json.encoder.FLOAT_REPR changed but no effect

Question

I am trying to make my JSON encoder dump floats with only 2 decimal precision. So '2.241' becomes '2.24'

I've read in this answer by Alex Martelli that you can overwrite the default FLOAT_REPR of json.encoder. I have tried the following:

>>> import json
>>> json.encoder.FLOAT_REPR = lambda o: format(o, '.2f')

But I dont get the same results:

>>> json.dumps(2.241)
'2.241'

And I can even verify the FLOAT_REPR is changed:

>>> print json.encoder.FLOAT_REPR
<function <lambda> at 0xb....>

And works as expected:

>>> json.encoder.FLOAT_REPR(2.241)
2.24

Why is the built-in JSON module not using the FLOAT_REPR when I can see that it has been overwritten and the solution should be working according to Alex Martelli?

I have tested this on two different computers, both running Python 2.7.6 on Ubuntu 14.0.4.

I found a comment in the mentioned question stating that "onkey patching json.encoder.FLOAT_REPR does not work if your Python runtime uses a C version of the JSON module" this can explain why it does not work. — Johannes valbjørn, Sep 13 '15 at 16:46

score 3 · Accepted Answer · edited Jan 25 '19 at 21:16

3

The problem occurs beause of the CPython speedups done by the c_make_encoder in json.encoder.

If you set it to None then the json.encoder.FLOAT_REPR trick works as explained in this answer on the same question:

The monkey-patch trick does not seem to work with the original simplejson module if the C speedups are installed:

My implementation can be seen in the jsonplustypes repository.

Note: This solution doesn't work on python 3.6+

edited Jan 25 '19 at 21:16

roschach

8,390
14
74
124

answered Sep 13 '15 at 20:35

Johannes valbjørn

398
3
13

1

Btw this "problem" exist both in builtin json and simplejson due to similar c speedup implementation – Johannes valbjørn Sep 13 '15 at 22:11
1

Not working in Python 3.6. – Nico Schlömer Jun 05 '18 at 12:58
edited the post to add the info and to bump the question. – Jean-François Fabre Jun 05 '18 at 13:01
@NicoSchlömer try this answer from https://stackoverflow.com/questions/1447287/format-floats-with-standard-json-module: https://stackoverflow.com/a/1733105/6451573 – Jean-François Fabre Jun 05 '18 at 22:13
For a simple monkey-patch solution that works with Python 3, see my answer at https://stackoverflow.com/questions/54370322/ – proski Sep 04 '21 at 14:53

json.encoder.FLOAT_REPR changed but no effect

1 Answers1

Linked