Decimal math

Summary

Does decimal math automatically.

Adds the required import of the the decimal module automatically.

Can be used either as an import hook or as a custom codec.

Source code

On Python-ideas, the following question was recently asked

Wouldn’t it be possible to have something along the lines of:

from decimal import TreatFloatsAsDecimal
@TreatFloatsAsDecimal
a = 0.1  # These are all now decimals
b = 0.2
c = 0.3
a + b == c # This now works

The answer is yes, using either an import hook or a custom encoding already implemented as an example. Here we show it in action using the ideas repl:

> python -m ideas -t decimal_math
from decimal import Decimal

Ideas Console version 0.0.30. [Python version: 3.7.9]

>>> 0.1 + 0.2 == 0.3
True
>>> 0.1 * 10 == 1
True
>>> 0.1
Decimal('0.1')
>>> 0.1 + 0.100
Decimal('0.200')

Warning

In the description below, setting PYTHONPATH in a terminal will only work as described if you install ideas in a normal (not virtual) environment.

Import hook

To use and test it easily as an import hook, you can do the following.

1. Create a file named usercustomize.py containing the following:

from ideas.examples import decimal_math
decimal_math.add_hook()

2. Assuming you are not in virtual environment, set the PYTHONPATH environment variable to the path where usercustomize.py is found. On Windows, this is most done by navigating where this file is found and typing: set PYTHONPATH=%cd%

You can now invoke your module doing the following:

python -c "import my_script"

Custom codec

Warning

Starting with Python 3.9, encodings cannot have an hyphen in their name such as:

# coding: decimal-math

Instead, they need to be normalized to using an underscore, as in:

# coding: decimal_math

To use and test it easily as a custom codec, you can do the following.

1. Create a file named usercustomize.py containing the following:

from ideas.examples import decimal_math
decimal_math.register()

2. Assuming you are not in virtual environment, set the PYTHONPATH environment variable to the path where usercustomize.py is found. On Windows, this is most done by navigating where this file is found and typing: set PYTHONPATH=%cd%

3. At the top of the module you wish to be run with the special codec, add the following two lines:

   # coding: decimal_math
   from decimal import Decimal
   

You can now invoke your module doing the following:

python my_script.py

This replaces any explicit float by a Decimal. It can be used either as a custom codec or import hook.

ideas.examples.decimal_math.add_hook(**_kwargs)

Creates and automatically adds the import hook in sys.meta_path

ideas.examples.decimal_math.source_init()

Adds required import

ideas.examples.decimal_math.transform_source(source, **_kwargs)

Simple transformation: replaces any explicit float by a Decimal.

By defining this function, we can also make use of Ideas’ console.