python - float64 with pandas to_csv

Question

I'm reading a CSV with float numbers like this:

Bob,0.085
Alice,0.005

And import into a dataframe, and write this dataframe to a new place

df = pd.read_csv(orig)
df.to_csv(pandasfile)

Now this pandasfile has:

Bob,0.085000000000000006
Alice,0.0050000000000000001

What happen? maybe I have to cast to a different type like float32 or something?

Im using pandas 0.9.0 and numpy 1.6.2.

Answer 1

As mentioned in the comments, it is a general floating point problem.

However you can use the float_format key word of to_csv to hide it:

df.to_csv('pandasfile.csv', float_format='%.3f')

or, if you don't want 0.0001 to be rounded to zero:

df.to_csv('pandasfile.csv', float_format='%g')

will give you:

Bob,0.085
Alice,0.005

in your output file.

For an explanation of %g, see Format Specification Mini-Language.