JSON Decoding error with Python2 + Anaconda: Decimal('0') is not JSON serializable

[drewbanin]

Issue

Issue description

The dbt test command can fail with an error like:

Decimal('0') is not JSON serializable

dbt explicitly implements a Decimal-aware JSON serializer, but it appears that there is more than one type of Decimal at play here. The agate library will optionally include the cdecimal version of Decimal if it is available (code here).

dbt does a check here that looks like:

        if isinstance(obj, Decimal):
            return float(obj)
        return super(JSONEncoder, self).default(obj)

here, Decimal is decimal.Decimal, but it could be that obj is of type cdecimal.Decimal. This leads to a confusing error.

Instead, dbt should check if the obj is of type decimal.Decimal or cdecimal.Decimal.

I think the Anaconda part of this is crucial, as I'm unsure how else someone might have cdecimal installed in their system. You can install it manually with pip install m3-cdecimal.

Results

"/Users/xxx/anaconda2/lib/python2.7/json/encoder.py", line 207, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/Users/xxx/anaconda2/lib/python2.7/json/encoder.py", line 270, in iterencode
    return _iterencode(o, 0)
  File "/Users/xxx/anaconda2/lib/python2.7/site-packages/dbt/utils.py", line 481, in default
    return super(JSONEncoder, self).default(obj)
  File "/Users/xxx/anaconda2/lib/python2.7/json/encoder.py", line 184, in default
    raise TypeError(repr(o) + " is not JSON serializable")
TypeError: Decimal('0') is not JSON serializable

System information

The output of dbt --version:

0.12.1

The operating system you're running on: OSX

The python version you're using (probably the output of python --version)
2.7.15 installed via Anaconda.