python3 で Decimal を JSON に変換

AWS の DynamoDB では、Decimal という型が使われます。この型を JSON に変換する方法です。

変換できない例

error.py

#! /usr/bin/python
#
import json
from decimal import Decimal

# ----------------------------------------------------------------
aa = [Decimal(3.4),Decimal(5.3)]
json_str = json.dumps(aa)
print(json_str)
# ----------------------------------------------------------------

実行結果

$ ./error.py 
Traceback (most recent call last):
  File "./error.py", line 8, in <module>
    json_str = json.dumps(aa)
  File "/usr/lib/python3.7/json/__init__.py", line 231, in dumps
    return _default_encoder.encode(obj)
  File "/usr/lib/python3.7/json/encoder.py", line 199, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/usr/lib/python3.7/json/encoder.py", line 257, in iterencode
    return _iterencode(o, 0)
  File "/usr/lib/python3.7/json/encoder.py", line 179, in default
    raise TypeError(f'Object of type {o.__class__.__name__} '
TypeError: Object of type Decimal is not JSON serializable

ちゃんと変換できる例

ok.py

#! /usr/bin/python
#
import json
from decimal import Decimal

# ----------------------------------------------------------------
def decimal_default_proc(obj):
	if isinstance(obj, Decimal):
		return float(obj)
	raise TypeError
# ----------------------------------------------------------------
aa = [Decimal(3.4),Decimal(5.3)]
json_str = json.dumps(aa,default=decimal_default_proc)
print(json_str)
# ----------------------------------------------------------------

実行結果

$ ./ok.py 
[3.4, 5.3]

確認したバージョン

$ python --version
Python 3.7.2