BUG 0.20.0rc1: Empty series for `var`/`std` for frame with non-numeric columns

[jcrist]

In [1]: import pandas as pd

In [2]: pd.__version__
Out[2]: '0.20.0rc1'

In [3]: df = pd.DataFrame({'int': [1, 2, 3, 4],
   ...:                    'float': [1., 2., 3., 4.],
   ...:                    'str': ['a', 'b', 'c', 'd']})

In [4]: df.mean()
Out[4]:
float    2.5
int      2.5
dtype: float64

In [5]: df.std()
Out[5]:
Series([], dtype: float64)

In [6]: df.var()
Out[6]:
Series([], dtype: float64)

[TomAugspurger]

This is strange... I can reproduce on 0.20rc1, but not master. Rebuilding my dev environment now.

[jorisvandenbossche]

@jcrist Thanks for the report!

[TomAugspurger]

Ah, so this is only an issue if bottleneck is not installed.

[jorisvandenbossche]

Hmm, I could reproduce this in my dev env, which has bottleneck installed ..

[TomAugspurger]

Huh: So running this in script.py

try:
    import bottleneck
    print("have bottleneck")
except ImportError:
    print("No bottleneck")

import pandas as pd
df = pd.DataFrame({'int': [1, 2, 3, 4], 'float': [1., 2., 3., 4.], 'str': ['a', 'b', 'c', 'd']})
print(df.std())

(py3) bash-4.4$ python script.py
have bottleneck
float    1.290994
int      1.290994
dtype: float64
(py3) bash-4.4$ pip uninstall -y bottleneck
Uninstalling Bottleneck-1.2.0:
  Successfully uninstalled Bottleneck-1.2.0
(py3) bash-4.4$ python script.py
No bottleneck
Series([], dtype: float64)
(py3) bash-4.4$

[jorisvandenbossche]

Hmm, OK, not sure why, but updated my env to latest master (previously was still from last week), and now it works correctly, so in accordance to your findings (as I have bottleneck installed), so ignore my above.

And can confirm it is indeed triggered by not using bottleneck:

In [5]: df.std()
Out[5]: 
float    1.290994
int      1.290994
dtype: float64

In [6]: pd.core.nanops._USE_BOTTLENECK = False

In [7]: df.std()
Out[7]: Series([], dtype: float64)

[TomAugspurger]

Here's the underlying issue, which gets swallowed due to how std is implemented using .apply

In [3]: pd.core.nanops.nanstd(pd.Series([1, 2, 3]))
Out[3]: 1.0

In [4]: pd.core.nanops._USE_BOTTLENECK = False

In [5]: pd.core.nanops.nanstd(pd.Series([1, 2, 3]))

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
/Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/core/nanops.py in f(values, axis, skipna, **kwds)
    106                 else:
--> 107                     result = alt(values, axis=axis, skipna=skipna, **kwds)
    108             except Exception:

/Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/core/nanops.py in nanvar(values, axis, skipna, ddof)
    396         values = values.copy()
--> 397         np.putmask(values, mask, 0)
    398

TypeError: putmask() argument 1 must be numpy.ndarray, not Series

During handling of the above exception, another exception occurred:

TypeError                                 Traceback (most recent call last)
/Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/core/nanops.py in f(values, axis, skipna, **kwds)
    106                 else:
--> 107                     result = alt(values, axis=axis, skipna=skipna, **kwds)
    108             except Exception:

/Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/core/nanops.py in nanstd(values, axis, skipna, ddof)
    375 def nanstd(values, axis=None, skipna=True, ddof=1):
--> 376     result = np.sqrt(nanvar(values, axis=axis, skipna=skipna, ddof=ddof))
    377     return _wrap_results(result, values.dtype)

/Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/core/nanops.py in _f(*args, **kwargs)
     49                 with np.errstate(invalid='ignore'):
---> 50                     return f(*args, **kwargs)
     51             except ValueError as e:

/Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/core/nanops.py in f(values, axis, skipna, **kwds)
    109                 try:
--> 110                     result = alt(values, axis=axis, skipna=skipna, **kwds)
    111                 except ValueError as e:

/Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/core/nanops.py in nanvar(values, axis, skipna, ddof)
    396         values = values.copy()
--> 397         np.putmask(values, mask, 0)
    398

TypeError: putmask() argument 1 must be numpy.ndarray, not Series

During handling of the above exception, another exception occurred:

TypeError                                 Traceback (most recent call last)
/Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/core/nanops.py in f(values, axis, skipna, **kwds)
    106                 else:
--> 107                     result = alt(values, axis=axis, skipna=skipna, **kwds)
    108             except Exception:

/Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/core/nanops.py in nanvar(values, axis, skipna, ddof)
    396         values = values.copy()
--> 397         np.putmask(values, mask, 0)
    398

TypeError: putmask() argument 1 must be numpy.ndarray, not Series

During handling of the above exception, another exception occurred:

TypeError                                 Traceback (most recent call last)
<ipython-input-5-7b66e79a9b96> in <module>()
----> 1 pd.core.nanops.nanstd(pd.Series([1, 2, 3]))

/Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/core/nanops.py in _f(*args, **kwargs)
     48             try:
     49                 with np.errstate(invalid='ignore'):
---> 50                     return f(*args, **kwargs)
     51             except ValueError as e:
     52                 # we want to transform an object array

/Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/core/nanops.py in f(values, axis, skipna, **kwds)
    108             except Exception:
    109                 try:
--> 110                     result = alt(values, axis=axis, skipna=skipna, **kwds)
    111                 except ValueError as e:
    112                     # we want to transform an object array

/Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/core/nanops.py in nanstd(values, axis, skipna, ddof)
    374 @bottleneck_switch(ddof=1)
    375 def nanstd(values, axis=None, skipna=True, ddof=1):
--> 376     result = np.sqrt(nanvar(values, axis=axis, skipna=skipna, ddof=ddof))
    377     return _wrap_results(result, values.dtype)
    378

/Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/core/nanops.py in _f(*args, **kwargs)
     48             try:
     49                 with np.errstate(invalid='ignore'):
---> 50                     return f(*args, **kwargs)
     51             except ValueError as e:
     52                 # we want to transform an object array

/Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/core/nanops.py in f(values, axis, skipna, **kwds)
    108             except Exception:
    109                 try:
--> 110                     result = alt(values, axis=axis, skipna=skipna, **kwds)
    111                 except ValueError as e:
    112                     # we want to transform an object array

/Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/pandas/pandas/core/nanops.py in nanvar(values, axis, skipna, ddof)
    395     if skipna:
    396         values = values.copy()
--> 397         np.putmask(values, mask, 0)
    398
    399     # xref GH10242

TypeError: putmask() argument 1 must be numpy.ndarray, not Series

Looking into it now.

[jreback]

pd.core.nanops.nanstd(pd.Series([1, 2, 3]))

@TomAugspurger these are NOT defined for Series and only accept arrays.

[TomAugspurger]

Right, I think that last traceback isn't the real failure.

Do you know what in your .agg PR changed that caused this? I still don't quite see what's going on.