[Solved] what is the ‘decimal.getcontext().copy()’ mean [closed]

Question

While I agree with many of the other comments here I figured I’d throw a bone.

The decimal module is a utility for doing arithmetic on numbers of arbitrary precision. This is used in some scientific contexts to do calculations on things that suffer from round-off errors. The classical example would be calculting the 9999th digit of pi. You just can’t do that with floats.

The decimal library uses the concept of a “context” to decide how much precision you really want. Since the more precision you want the slower the calculations will run. The decimal.getcontext() part returns the default context. But through a crazy “gotcha” (that I’ve actually fallen prey to) this is actually just a pointer. So if you were to change the context with a decimal.setcontext('foo') you would actually change a as well. Handily the module has a .copy() method.

The original programmer probably did this to save a copy of the context for later use. Perhaps s/he wanted to perform a part of the computation at a lower precision to save time and then wanted to perform the difficult part at a higher precision.

Although in python > 2.5 this is best done with a context manager like ‘with’

from decimal import localcontext

with localcontext() as ctx:
    ctx.prec -= 5   # Perform a low precision calculation
    s = calculate_something()
s = +s  # Round the final result back to the default precision

But without the rest of the relevant code my guess is no better than yours.

While I have worked with the decimal module before all of this is easily found in the python docs here. Try to go there and poke around first for questions like this … if you can figure it out yourself you’ll remember it longer. And you’ll get that warm feeling in your head that the rest of us feel when we answer questions.

Accepted Answer