Python JSON parse_float=decimal.Decimal not working

Question

I have a string with a floating point number in it, but I can’t get JSON to load it as a decimal.

x = u'{"14": [4.5899999999999999, "susan"]}'
json.loads(x, parse_float = decimal.Decimal)

This returns:

{u'14': [Decimal('4.5899999999999999'), u'susan']}

Any idea how I can make it into the actual “4.59”?

Asked By: victor

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Source

Answer 1

You need to define a function that performs whatever rounding you desire, then uses the altered string to build the Decimal. Your current solution does work perfectly well: it just does exactly what you tell it to, i.e., use the entire string, as opposed to what you desire (and have not told either the code, or us;-).

E.g.:

>>> def doit(s): return decimal.Decimal(str(round(float(s), 2)))
... 
>>> json.loads(x, parse_float=doit)
{u'14': [Decimal('4.59'), u'susan']}
>>>

Answered By: Alex Martelli

Answer 2

You can’t. That number isn’t 4.59, it’s 4.589999999999999999, as far as the json parser knows. You’d need to add some more complicated logic that rounds numbers like that, as a wrapper around decimal.Decimal.

Answered By: inklesspen

Answer 3

I know this is over a decade later, but just to refine the answer from Alex Martin a bit for anyone coming across this as I did on Google. We can simplify his function to:

json.loads(x, parse_float=lambda x: round(decimal.Decimal(x),2))

The reason is that the value passed into the parse_float callable by json.loads is already a str. Also, this code would not truncate the input value at 64 bits as his would (parsing to a float first will truncate at 64 bit precision).

Obviously, you could use a function rather than a lambda too. I just used the lambda here for brevity.

Answered By: UglyBug

Python JSON parse_float=decimal.Decimal not working

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