pandas.dataframe.values floating number changes automatically

[snowflake01986]

Code Sample, a copy-pastable example if possible

test.xlsx

# Your code here
import pandas as pd
df = pd.read_excel('test.xlsx', 'test1', header=0, index_col=None)
print(df.values)

Problem description

I loaded a pandas dataframe from the attached test.xlsx, of which the content is as follows:
name c1 c2
0 r1 0.014 0.000-0.054
1 r2 0.984 0.025-1.785
As we can see, the c1 columns has been well rounded. For some reasons, I needed only the values numpy.darray, but the floating precision expands undesirably and changes a little as follows:

array([['r1', 0.013999999999999999, '0.000-0.054'],
['r2', 0.9840000000000001, '0.025-1.785']], dtype=object)

what is odd is that I have some other similar tables which resulted in the expected results. So this really beyond me.

Expected Output

What I wanted was the perfectly correspondance of dataframe:
array([['r1', 0.0134, '0.000-0.054'],
['r2', 0.984, '0.025-1.785']], dtype=object)

Output of pd.show_versions()

INSTALLED VERSIONS

commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Linux
OS-release: 4.8.0-59-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: zh_CN.utf8
LANG: en_US.UTF-8
LOCALE: zh_CN.UTF-8

pandas: 0.19.1
nose: None
pip: 10.0.1
setuptools: 26.1.1
Cython: None
numpy: 1.13.3
scipy: 0.18.1
statsmodels: None
xarray: None
IPython: 6.2.1
sphinx: None
patsy: None
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: 2.1.0
openpyxl: None
xlrd: 1.0.0
xlwt: 1.3.0
xlsxwriter: 0.7.3
lxml: None
bs4: 4.5.1
html5lib: 1.0b10
httplib2: 0.9.1
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.8
boto: None
pandas_datareader: None

[gfyoung]

I suspect the discrepancies are rooted in the Excel engine. Reading this as a CSV does not have that rounding problem as you describe.

[gfyoung]

Actually, that assessment was incorrect. The issue persists even with CSV. The crux of the problem actually lies with our internal converter to float (or numeric in general):

import pandas.util.testing as tm
import pandas._libs.lib as lib
import numpy as np

inp = np.array(["0.014", "0.984"], dtype=object)
exp = np.array([0.014, 0.984])

tm.assert_numpy_array_equal(lib.maybe_convert_numeric(inp, set(), False), exp)
...
AssertionError: numpy array are different

numpy array values are different (100.0 %)
[left]:  [0.013999999999999999, 0.9840000000000001]
[right]: [0.014, 0.984]

[david-liu-brattle-1]

@gfyoung
The string parsing functions seem to call a custom built xstrtod function

pandas/pandas/_libs/src/parser/tokenizer.c

Lines 1532 to 1534 in bb43726

	double xstrtod(const char *str, char **endptr, char decimal, char sci,
	char tsep, int skip_trailing) {
	double number;

which does a fine job of evaluating the string but the issue here is it's not evaluating it exactly as python (or numpy) is evaluating it. float('0.014')==0.014==np.fromstring(b'0.014',sep=' ')[0] but the xstrtod('0.014') != 0.014. For consistency's sake I think it makes sense that a number read in by pandas as string should be evaluated and written back out as the same number. (currently 0.014 is written back out as 0.0139999999 after being evaluated). It's a fluke that this issue isn't being picked up by any of the tests. For example, if "0.014" would make the following fail if it in the array:

pandas/pandas/tests/dtypes/test_inference.py

Lines 398 to 405 in bb43726

	def test_maybe_convert_numeric_post_floatify_nan(self, coerce):
	# see gh-13314
	data = np.array(['1.200', '-999.000', '4.500'], dtype=object)
	expected = np.array([1.2, np.nan, 4.5], dtype=np.float64)
	nan_values = {-999, -999.0}
	out = lib.maybe_convert_numeric(data, nan_values, coerce)
	tm.assert_numpy_array_equal(out, expected)

This seems like a fairly straightforward fix of replacing xstrtod with the Python float evaluator PyOS_string_to_double from Python.h, unless there is a good reason to stick with the original xstrtod?

As a side note, the almost identical xstrtod is defined again in parser_helper.h, which seems like an oversight.

pandas/pandas/_libs/src/parse_helper.h

Lines 148 to 151 in bb43726

	static double xstrtod(const char *str, char **endptr, char decimal, char sci,
	int skip_trailing, int *maybe_int) {
	double number;
	int exponent;

[gfyoung]

As a side note, the almost identical xstrtod is defined again in parser_helper.h, which seems like an oversight.

Weird...I think it would be good to see if we could unify the two.

This seems like a fairly straightforward fix of replacing xstrtod with the Python float evaluator PyOS_string_to_double from Python.h, unless there is a good reason to stick with the original xstrtod

I'm not sure what the reason was implementing our own. However, I would encourage you to investigate the consequences of doing so, both from an accuracy and performance perspective.

[seb-emmot]

Have there been any progress on this issue?
I experience the same issue when importing floats from CSV data. This makes it very difficult to verify that my data transform operations work as expected.

I'm using Pandas 1.1.2

[seb-emmot]

Have there been any progress on this issue?
I experience the same issue when importing floats from CSV data. This makes it very difficult to verify that my data transform operations work as expected.

I'm using Pandas 1.1.2

It seems like I found a solution for this, at least when using the read_csv functionality. float_precision='round_trip' as argument to the read_csv function. Based on https://stackoverflow.com/questions/36909368/precision-lost-while-using-read-csv-in-pandas

Not sure if this solves the original issue that @snowflake01986 had though.