Why does one use of iloc() give a SettingWithCopyWarning, but the other doesn't?

Question

Inside a method from a class i use this statement:

self.__datacontainer.iloc[-1][\'c\'] = value

Doing this i get a \"SettingWithCopyWarnin

Answer 1

Chain indexing

As the documentation and a couple of other answers on this site ([1], [2]) suggest, chain indexing is considered bad practice and should be avoided.

Since there doesn't seem to be a graceful way of making assignments using integer position based indexing (i.e. .iloc) without violating the chain indexing rule (as of pandas v0.23.4), it is advised to instead use label based indexing (i.e. .loc) for assignment purposes whenever possible.

However, if you absolutely need to access data by row number you can

df.iloc[-1, df.columns.get_loc('c')] = 42

or

df.iloc[[-1, 1], df.columns.get_indexer(['a', 'c'])] = 42

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Pandas behaving oddly

From my understanding you're absolutely right to expect the warning when trying to reproduce the error artificially.

What I've found so far is that it depends on how a dataframe is constructed

df = pd.DataFrame({'a': [4, 5, 6], 'c': [3, 2, 1]})
df.iloc[-1]['c'] = 42 # no warning

df = pd.DataFrame({'a': ['x', 'y', 'z'], 'c': ['t', 'u', 'v']})
df.iloc[-1]['c'] = 'f' # no warning

df = pd.DataFrame({'a': ['x', 'y', 'z'], 'c': [3, 2, 1]})
df.iloc[-1]['c'] = 42 # SettingWithCopyWarning: ...

It seems that pandas (at least v0.23.4) handles mixed-type and single-type dataframes differently when it comes to chain assignments [3]

def _check_is_chained_assignment_possible(self):
    """
    Check if we are a view, have a cacher, and are of mixed type.
    If so, then force a setitem_copy check.
    Should be called just near setting a value
    Will return a boolean if it we are a view and are cached, but a
    single-dtype meaning that the cacher should be updated following
    setting.
    """
    if self._is_view and self._is_cached:
        ref = self._get_cacher()
        if ref is not None and ref._is_mixed_type:
            self._check_setitem_copy(stacklevel=4, t='referant',
                                     force=True)
        return True
    elif self._is_copy:
        self._check_setitem_copy(stacklevel=4, t='referant')
    return False

which appears really odd to me although I'm not sure if it's not expected.

However, there's an old bug with a similar behavour.

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UPDATE

According to the developers the above behaviour is expected.