How do I convert a string into a decimal number for arithmetic manipulation in Python?

Question

Similar posts such as the following do not answer my question. Convert a string to integer with decimal in Python

Consider the following Python code.

>>> import decimal
>>> s = '23.456'
>>> d = decimal.Decimal(s)
>>> d
Decimal('23.456')           # How do I represent this as simply 23.456?
>>> d - 1
22                          # How do I obtain the output to be 22.456?

How do I convert a string to a decimal number, so I am able to perform arithmetic functions on it and obtain an output with the correct precision?

Answer 1

If you want to stay in decimal numbers, safest is to convert everything:

>>> s = '23.456'
>>> d = decimal.Decimal(s)

>>> d - decimal.Decimal('1')
Decimal('22.456')
>>> d - decimal.Decimal('1.0')
Decimal('22.456')

In Python 2.7, there's an implicit conversion for integers, but not floats.

>>> d - 1
Decimal('22.456')
>>> d - 1.0
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for -: 'Decimal' and 'float'

Answer 2

Is the Decimal required for your computations? The Decimal fixed point and floating point arithmetic doc outlines their differences. If not, you could just do

 d = float('23.456')
 d
 23.456

 d - 1
 22.456

Oddly enough re Decimal , I get this interactively

d = decimal.Decimal('23.456')

d
Decimal('23.456')
d - 1
Decimal('22.456')

But when I print it, I get the values

print d
23.456
print d-1
22.456

Answer 3

Use the bultin float function:

>>> d = float('23.456')
>>> d
23.456
>>> d - 1
22.456

See the docs here: http://docs.python.org/library/functions.html#float

Answer 4

My Python seems to do it differently:

>>> s = '23.456'
>>> d = decimal.Decimal(s)
>>> d
Decimal('23.456')
>>> d-1
Decimal('22.456')

What version/OS are you using?

Answer 5

Are you specifically TRYING specifically to use the Decimal arbitrary precision library or are you just struggling to convert a string to a Python float?

If you are TRYING to use Decimal:

>>> import decimal
>>> s1='23.456'
>>> s2='1.0'
>>> decimal.Decimal(s1) - decimal.Decimal(s2)
Decimal('22.456')
>>> s1='23.456'
>>> s2='1'
>>> decimal.Decimal(s1) - decimal.Decimal(s2)
Decimal('22.456')

Or, what I think is more likely, you are trying to just convert a string to a Python floating point value:

>>> s1='23.456'
>>> s2='1'
>>> float(s1)-float(s2)
22.456
>>> float(s1)-1
22.456
>>> float(s1)-1.0
22.456

Answer 6

If using float, when the number gets too large -- x = 29345678.91 for example -- you get results that you might not expect. In this case, float(x) becomes 2.934567891E7 which seems undesirable especially if working with financial numbers.

Answer 7

In python, to convert a value string to float just do it:

num = "29.0"
print (float(num))

To convert string to decimal

from decimal import Decimal
num = "29.0"
print (Decimal(num))