Pandas DataFrame columns.values gives a reference to the original column instead of a copy

Code Sample, a copy-pastable example if possible

import pandas as pd
import numpy as np

print "pd.__version__: %s" % pd.__version__
print "\n"

df = pd.DataFrame(np.arange(0, 3, 0.5), columns=["a"])

# copy the values of a columns
vals_copy = df.a.values

print "vals_copy max value before '-=' operation: %f" % vals_copy.max()
print "DataFrame Column max value before '-=' operation: %f " % df.a.max()
print "\n"

# '-=' opperation on the values copy
vals_copy -= vals_copy.max()

print "vals_copy max value after '-=' operation: %f" % vals_copy.max()
print "DataFrame Column max value after '-=' operation: %f" % df.a.max()
print "\n"

if vals_copy.max() == df.a.max():
    print "This is a bug! The DataFrame column should not be effected."

"""
Output:
pd.__version__: 0.20.3

vals_copy max value before '-=' operation: 2.500000
DataFrame Column max value before '-=' operation: 2.500000 

vals_copy max value after '-=' operation: 0.000000
DataFrame Column max value after '-=' operation: 0.000000

This is a bug! The DataFrame column should not be effected.
"""

Problem description

Changing the 'values' of a DataFrame columns should not effect the original DataFrame.

Expected Output

Output of `pd.show_versions()`

[paste the output of ``pd.show_versions()`` here below this line] INSTALLED VERSIONS ------------------ commit: None python: 2.7.13.final.0 python-bits: 64 OS: Linux OS-release: 4.2.0-23-generic machine: x86_64 processor: x86_64 byteorder: little LC_ALL: en_US.UTF-8 LANG: en_US.UTF-8 LOCALE: en_US.UTF-8 pandas: 0.20.3 pytest: 2.8.1 pip: 9.0.1 setuptools: 35.0.2 Cython: 0.23.4 numpy: 1.12.1 scipy: 0.19.1 xarray: None IPython: 4.0.1 sphinx: 1.3.1 patsy: 0.4.0 dateutil: 2.6.0 pytz: 2016.10 blosc: None bottleneck: 1.0.0 tables: 3.2.2 numexpr: 2.6.2 feather: None matplotlib: 1.5.0 openpyxl: 2.2.6 xlrd: 0.9.4 xlwt: 1.0.0 xlsxwriter: 0.7.7 lxml: 3.4.4 bs4: 4.4.1 html5lib: None sqlalchemy: 1.0.9 pymysql: None psycopg2: 2.6.1 (dt dec pq3 ext lo64) jinja2: 2.8 s3fs: None pandas_gbq: None pandas_datareader: None

Comment From: jreback

copy the values of a columns vals_copy = df.a.values

this is not a copy at all, but just a reference to the original.