NB: this question is about significant figures. It is not a question about "digits after the decimal point" or anything like that.
EDIT: This question is not a duplicate of Significant figures in the decimal module. The two questions are asking about entirely different problems. I want to know why the function about does not return the desired value for a specific input. None of the answers to Significant figures in the decimal module address this question.
The following function is supposed to return a string representation of a float with the specified number of significant figures:
import decimal
def to_sigfigs(value, sigfigs):
return str(decimal.Context(prec=sigfigs).create_decimal(value))
At first glance, it seems to work:
print to_sigfigs(0.000003141592653589793, 5)
# 0.0000031416
print to_sigfigs(0.000001, 5)
# 0.0000010000
print to_sigfigs(3.141592653589793, 5)
# 3.1416
...but
print to_sigfigs(1.0, 5)
# 1
The desired output for the last expression (IOW, the 5-significant figure representation of 1.0) is the string '1.0000'. The actual output is the string '1'.
Am I misunderstanding something or is this a bug in decimal?
Is it a bug? I don't know, I don't think the documentation is tight enough to make that determination. It certainly is a surprising result.
It is possible to fix your own function with a little more logic.
def to_sigfigs(value, sigfigs):
sign, digits, exponent = decimal.Context(prec=sigfigs).create_decimal(value).as_tuple()
if len(digits) < sigfigs:
missing = sigfigs - len(digits)
digits = digits + (0,) * missing
exponent -= missing
return str(decimal.Decimal((sign, digits, exponent)))
The precision of a context is a maximum precision; if an operation would produce a Decimal with less digits than the context's precision, it is not padded out to the context's precision.
When you call to_sigfigs(0.000001, 5), 0.000001 already has some rounding error due to the conversion from source code to binary floating point. It's actually 9.99999999999999954748111825886258685613938723690807819366455078125E-7. Rounding that to 5 significant figures gives decimal.Decimal("0.0000010000").
On the other hand, 1 is exactly representable in binary floating point, so 1.0 is exactly 1. Since only 1 digit is needed to represent this in decimal, the context's precision doesn't require any rounding, and you get a 1-digit Decimal result.
来源：https://stackoverflow.com/questions/34211451/why-doesnt-python-decimal-library-return-the-specified-number-of-signficant-fig