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

output.loc[row_id, col_id]

and I'd like to set this value to an empty list, [].

output.loc[row_id, col_id] = []

Unfortunately, I get an error saying the size of my keys and values do not match (Pandas thinks I'm trying to set values with not to an iterable).

Is there a way to do this?

Thanks!

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).

---

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).

---

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

Answer 2

The answer by MishaTeplitskiy works when the index label is 0. More generally, if you want to assign an array x to an element of a DataFrame df with row r and column c, you can use:

df.loc[[r], c] = pd.Series([x], index = [r])