Dataframe that is view and copy
```python
In [1]: import pandas as pd

In [2]: df = pd.DataFrame({'a': range(30), 'b': range(30})

In [3]: df2 = df[::2]

In [4]: df2.is_copy
Out[4]: <weakref at 0x7ffffb15e0c78; to 'DataFrama' at 0x7fffb16135c0>

In [5]: df2._is_view
Out[5]: True

In [6]: df2 += 2

In [7]: df2.iloc[0]
a    2
b    2
Name: O, dtype: int64

In [8]: df.iloc[0]
a    0
b    0
Name: O, dtype: int64
```

#### Problem description
`df2` seems to be both a view *and* a copy. Everything I've read in the documentation and on stackoverflow presents the view and copy concepts as a "versus", implying that you can have one xor the other.  Is this a bug?  If not, please explain.

The observed behavior, as I understand the "definitions", is that `df2` is actually a copy and *not* a view.

#### Output of ``pd.show_versions()``

<details>

commit: None
python: 3.6.4.final.0
python-bits: 64
OS: Linux
OS-release: 3.10.0-862.3.2.el7.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.23.4
pytest: 3.9.1
pip: 9.0.1
setuptools: 40.4.3
Cython: 0.26
numpy: 1.16.0
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: 1.7.9
patsy: None
dateutil: 2.7.5
pytz: 2018.9
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.2.3
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: 4.6.3
html5lib: 1.0.1
sqlalchemy: 1.2.13
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None


</details>
