[Python-Dev] return type of complex (original) (raw)

Devin Jeanpierre jeanpierreda at gmail.com
Sun Oct 21 07:06:08 CEST 2012

On Sat, Oct 20, 2012 at 10:57 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:

PEP 3141 is indeed the driver for these changes, and it's based on the Python 3.x numeric tower consisting of strict supersets: Complex > Real > Rational > Integral

Pedant mode: That numeric tower is wrong as it applies to Python -- we have some rational types that can represent some numbers that can't be represented by our complex and """real""" types.

>>> int(float(10**100)) == 10**100
False

(Also, floats aren't reals and no computer can store any number that is not rational and we should stop pretending they can. (Complex numbers get a free pass because "complex numbers with rational real and imaginary parts" is not a memorable name for a type.))

Pedant mode deactivated

If an operation at one level of the tower produces a result in one of the larger supersets, then that's what it will do rather than throwing TypeError. int / int promoting to float is one example, as is raising a negative number to a fractional power promoting to complex.

The general philosophy is described in http://www.python.org/dev/peps/pep-3141/#implementing-the-arithmetic-operations It sounds like cmath (or, more specifically, the "D" conversion code) missed out on the associated updates).

No, no no. The return type of conversion methods has nothing to do with the interoperability of types in arithmetic. If it did there would've been a systematic purging of old behaviors in these conversion functions, and there evidently hasn't been because float(), cmath, and math all behave "wrong", and those are collectively fairly hard to miss.

Since float(), cmath, and math all behave the same, I'd assert that it's complex() (and int()) that is weird and unusual.

class A: ... def complex(self): ... return 1 ... def float(self): ... return 1 ... complex(A()) (1+0j) float(A()) Traceback (most recent call last): File "", line 1, in TypeError: float returned non-float (type int) cmath.sqrt(A()) Traceback (most recent call last): File "", line 1, in TypeError: complex should return a complex object math.sqrt(A()) Traceback (most recent call last): File "", line 1, in TypeError: nb_float should return float object

-- Devin

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