SettingWithCopyWarning single- vs mixed-type dataframes

[ayorgo]

Problem description

It seems that pandas (at least v0.23.4) handles mixed-type and single-type dataframes differently when it comes to throwing SettingWithCopyWarning

    df = pd.DataFrame({'a': [4, 5, 6], 'c': [3, 2, 1]})
    df._is_view # False
    df._is_cached # False
    df._is_mixed_type # False

    df.iloc[-1]['c'] = 42 # no warning

    df = pd.DataFrame({'a': ['x', 'y', 'z'], 'c': ['t', 'u', 'v']})
    df._is_view # False
    df._is_cached # False
    df._is_mixed_type # False

    df.iloc[-1]['c'] = 'f' # no warning

    df = pd.DataFrame({'a': ['x', 'y', 'z'], 'c': [3, 2, 1]})
    df._is_view # False
    df._is_cached # False
    df._is_mixed_type # True

    df.iloc[-1]['c'] = 42 # SettingWithCopyWarning: ...

Expected Output

SettingWithCopyWarning should appear in all three cases above.

Output of pd.show_versions()

INSTALLED VERSIONS

commit: None
python: 3.6.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.15.0-38-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8
LOCALE: en_GB.UTF-8

pandas: 0.23.4
pytest: 3.2.1
pip: 18.0
setuptools: 40.2.0
Cython: 0.26.1
numpy: 1.13.3
scipy: 0.19.1
pyarrow: None
xarray: None
IPython: 6.5.0
sphinx: 1.6.3
patsy: 0.4.1
dateutil: 2.7.3
pytz: 2017.2
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.2
feather: None
matplotlib: 3.0.1
openpyxl: 2.4.8
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.2
lxml: 4.1.0
bs4: 4.6.0
html5lib: 1.0.1
sqlalchemy: 1.1.13
pymysql: None
psycopg2: 2.7.4 (dt dec pq3 ext lo64)
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

[jreback]

a non mixed type frame is a pure view and is not subject to this warning when set with a compatible value

this is by definition: please read the documentation: http://pandas.pydata.org/pandas-docs/stable/indexing.html?highlight=settingwithcopy#returning-a-view-versus-a-copy

[ayorgo]

Thanks for the prompt and definitive reply @jreback.

However, I'm struggling to find the line in the docs that says "a non-mixed type frame is a pure view".
Could you please point out the exact line in the docs that says that?

Thank you.

[jreback]

what exactly is the issue? the warning happens when it’s relevant. the docs point out when it happens. they don’t point out when it doesn’t which is most of the time.