PEP proposal for round(x,n) enhancement

Christopher Smith csmith at
Mon Sep 10 23:49:07 CEST 2001

Dear List,

It was suggested that I see what type of interest there might be in 
this idea before posting it as a PEP.

I would like to see an additional syntax added to the round function.

At present the syntax for round is round(x,n) where n can be a float or 
integer, (positive or negative) that indicates the digit to which the 
number x should be rounded as measured from the one's digit: 
round(12345,-2) returns 12300 while round(1.2345,2) returns 1.23.  

The enhancement that I would like to see is the ability to indicate the 
number of digits counting *from the 1st non-zero digit* that should be 
reported.  These digits are often called the significant digits.  Both 
12345 and 1.2345 when rounded to the 3rd significant digit are 12300 
and 1.23 but instead of having to inspect the numbers to figure out 
what digit to round them to, an argument that would be interpreted as 
"the 3rd digit counting from the first non-zero digit" would do the job 

How to add this syntax to the round function:

round(x,n) will test to see if there is a decimal portion to n.  If 
there is, n will be multiplied by 10 and the integer of the resulting 
value will be the number of digits to which x will be rounded.  This 
can be simulated with the function Round as shown below:

from math import floor,log10
def Round(x,n):
	if n%1:
		return round(x,sigdig-1-floor(log10(abs(x))))
		return round(x,n)
Example input/output:
>>> print Round(1.23456789019,1.1) #11 digits
>>> print Round(12345,.3)
>>> print Round(1.2345,.3)

Anticipated problems

-the floor() portion of the solution returns a floating point value;
when implemented, this could be changed to int(floor()) to avoid 
coercion issues.

-Using the syntax as stated above is the simplest way I can think to 
allow for an arbitrary number of digits to be specified.  (The simplest 
method which would allow for the range 1-9 would be to use the 10ths 
place only.)  The only quirky thing that would be introduced with the 
present proposal would be that to specify a multiple of 10 you would 
have to make n=1.0x or 2.0x where 'x' specifies some digit in the 
1/100ths place to create a non-zero decimal portion to the number.  
This x would be lost in the conversion int(10*n).  If someone were 
rounding their data, it is doubtful that they would be rounding to more 
than 5 digits so this issue wouldn't arise often.

Open for feedback :-)


Christopher P. Smith
The Blake School
Minneapolis, MN

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