Arbitrary precision of square roots

I was quite disappointed when decimal.Decimal(math.sqrt(2)) yielded

Decimal('1.4142135623730951454746218587388284504413604736328125')

and the digits after the 15th decimal place turned out wrong. (Despite happily giving you much more than 15 digits!)

How can I get the first m correct digits in the decimal expansion of sqrt(n) in Python?

Answer

Use the sqrt method on Decimal

>>> from decimal import *
>>> getcontext().prec = 100  # Change the precision
>>> Decimal(2).sqrt()
Decimal('1.414213562373095048801688724209698078569671875376948073176679737990732478462107038850387534327641573')