Pandas DataFrame with tuple of strings as index
indexing, pandas, python
Solution
Your tuple in the selection brackets is seen as a sequence containing the elements you want to retrieve. It's like you would have passed `['1', 'b']` as argument. Thus the KeyError message: pandas tries to find the key `'1'` and obviously doesn't find it.
That's why it works when you add additional brackets, as now the argument becomes a sequence of one element - your tuple.
You should avoid dealing with ambiguities around list and tuple arguments in selection. The behavior can be also different depending on the index being a simple index or a multiindex.
In any case, if you ask about recommendations here, the one I see is that you should try to not build simple indexes made of tuples: pandas will work better and will be more powerful to use if you actually build a multiindex instead:
df = pd.DataFrame(columns=['Col 1', 'Col 2', 'Col 3'],
index=pd.MultiIndex.from_tuples([('1', 'a'), ('2', 'a'), ('1', 'b'), ('2', 'b')]))
df['Col 2'].loc[('1', 'b')] = 6
df['Col 2'].loc[('1', 'b')]
Out[13]: 6
df
Out[14]:
Col 1 Col 2 Col 3
1 a NaN NaN NaN
2 a NaN NaN NaN
1 b NaN 6 NaN
2 b NaN NaN NaN
Problem
I'm sensing some weird `pandas` behavior here. I have a dataframe that looks like ``` df = pd.DataFrame(columns=['Col 1', 'Col 2', 'Col 3'], index=[('1', 'a'), ('2', 'a'), ('1', 'b'), ('2', 'b')]) In [14]: df Out[14]: Col 1 Col 2 Col 3 (1, a) NaN NaN NaN (2, a) NaN NaN NaN (1, b) NaN NaN NaN (2, b) NaN NaN NaN ``` I can set the value of an arbitrary element ``` In [15]: df['Col 2'].loc[('1', 'b')] = 6 In [16]: df Out[16]: Col 1 Col 2 Col 3 (1, a) NaN NaN NaN (2, a) NaN NaN NaN (1, b) NaN 6 NaN (2, b) NaN NaN NaN ``` But when I go to reference the element that I just set using the same syntax, I get ``` In [17]: df['Col 2'].loc[('1', 'b')] KeyError: 'the label [1] is not in the [index]' ``` Can someone tell me what I'm doing wrong or why this behavior occurs? Am I simply not allowed to set the index as a multi-element tuple? Edit Apparently, wrapping the tuple index in a list works. ``` In [38]: df['Col 2'].loc[[('1', 'b')]] Out[38]: (1, b) 6 Name: Col 2, dtype: object ``` Although I'm still getting some weird behavior in my actual use case so it'd be nice to know if this is not recommended usage.