complex math module cmath to have the same property with respect to __complex__. But this isn't so: >>> class test2(object): ... def __complex__(self): ... return -3 + 0j ... >>> from cmath import sqrt as csqrt >>> csqrt(test2()) Traceback (most recent call last): File "", line 1, in TypeError: a float is required The real surprise is that the cmath functions *will* call the __float__ method, if it's available: >>> csqrt(test1()) (1.7320508075688772+0j) This patch expands the PyComplex_AsCComplex method so that it looks for a __complex__ method before looking for the __float__ method. This `fixes' the above behaviour. Should it be a documented feature that the math functions will make use of __float__? If so, and if the patched behaviour seems desirable, I'll add suitable tests to test_math and test_cmath.
