Thomas Fischbacher <tfish@google.com> added the comment: Tim, the problem may well be simply due to the documentation of math.isfinite() being off here. This is what we currently have: https://docs.python.org/3/library/math.html#math.isfinite === math.isfinite(x) Return True if x is neither an infinity nor a NaN, and False otherwise. (Note that 0.0 is considered finite.) New in version 3.2. === If this were re-worded as follows (and corresponding changes were made to other such functions), everyone would know what the expectations and behavior are: === math.isfinite(x) If `x` is a `float` instance, this evaluates to `True` if `x` is neither a float infinity nor a NaN, and `False` otherwise. If `x` is not a `float` instance, this is evaluates to `math.isfinite(float(x))`. New in version 3.2. === This would be an accurate defining description of the actual behavior. Note that, "thanks to PEP-484", this abbreviation would currently be ambiguous though: === math.isfinite(x) If `x` is a float, this evaluates to `True` if `x` is neither a float infinity nor a NaN, and `False` otherwise. If `x` is not a float, this is evaluates to `math.isfinite(float(x))`. New in version 3.2. === ("ambiguous" since "float" means different things as a static type and as a numbers class - and it is not clear what would be referred to here). Changing/generalizing the behavior might potentially be an interesting other proposal, but I would argue that then one would want to change the behavior of quite a few other functions here as well, and all this should then perhaps go into some other `xmath` (or so) module - bit like it is with `cmath`. However, since the Python philosophy is to not rely on bureaucracy to enforce contracts (as C++, Java, etc. do it), but instead to rely on people's ability to define their own contracts, making the math.isfinite() contract more accurate w.r.t. actual behavior in the CPython implementation via extra clarification looks like a good thing to do, no? ---------- _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue47121> _______________________________________