from decimal import *
Pi=Decimal(3.141592653589793238462643383279502884197169399373)
print(Pi)
Actual output:
3.141592653589793115997963468544185161590576171875
Output should be:
3.141592653589793238462643383279502884197169399373
Why does the value change?
You're passing in a floating-point number to the Decimal constructor, and floating-point numbers are inherently imprecise (see also the Python manual).
To pass in a precise number to the Decimal constructor, pass it in as a string.
>>> from decimal import Decimal
# bad
>>> Decimal(3.141592653589793238462643383279502884197169399373)
Decimal('3.141592653589793115997963468544185161590576171875')
# good
>>> Decimal('3.141592653589793238462643383279502884197169399373')
Decimal('3.141592653589793238462643383279502884197169399373')
If you have a floating-point variable, you can cast it to a string first, then to a Decimal to avoid some floating-point imprecision:
>>> a = 0.1 + 0.2
0.30000000000000004
>>> Decimal(a)
Decimal('0.3000000000000000444089209850062616169452667236328125')
>>> Decimal(str(a))
Decimal('0.30000000000000004')
>>>
If you need full precision, just work with Decimals all the way:
>>> Decimal("0.1") + Decimal("0.2")
Decimal('0.3')
You should pass a string to Decimal(), not a float, floats are imprecise to begin with. Also, note the following from the Python docs
Unlike hardware based binary floating point, the decimal module has a user alterable precision (defaulting to 28 places) which can be as large as needed for a given problem
from decimal import *
getcontext().prec = 100 #precision
pi = Decimal("3.141592653589793238462643383279502884197169399373")
print(pi) #3.141592653589793238462643383279502884197169399373