How to determine whether a Pandas Column contains a particular value

I am trying to determine whether there is an entry in a Pandas column that has a particular value. I tried to do this with if x in df['id']. I thought this was working, except when I fed it a value that I knew was not in the column 43 in df['id'] it still returned True. When I subset to a data frame only containing entries matching the missing id df[df['id'] == 43] there are, obviously, no entries in it. How to I determine if a column in a Pandas data frame contains a particular value and why doesn't my current method work? (FYI, I have the same problem when I use the implementation in this answer to a similar question).

in of a Series checks whether the value is in the index:

    In [11]: s = pd.Series(list('abc'))

    In [12]: s
    0    a
    1    b
    2    c
    dtype: object

    In [13]: 1 in s
    Out[13]: True

    In [14]: 'a' in s
    Out[14]: False

One option is to see if it's in unique values:

    In [21]: s.unique()
    Out[21]: array(['a', 'b', 'c'], dtype=object)

    In [22]: 'a' in s.unique()
    Out[22]: True

or a python set:

    In [23]: set(s)
    Out[23]: {'a', 'b', 'c'}

    In [24]: 'a' in set(s)
    Out[24]: True

As pointed out by @DSM, it may be more efficient (especially if you're just doing this for one value) to just use in directly on the values:

    In [31]: s.values
    Out[31]: array(['a', 'b', 'c'], dtype=object)

    In [32]: 'a' in s.values
    Out[32]: True