Create and set an element of a Pandas DataFrame to a list

Question

I have a Pandas DataFrame that I\'m creating row-by-row (I know, I know, it\'s not Pandorable/Pythonic..). I\'m creating elements using .loc like so

Answer 1

You need to make sure two things:

1. there is precisely one entry for that loc,
2. the column has dtype object (actually, on testing this seems not to be an issue).

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A hacky way to do this is to use a Series with []:

In [11]: df = pd.DataFrame([[1, 2], [3, 4]], columns=['A', 'B'])

In [12]: df.loc[[0], 'A'] = pd.Series([[]])

In [13]: df
Out[13]:
    A  B
0  []  2
1   3  4

pandas doesn't really want you use [] as elements because it's usually not so efficient and makes aggregations more complicated (and un-cythonisable).

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In general you don't want to build up DataFrames cell-by-cell, there is (almost?) always a better way.