Python - round a number to significant digits

round_sd.py

from math import log10, floor
from decimal import Decimal as Dec

# Note: the method spec uses a union type float | Dec introduced in Python 3.10
# Remove the type definitions to make it work with earlier versions.

def round_sd(x: float | Dec, sd: int = 3):
    """Round a value to a specified amount of significant digits `sd`.
    """
    return round(x, sd - int(floor(log10(abs(x)))) - 1)

# 
# Examples
# round_sd(1.01235, 3)   ->        1.01
# round_sd(-1.01235, 3)  ->        -1.01
# round_sd(1670.0123, 3)  ->       1670.0
# round_sd(0.00000012345, 3)  ->   1.23e-07
#