Question

I have a dictionary that I want to write to a csv file, but the floats in the dictionary are rounded off when I write them to the file. I want to keep the maximum precision.

Where does the rounding occur and how can I prevent it?

What I did

I followed the DictWriter example here and I’m running Python 2.6.1 on Mac (10.6 – Snow Leopard).

---

# my import statements
import sys
import csv

Here is what my dictionary (d) contains:

>>> d = runtime.__dict__
>>> d
{'time_final': 1323494016.8556759,
'time_init': 1323493818.0042379,
'time_lapsed': 198.85143804550171}

The values are indeed floats:

>>> type(runtime.time_init)
<type 'float'>

Then I setup my writer and write the header and values:

f = open(log_filename,'w')
fieldnames = ('time_init', 'time_final', 'time_lapsed')
myWriter = csv.DictWriter(f, fieldnames=fieldnames)
headers = dict( (n,n) for n in fieldnames )
myWriter.writerow(headers)
myWriter.writerow(d)
f.close()

But when I look in the output file, I get rounded numbers (i.e., floats):

time_init,time_final,time_lapsed
1323493818.0,1323494016.86,198.851438046

< EOF >

Answer 1

It looks like csv is using float.__str__ rather than float.__repr__:

>>> print repr(1323494016.855676)
1323494016.855676
>>> print str(1323494016.855676)
1323494016.86

Looking at the csv source, this appears to be a hardwired behavior. A workaround is to cast all of the float values to their repr before csv gets to it. Use something like: d = dict((k, repr(v)) for k, v in d.items()).

Here’s a worked-out example:

import sys, csv

d = {'time_final': 1323494016.8556759,
     'time_init': 1323493818.0042379,
     'time_lapsed': 198.85143804550171
}

d = dict((k, repr(v)) for k, v in d.items())

fieldnames = ('time_init', 'time_final', 'time_lapsed')
myWriter = csv.DictWriter(sys.stdout, fieldnames=fieldnames)
headers = dict( (n,n) for n in fieldnames )
myWriter.writerow(headers)
myWriter.writerow(d)

This code produces the following output:

time_init,time_final,time_lapsed
1323493818.0042379,1323494016.8556759,198.85143804550171

A more refined approach will take care to only make replacements for floats:

d = dict((k, (repr(v) if isinstance(v, float) else str(v))) for k, v in d.items())

Note, I’ve just fixed this issue for Py2.7.3, so it shouldn’t be a problem in the future. See http://hg.python.org/cpython/rev/bf7329190ca6