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

Steven D'Aprano steve at pearwood.info
Sun Oct 21 03:53:10 CEST 2012

On 21/10/12 06:28, Tres Seaver wrote:

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On 10/19/2012 07:35 PM, Greg Ewing wrote: Antonio Cuni wrote:

Traceback (most recent call last): File "", line 1, in TypeError: complex should return a complex object

i.e., the complex constructor does not check that complex returns an actual complex, while the cmath functions do. Looks to me like cmath is being excessively finicky here. Why shouldn't a float be usable in any context expecting a complex? Exactly: float is perfectly Liskov-substituable for complex; only applications which do explicit type sniffing can tell the difference, which makes the sniffing bogus.

But float is not numerically substitutable for complex, which is why type-sniffing is not bogus at all. If you have an application which assumes numeric quantities represent real values, then you need to distinguish between real-valued and complex-valued arithmetic, and treating floats as implicitly complex is the wrong thing to do.

Since most applications are based on real-values, implicit promotion to complex is in my opinion an anti-feature.

Python 2.x legitimately distinguished between floats and complex, e.g.:

py> (-100.0)**0.5 Traceback (most recent call last): File "", line 1, in ValueError: negative number cannot be raised to a fractional power

If you wanted a complex result, you can explicitly ask for one:

py> (-100.0+0j)**0.5 (6.123031769111886e-16+10j)

I see that behaviour is changed in Python 3. Was this discussed before being changed? I see a single sample docstring buried in PEP 3141 that:

"""a**b; should promote to float or complex when necessary."""

but I can't find any discussion of the consequences of this for the majority of users who would be surprised by the unexpected appearance of a "j" inside their numbers.

Nor is there any hint in the docs for pow and ** that they will promote floats to complex:

http://docs.python.org/dev/library/functions.html#pow

http://docs.python.org/dev/library/stdtypes.html#numeric-types-int-float-complex

-- Steven

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