In[25]:%timeit 12345 in df.CID
Out[25]:89.8 µs ± 254 ns per loop (mean ± std. dev. of 7 runs, 10000 loops each)

In[26]:%timeit 12345 in df['CID']
Out[26]:42.3 µs ± 334 ns per loop (mean ± std. dev. of 7 runs, 10000 loops each)

In[27]:type( df.CID)
Out[27]: pandas.core.series.Series

In[28]:type( df['CID'])
Out[28]: pandas.core.series.Series

df = pd.DataFrame({'A': [1, 2, 3]})
df.A

0    1
1    2
2    3
Name: A, dtype: int64

df.pop
# <bound method NDFrame.pop of ...>

df['pop'] = [4, 5, 6]
df
   A  pop
0  1    4
1  2    5
2  3    6

df.pop
# <bound method NDFrame.pop of ...>

df['pop']

0    4
1    5
2    6
Name: pop, dtype: int64