BUG: Replace in `string` series with NA

[albertotb]

Code Sample, a copy-pastable example if possible

import pandas as pd
import numpy as np

rep = {'one': '1', 'two': '2'}
a = pd.Series(['one', 'two'], dtype='string')
b = pd.Series(['one', 'two', np.nan])
c = pd.Series(['one', 'two', np.nan], dtype='string')


# A: this works
a.replace(to_replace=rep)

# B: this also works
b.replace(to_replace=rep)

# C: this throws exception
c.replace(to_replace=rep)
# TypeError: Cannot compare types 'ndarray(dtype=object)' and 'str'

Problem description

pandas.Series.replace cannot be used in series of type string that contain <NA>

Expected Output

I would expect C to output the same as B but with string dtype

Output of pd.show_versions()

INSTALLED VERSIONS

commit : None
python : 3.7.4.final.0
python-bits : 64
OS : Linux
OS-release : 4.4.0-112-generic
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : es_ES.UTF-8
LOCALE : es_ES.UTF-8

pandas : 1.0.1
numpy : 1.18.1
pytz : 2019.3
dateutil : 2.8.1
pip : 19.3.1
setuptools : 41.4.0
Cython : None
pytest : None
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 2.11.1
IPython : 7.8.0
pandas_datareader: None
bs4 : None
bottleneck : None
fastparquet : None
gcsfs : None
lxml.etree : None
matplotlib : 3.1.3
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pytables : None
pytest : None
pyxlsb : None
s3fs : None
scipy : 1.4.1
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : 1.2.0
xlwt : None
xlsxwriter : None
numba : None

[jorisvandenbossche]

@albertotb Thanks for the report!

Slightly related to #32075

It seems this works with scalars:

In [18]: c.replace('one', '1')                                                                                                                                                                                     
Out[18]: 
0       1
1     two
2    <NA>
dtype: string

but not with lists/dicts

[chrispe]

Hi, can I try to pick this up?

[jorisvandenbossche]

Yes, that would be welcome!

[chrispe]

So, here's a few things I've noticed so far:

• When the dtype of a pd.Series is defined as a string, then the nan values of the array (which is to be stored under a series object) are converted topd.NA.
• When a dictionary/list is used to define the replacements (see variable rep in the first example) a different function is called underneath to apply those replacements. That's why the version specified by @jorisvandenbossche is working fine. Apparently, the replace function is very different in that case (not sure yet why that is though).
• The function used to find where to apply the replacements is using operator.eq. That operator fails to make the element-wise comparison between a value and pd.NA and returns a single scalar (instead of an array consisted of boolean values). And that's why the TypeError exception is raised.

I'm now investigating what's the best way of resolving this.

[chrispe]

I also noticed another issue, have a look at this code:

import pandas as pd
import numpy as np

replacements = {'one': '1', 'two': '2'}

series_a = pd.Series(['one', 'two'], dtype='string')
for rep in replacements:
    series_a = series_a.replace(rep, replacements[rep])

series_b = pd.Series(['one', 'two'], dtype='string')
series_b = series_b.replace(to_replace=replacements)

It produces the following output:
In [4]: series_a
Out[4]:
0 1
1 2
dtype: string
In [5]: series_b
Out[5]:
0 1
1 2
dtype: object

Shouldn't they both return a pd.Series with a dtype of string?

[chrispe]

Here's an explanation of my current solution (chrispe@47f6676). In order to enable comparison with arrays containing pd.NA, I make sure to replace all of the missing values with np.na:

# Replace all definitions of missing values (isna=True) to a numpy.nan
# Where x is an array of values
x = np.where(isna(x), np.nan, x)