Dataframe.mean with numeric_only=False results in error with strings

[smangham]

Code Sample, a copy-pastable example if possible

import pandas as pd
df = pd.DataFrame({
    'A': [0, 1, 2], 'B': ['a', 'b', 'c'], 'C': [4, 5, 6]
})
df.mean(axis=0, numeric_only=False, skipna=False)

Problem description

Instead of outputting a NaN for a non-numeric column of strings when trying to take the mean, it instead throws TypeError: could not convert string 'abc' to float.

Expected Output

I would expect this to output a series with values [1, NaN, 5].

My work-around is currently df.apply(pd.to_numeric, args=['coerce']).mean(axis=0, skipna=False), which outputs the expected result.

Output of pd.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 3.6.7.final.0 python-bits: 64 OS: Linux OS-release: 4.15.0-51-generic machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: en_GB.UTF-8 LOCALE: en_GB.UTF-8

pandas: 0.24.2
pytest: None
pip: 19.1.1
setuptools: 40.6.3
Cython: 0.29.2
numpy: 1.16.0
scipy: 1.2.1
pyarrow: 0.12.0
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.7.5
pytz: 2018.9
blosc: None
bottleneck: None
tables: 3.4.4
numexpr: 2.6.9
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml.etree: None
bs4: None
html5lib: None
sqlalchemy: 1.2.16
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None

[TomAugspurger]

I'm not sure I agree with NaN being the expected output for the mean of string columns.

[smangham]

Fair enough, though it seems to me within the context of numeric_only=False it would be a reasonable response, as the average of a non-numeric column is not (usually) a number.

I was expecting it to mostly be useful for making sure the output series length matched the column index length, taking non-numeric columns into account.

Is it the case that numeric_only is instead really intended for custom types where you've defined addition and float conversion yourself?

[TomAugspurger]

On Tue, Jun 18, 2019 at 11:27 AM Sam Mangham ***@***.***> wrote: Fair enough, though it seems to me within the context of numeric_only=False it would be a reasonable response, as the average of a non-numeric column is not (usually) a number.

I interpret numeric_only as an option to exclude non-numeric columns, so that the aggfunc isn't called on them. I don't think it would be appropriate for `np.mean(string_series)`, called via `DataFrame.mean(numeric_only=False)`, to differ. IMO an exception is appropriate for both.

I was expecting it to mostly be useful for making sure the output series length matched the column index length, taking non-numeric columns into account.

I'd recommend reindexing afterwards.

Is it the case that numeric_only is instead really intended for custom types where you've defined addition and float conversion yourself?

It's a bit hazy, but something like that. Extension arrays, for example, indicate whether they should be considered numeric.

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[smangham]

Would elaborating on the documentation to make the behaviour of numeric_only=False more explicit (i.e. non-numeric dtypes that do not support both the + and / operators will result in an exception) be a reasonable response?

[jreback]

yes and/or doc-string additions

[sergeny]

We have found a much bigger problem:

pd.Series(['1', '2', '3', '4']).mean() returns 308.5 instead of 2.5 (pandas 0.23.4).
This is because it divides 1234 by 4.

[TomAugspurger]

@sergeny that sounds like a different issue.

[jsignell]

It seems to me that a good behavior would be to just include a bit more information on the error. Maybe something like:

TypeError: Could not convert ['abc'] to numeric. Select only valid columns before calling the reduction or drop nuisance columns with 'numeric_only=True'.