| msg93504 - (view) |
Author: Benjamin Peterson (benjamin.peterson) *  |
Date: 2009-10-03 18:59 |
| The docs for 'g' say "This prints the number as a fixed-point number, unless the number is too large." Could you please explain what exactly constitutes "too large"? |
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| msg93507 - (view) |
Author: Mark Dickinson (mark.dickinson) * |
Date: 2009-10-03 20:25 |
| For 'g' formatting (either {} style or %-style) with precision p >= 1, I believe fixed- point is used if and only if the formatted value satisfies 1e-4 <= abs(formatted_value) < 10**precision. Note that the 'formatted value' refers to the value rounded to p significant figures, not the original value of the number being formatted. For example, in '%.6g' % 9.999999e-5, the float rounded to 6 places is 1e-4, so the result is '0.0001', using fixed-point. But '%.6g' % 9.999994e-5 gives '9.99999e-05'. |
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| msg93692 - (view) |
Author: Eric V. Smith (eric.smith) * |
Date: 2009-10-07 12:26 |
| Mark's really the expert here, so I trust his description. Is his description layman-speak enough for the docs? |
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| msg93695 - (view) |
Author: Mark Dickinson (mark.dickinson) * |
Date: 2009-10-07 12:46 |
| That's amusing. The moment I submitted the comment above I remember thinking 'Hmm. I could have explained that better.' I'll try to come up with a doc patch. Stealing the issue from Eric. |
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| msg93752 - (view) |
Author: Mark Dickinson (mark.dickinson) * |
Date: 2009-10-08 17:37 |
| Benjamin, which docs are you looking at here? The ones at: http://docs.python.org/library/stdtypes.html#string-formatting-operations say: """Floating point format. Uses lowercase exponential format if exponent is less than -4 or not less than precision, decimal format otherwise.""" which seem quite specific to me (though it could be made clearly exactly *what* exponent is meant here). |
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| msg93753 - (view) |
Author: Mark Dickinson (mark.dickinson) * |
Date: 2009-10-08 17:39 |
| Ah. Found them. http://docs.python.org/library/string.html#formatspec |
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| msg93754 - (view) |
Author: Mark Dickinson (mark.dickinson) * |
Date: 2009-10-08 18:06 |
| Proposed revision: General format. For a given precision ``p >= 1``, this rounds the number to ``p`` significant digits and then formats the result in either fixed-point format or in scientific notation, depending on its magnitude. The precise rules are as follows: suppose that the result formatted with presentation type ``e`` and precision ``p-1`` would have exponent ``exp``. Then if ``-4 <= exp < p``, the number is formatted as if with presentation type ``f`` and precision chosen in such a way as to give ``p`` significant digits. Otherwise, the number is formatted as if with presentation type ``e`` and precision ``p-1``. A precision of ``0`` is treated as equivalent to a precision of ``1``. |
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| msg93755 - (view) |
Author: Eric V. Smith (eric.smith) * |
Date: 2009-10-08 18:20 |
| That wording is okay with me. Maybe run it by Georg to see if he has any suggestions? Or just check it in and see if anyone complains. But I'm okay with it as-is. Thanks for doing this. Eric. |
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| msg93756 - (view) |
Author: Mark Dickinson (mark.dickinson) * |
Date: 2009-10-08 18:24 |
| Thanks, Eric. I noticed that the description was missing details of what happens to trailing zeros (i.e., they're removed) and what happens to special values: infinities, zeros, nans. Here's the revised revised text: General format. For a given precision ``p >= 1``, this rounds the number to ``p`` significant digits and then formats the result in either fixed-point format or in scientific notation, depending on its magnitude. The precise rules are as follows: suppose that the result formatted with presentation type ``e`` and precision ``p-1`` would have exponent ``exp``. Then if ``-4 <= exp < p``, the number is formatted as if with presentation type ``f`` and precision ``p-1-exp``. Otherwise, the number is formatted as if with presentation type ``e`` and precision ``p-1``. In both cases trailing zeros are removed from the significand, as is the decimal point if there are no remaining digits following it. Postive and negative infinity, positive and negative zero, and nans, are formatted as ``inf``, ``-inf``, ``0``, ``-0`` and ``nan`` respectively. A precision of ``0`` is treated as equivalent to a precision of ``1``. |
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| msg93757 - (view) |
Author: Eric V. Smith (eric.smith) * |
Date: 2009-10-08 18:27 |
| I obviously hadn't thought of those cases, either. This version looks good(er) to me. |
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| msg93760 - (view) |
Author: Georg Brandl (georg.brandl) * |
Date: 2009-10-08 19:52 |
| The "as if with" feels strange, otherwise it is fine with me. |
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| msg93761 - (view) |
Author: Mark Dickinson (mark.dickinson) * |
Date: 2009-10-08 19:58 |
| Okay. I'll replace the 'as if with' with simply 'with'. That doesn't quite meet my favoured level of pedantry any more, but it does read better and it's not going to lead to any confusion. :) |
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| msg93762 - (view) |
Author: R. David Murray (r.david.murray) * |
Date: 2009-10-08 20:03 |
| I think the 'as if with' is probably valid English, but if you want to make it sound better yet retain the same level of pedantry you could instead say, e.g.: "in the same way as presentation type e using precision p-1". |
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| msg93763 - (view) |
Author: Mark Dickinson (mark.dickinson) * |
Date: 2009-10-08 20:08 |
| Thanks Georg and Eric for the feedback! New wording committed in r75289 (trunk), r75290 (py3k), r75291 (release31-maint). Leaving open for the backport to release26-maint. BTW, it's much easier to describe the rules in Python than in prose: def strip_zero_dot(sig): return sig.rstrip('0').rstrip('.') if '.' in sig else sig def gformat(x, p=6): if math.isinf(x) or math.isnan(x) or not x: return '{:.0f}'.format(x) p = max(p, 1) sig, exp = '{:.{}e}'.format(x, p-1).split('e') if -4 <= int(exp) < p: return strip_zero_dot('{:.{}f}'.format(x, p-1-int(exp))) else: return strip_zero_dot(sig) + 'e' + exp |
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| msg94566 - (view) |
Author: Mark Dickinson (mark.dickinson) * |
Date: 2009-10-27 17:07 |
| Looks like this was merged to release26-maint by Georg in r75790. Thanks, Georg! |
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