在pandas 0.10.1上使用pandas.read_csv指定dtype float32

Question

I'm attempting to read a simple space-separated file with pandas read_csv method. However, pandas doesn't seem to be obeying my dtype argument. Maybe I'm incorrectly specifying it?

I've distilled down my somewhat complicated call to read_csv to this simple test case. I'm actually using the converters argument in my 'real' scenario but I removed this for simplicity.

Below is my ipython session:

>>> cat test.out
a b
0.76398 0.81394
0.32136 0.91063
>>> import pandas
>>> import numpy
>>> x = pandas.read_csv('test.out', dtype={'a': numpy.float32}, delim_whitespace=True)
>>> x
         a        b
0  0.76398  0.81394
1  0.32136  0.91063
>>> x.a.dtype
dtype('float64')

I've also tried this using this with a dtype of numpy.int32 or numpy.int64 . These choices result in an exception:

AttributeError: 'NoneType' object has no attribute 'dtype'

I'm assuming the AttributeError is because pandas will not automatically try to convert/truncate the float values into an integer?

I'm running on a 32-bit machine with a 32-bit version of Python.

>>> !uname -a
Linux ubuntu 3.0.0-13-generic #22-Ubuntu SMP Wed Nov 2 13:25:36 UTC 2011 i686 i686 i386 GNU/Linux
>>> import platform
>>> platform.architecture()
('32bit', 'ELF')
>>> pandas.__version__
'0.10.1'

Answer 1

0.10.1 doesn't really support float32 very much

see this http://pandas.pydata.org/pandas-docs/dev/whatsnew.html#dtype-specification

you can do this in 0.11 like this:

# dont' use dtype converters explicity for the columns you care about
# they will be converted to float64 if possible, or object if they cannot
df = pd.read_csv('test.csv'.....)

#### this is optional and related to the issue you posted ####
# force anything that is not a numeric to nan
# columns are the list of columns that you are interesetd in
df[columns] = df[columns].convert_objects(convert_numeric=True)


    # astype
    df[columns] = df[columns].astype('float32')

see http://pandas.pydata.org/pandas-docs/dev/basics.html#object-conversion

Its not as efficient as doing it directly in read_csv (but that requires
 some low-level changes)

I have confirmed that with 0.11-dev, this DOES work (on 32-bit and 64-bit, results are the same)

In [5]: x = pd.read_csv(StringIO.StringIO(data), dtype={'a': np.float32}, delim_whitespace=True)

In [6]: x
Out[6]: 
         a        b
0  0.76398  0.81394
1  0.32136  0.91063

In [7]: x.dtypes
Out[7]: 
a    float32
b    float64
dtype: object

In [8]: pd.__version__
Out[8]: '0.11.0.dev-385ff82'

In [9]: quit()
vagrant@precise32:~/pandas$ uname -a
Linux precise32 3.2.0-23-generic-pae #36-Ubuntu SMP Tue Apr 10 22:19:09 UTC 2012 i686 i686 i386 GNU/Linux

Answer 2

In [22]: df.a.dtype = pd.np.float32

In [23]: df.a.dtype
Out[23]: dtype('float32')

the above works fine for me under pandas 0.10.1