DataFrame.update() changes type of boolean column to Object.

[Sereger13]

After applying DataFrame.update() on a DataFrame that has boolean column the type of that column changes to Object and filtering stops working as negate operator (~) produces '-1' series instead of 'True'.

Code Sample, a copy-pastable example if possible

df = pd.DataFrame({'a': [1, 2, 3], 'b': ['notset'] * 3, 'c': [False] * 3})
df.set_index('a', inplace=True)

df_update_from = pd.DataFrame({'a': [2, 3], 'b': ['updated2', 'updated3'], 'c': [False, False]})
df_update_from.set_index('a', inplace=True)
df.update(df_update_from)
~df.c

Expected Output

a
1 True
2 True
3 True
Name: c, dtype: bool

Actual Output

a
1 -1
2 -1
3 -1
Name: c, dtype: object

output of pd.show_versions()

INSTALLED VERSIONS

commit: None
python: 2.7.9.final.0
python-bits: 64
OS: Linux
OS-release: 2.6.18-238.9.1.el5
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US

pandas: 0.16.2
nose: 1.3.4
Cython: 0.22
numpy: 1.9.2
scipy: 0.15.1
statsmodels: 0.6.1
IPython: 3.0.0
sphinx: 1.3.1
patsy: 0.3.0
dateutil: 2.4.1
pytz: 2016.3
bottleneck: None
tables: 3.1.1
numexpr: 2.3.1
matplotlib: 1.4.3
openpyxl: 1.8.5
xlrd: 0.9.3
xlwt: 0.7.5
xlsxwriter: 0.6.7
lxml: 3.4.2
bs4: 4.3.2
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.0.9
pymysql: None
psycopg2: None

[jorisvandenbossche]

@Sereger13 Thanks for the report.

This is the result of the provided dataframe in update being reindexed to the index/columns of the calling dataframe, which can introduce NaN values if both are not aligned, which in turn can cause dtype changes.
For example, you see this for integer dtypes as well:

In [28]: df2 = pd.DataFrame({'a':[1,2,3]})

In [29]: df_update_from2= pd.DataFrame({'a':[20,30]}, index=[1,2])

In [30]: df2.dtypes
Out[30]: 
a    int64
dtype: object

In [32]: df2.update(df_update_from2)

In [33]: df2
Out[33]: 
      a
0   1.0
1  20.0
2  30.0

In [34]: df2.dtypes
Out[34]: 
a    float64
dtype: object

Maybe we have to check if the data can be cast again to the original dtypes after updating.

[jreback]

this is a dupe of #4094. .update() is in general not very dtype friendly. pull-requests are welcome.