BUG: Float64Dtype and isnull() functions unexpected behavior

[jayantsahewal]

Pandas version checks

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• I have confirmed this bug exists on the main branch of pandas.

Reproducible Example

import pandas as pd
import numpy as np

def weird_behavior(df, cols):
    print(df.dtypes.tolist())
    for col in cols:
        print(df[col].isna().sum(), df[col].apply(pd.isna).sum())
        print(df[col].isnull().sum(), df[col].apply(pd.isnull).sum())
    print()

test_1_df = pd.DataFrame(data=[[1.0, 0.0], [1.0, 1], [1.0, 1], [4.0, 0], [0.0, 0]], columns=['a', 'b'], dtype=pd.Float64Dtype())
test_1_df['c'] = test_1_df['a'] / test_1_df['b']
test_1_df['d'] = float('inf')
test_1_df['e'] = np.nan
test_2_df = test_1_df.astype(pd.Float64Dtype())
test_3_df = test_1_df.astype(np.float64)
cols = ['c', 'd', 'e']
weird_behavior(test_1_df, cols)
weird_behavior(test_2_df, cols)
weird_behavior(test_3_df, cols)

test_4_df = pd.DataFrame(data=[[1.0, 0.0], [1.0, 1], [1.0, 1], [4.0, 0], [0.0, 0]], columns=['a', 'b'])
test_4_df['c'] = test_4_df['a'] / test_4_df['b']
test_4_df['d'] = float('inf')
test_4_df['e'] = np.nan
test_5_df = test_4_df.astype(pd.Float64Dtype())
test_6_df = test_4_df.astype(np.float64)
cols = ['c', 'd', 'e']
weird_behavior(test_4_df, cols)
weird_behavior(test_5_df, cols)
weird_behavior(test_6_df, cols)

[Float64Dtype(), Float64Dtype(), Float64Dtype(), dtype('float64'), dtype('float64')]
0 1
0 1
0 0
0 0
5 5
5 5

[Float64Dtype(), Float64Dtype(), Float64Dtype(), Float64Dtype(), Float64Dtype()]
0 1
0 1
0 0
0 0
5 5
5 5

[dtype('float64'), dtype('float64'), dtype('float64'), dtype('float64'), dtype('float64')]
1 1
1 1
0 0
0 0
5 5
5 5

[dtype('float64'), dtype('float64'), dtype('float64'), dtype('float64'), dtype('float64')]
1 1
1 1
0 0
0 0
5 5
5 5

[Float64Dtype(), Float64Dtype(), Float64Dtype(), Float64Dtype(), Float64Dtype()]
1 1
1 1
0 0
0 0
5 5
5 5

[dtype('float64'), dtype('float64'), dtype('float64'), dtype('float64'), dtype('float64')]
1 1
1 1
0 0
0 0
5 5
5 5

### Issue Description

Specifying `dtype=pd.Float64Dtype()` while creating a pandas dataFrame leads to inconsistent results in using `series.isnull()` vs. `series.apply(pd.isnull)`. 

If I don't specify the dtype while creating a pandas dataFrame, it works as expected in using `series.isnull()` vs. `series.apply(pd.isnull)`

### Expected Behavior

1 1
1 1
0 0
0 0
5 5
5 5

### Installed Versions

<details>

commit           : 965ceca9fd796940050d6fc817707bba1c4f9bff
python           : 3.10.8.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.10.157-139.675.amzn2.x86_64
Version          : #1 SMP Thu Dec 8 01:29:11 UTC 2022
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 2.0.2
numpy            : 1.23.4
pytz             : 2022.7
dateutil         : 2.8.2
setuptools       : 65.6.3
pip              : 23.1
Cython           : 0.29.33
pytest           : 7.2.0
hypothesis       : None
sphinx           : 6.1.2
blosc            : None
feather          : None
xlsxwriter       : 3.0.6
lxml.etree       : 4.9.2
html5lib         : None
pymysql          : 1.0.3
psycopg2         : 2.9.3
jinja2           : 3.1.2
IPython          : 7.34.0
pandas_datareader: None
bs4              : 4.11.1
bottleneck       : 1.3.5
brotli           : 
fastparquet      : None
fsspec           : 2022.11.0
gcsfs            : None
matplotlib       : 3.6.2
numba            : 0.56.4
numexpr          : 2.7.3
odfpy            : None
openpyxl         : 3.0.10
pandas_gbq       : None
pyarrow          : 8.0.0
pyreadstat       : None
pyxlsb           : None
s3fs             : 0.4.2
scipy            : 1.10.0
snappy           : None
sqlalchemy       : 1.4.46
tables           : 3.7.0
tabulate         : None
xarray           : None
xlrd             : None
zstandard        : None
tzdata           : 2023.3
qtpy             : 2.3.0
pyqt5            : None

</details>

[jbrockmendel]

xref #32265

[cmillani]

I just had an issue related to this while using isna on a DF.

