python - 为什么 %e 在格式字符串中的行为与 %g 不同？

Question

我已经在 Python2、Python3 和 C 中尝试过这个:

为什么这些格式字符串返回的数字精度不同？

>>> "%.3e" % 123456789
'1.235e+08' 
>>> "%.3g" % 123456789
'1.23e+08' 

Answer 1

来自Python 3.7 documentation :

'e':

Exponent notation. Prints the number in scientific notation using the letter ‘e’ to indicate the exponent. The default precision is 6.

'g':

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 with presentation type 'f' and precision p-1-exp. Otherwise, the number is formatted with presentation type 'e' and precision p-1. In both cases insignificant trailing zeros are removed from the significand, and the decimal point is also removed if there are no remaining digits following it, unless the '#' option is used.

Positive and negative infinity, positive and negative zero, and nans, are formatted as inf, -inf, 0, -0 and nan respectively, regardless of the precision.

A precision of 0 is treated as equivalent to a precision of 1. The default precision is 6.

具有替代值:

>>> "%.3e" % 123
'1.230e+02'
>>> "%.3g" % 123
'123'
>>> "%.3e" % 1234
'1.234e+03'
>>> "%.3g" % 1234
'1.23e+03'

区别显然在于如何指定精度。 g似乎使用精度作为精度的正常定义，而 e使用小数点后 的位数。