Round off dict values to 2 decimals

Question

I\'m having a hard time rounding off values in dicts. What I have is a list of dicts like this:

y = [{\'a\': 80.0, \'b\': 0.0786235, \'c\': 10.0, \'d\': 10.6         


        

Answer 1

JSONEncoder uses repr, and repr prints floats with all their available precision. The only possible solutions are to inherit from JSONEncoder and round while actually converting the values to a string (which implies to copy and adapt some code from the json.encoder module), or else wrap the floats into your own type RoundedFloat and register a serializer for that. Also note that repr's behaviour depends on the Python version used.

As often with non-obvious behaviour, the observation during debugging can trick you: print uses str(), and str() rounds at a certain point, unlike repr() which shows the naked ugliness of floating point maths.

The proof is in the code:

>>> class F(float):
...     def __str__(self): return "str"
...     def __repr__(self): return "repr"
...     
... 
>>> print F(1)
str
>>> F(1)
repr
>>> repr(1-1e-15)
'0.999999999999999'
>>> str(1-1e-15)
'1.0'