On float64 dtype NaN is considered True:

import numpy as np
import pandas as pd
df = pd.DataFrame({'a': [5, 0], 'b': [np.nan, 12]})
df = df.astype('float64')
df['c'] = df['a'] * np.inf
print(df.isna())

Results:

    a      b      c
0  False   True  False
1  False  False   True

Simply changing types:

df = df.astype('Float64')
df['c'] = df['a'] * np.inf
print(df.isna())

Changes the result of the columns C row 1:

     a      b      c
0  False   True  False
1  False  False  False

We solved this issue using numpy.isnan, but since Pandas is allowing Arrow as a backend this approach does not seems good, and I believe that consistency between Float64 and float64 would be a good choice.

I know little about the internals, but would be happy to help with if this is not a desired behavior! If this is desired, it would be nice to document and guide users :)

[rachtsingh]

@cmillani

You can fix this in your own codebase at the moment by making the following change:

import numpy as np
from pandas.core.arrays.floating import FloatingArray
FloatingArray.oldisna = FloatingArray.isna
def newisna(self: FloatingArray) -> np.ndarray:
    return np.isnan(self._data) | self._mask.copy()
FloatingArray.isna = newisna

For us this is part of our default pandas monkey module that is loaded in all code.

[rachtsingh]

For anyone else seeing this, also worth ing fillna:

from pandas.core.arrays.masked import BaseMaskedArrayT, validate_fillna_kwargs, is_array_like, missing
FloatingArray.oldfillna = FloatingArray.fillna
def newfillna(
    self: BaseMaskedArrayT, value=None, method=None, limit=None
) -> BaseMaskedArrayT:
    value, method = validate_fillna_kwargs(value, method)

    mask = (self._mask | np.isnan(self._data)).copy()

    if is_array_like(value):
        if len(value) != len(self):
            raise ValueError(
                f"Length of 'value' does not match. Got ({len(value)}) "
                f" expected {len(self)}"
            )
        value = value[mask]

    if mask.any():
        if method is not None:
            func = missing.get_fill_func(method, ndim=self.ndim)
            npvalues = self._data.copy().T
            new_mask = mask.T
            func(npvalues, limit=limit, mask=new_mask)
            return type(self)(npvalues.T, new_mask.T)
        else:
            # fill with value
            new_values = self.copy()
            new_values[mask] = value
    else:
        new_values = self.copy()
    return new_values
FloatingArray.fillna = newfillna

[DeGiorgiMarcello]

Hi, I have the same problem when filling nans generated by a 0/0 division:

In [2]: df = pd.DataFrame({"A":[1.,0,0,pd.NA],"B":[0,0,2,1]}, dtype="Float32")

In [3]: df
Out[3]: 
   A    B
0   1.0  0.0
1   0.0  0.0
2   0.0  2.0
3  <NA>  1.0

In [4]: z = df.A /df.B

In [5]: z
Out[5]: 
0    inf
1    NaN
2    0.0
3   <NA>
dtype: Float32

In [6]: z.fillna(0.)
Out[6]: 
0    inf
1    NaN
2    0.0
3    0.0
dtype: Float32

When filling the nan, the 0/0 nan is not filled.

Casting to float32 and recasting to Float32 solves the problem:

In [14]: z.astype("float32").astype("Float32").fillna(0.)
Out[14]: 
0    inf
1    0.0
2    0.0
3    0.0
dtype: Float32

[glaucouri]

Some more info on this behavior: #32265

The v2.0 changelog mention this: https://pandas.pydata.org/docs/whatsnew/v2.0.0.html#

Division by zero with ArrowDtype dtypes returns -inf, nan, or inf depending on the numerator, instead of raising (GH 51541)

To unify the nan values I've found another solution:

z.where(z==z, pd.NA)

This replaces all np.nan with the pd.NA values

[arobrien]

This has tripped me up and I consider it currently broken with pyarrow backend.

My current workarounds are to use df.astype(float).isna() or np.isnan(df), which give the expected behaviour.

Simple reproduction:

a = pd.DataFrame({'a': [1.0, 0.0, np.nan]})
b = a.convert_dtypes(dtype_backend='pyarrow')

# inconsistent behaviour for 0/0 but consistent for nan/0
display((a / 0).isna())
display((b / 0).isna())

# consistent behaviour 0/0 and nan/0 both yield True.
display(np.isnan(a / 0))
display(np.isnan(b / 0))
```