Pandas DataFrame stack multiple column values into single column

You can melt your dataframe:

>>> keys = [c for c in df if c.startswith('key.')]
>>> pd.melt(df, id_vars="topic", value_vars=keys, value_name="key")

   topic variable  key
0      8    key.0  abc
1      9    key.0  xab
2      8    key.1  def
3      9    key.1  xcd
4      8    key.2  ghi
5      9    key.2  xef

It also gives you the source of the key.

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From v0.20, melt is a first class function of the pd.DataFrame class:

>>> df.melt('topic', value_name="key").drop('variable', 1)

   topic  key
0      8  abc
1      9  xab
2      8  def
3      9  xcd
4      8  ghi
5      9  xef