[Python-Dev] Two proposed changes to float formatting (original) (raw)

Eric Smith eric at trueblade.com
Sun Apr 26 18:59:14 CEST 2009

Mark Dickinson wrote:

I'd like to propose two minor changes to float and complex formatting, for 3.1. I don't think either change should prove particularly disruptive.

(1) Currently, '%f' formatting automatically changes to '%g' formatting for numbers larger than 1e50. For example: ... I propose removing this feature for 3.1

I'm +1 on this.

I have a suspicion that at least part of the motivation for the '%f' -> '%g' switch is that it means the implementation can use a fixed-size buffer. But Eric has fixed this (in 3.1, at least) and the buffer is now dynamically allocated, so this isn't a concern any more.

I agree that this is a big part of the reason it was done. There's still some work to be done in the fallback code which we use if we can't use Gay's implementation of _Py_dg_dtoa. But it's reasonably easy to calculate the maximum buffer size needed given the precision, for passing on to PyOS_snprintf. (At least I think that sentence is true, I'll very with Mark offline).

Other reasons not to switch from '%f' to '%g' in this way:

- the change isn't gentle: as you go over the 1e50 boundary, the number of significant digits produced suddenly changes from 56 to 6; it would make more sense to me if it stayed fixed at 56 sig digits for numbers larger than 1e50.

This is the big reason for me.

- float formatting is already quite complicated enough; no need to add to the mental complexity

And this, too.

(2) complex str and repr don't behave like float str and repr, in that the float version always adds a trailing '.0' (unless there's an exponent), but the complex version doesn't: ... I propose changing the complex str and repr to behave like the float version. That is, repr(4. + 10.j) should be "(4.0 + 10.0j)" rather than "(4+10j)".

I'm +0.5 on this. I'd probably be +1 if I were a big complex user. Also, I'm not sure about the spaces around the sign. If we do want the spaces there, we can get rid of Py_DTSF_SIGN, since that's the only place it's used and we won't be able to use it for complex going forward.

Eric.

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