Getting TypeError: reduction operation 'argmax' not allowed for this dtype when trying to use idxmax()

Question

When using the idxmax() function in Pandas, I keep receiving this error.

Traceback (most recent call last):
  File \"/Users/username/College/yea         


        

Answer 1

If NaN are present (and we can sort of see this by the stack trace) then when you think you are working with a data frame of numerics, you could well have mixed types, and in particular, a string among numerics. Let me give you 3 code examples, the first 2 work, the last doesn't and is likely your case.

This represents all numeric data, it will work with idxmax

the_dict = {}
the_dict['a'] = [0.1, 0.2, 0.5]
the_dict['b'] = [0.3, 0.4, 0.6]
the_dict['c'] = [0.25, 0.3, 0.9]
the_dict['d'] = [0.2, 0.1, 0.4]
the_df = pd.DataFrame(the_dict)

This represents a numeric nan, it will work idxmax

the_dict = {}
the_dict['a'] = [0.1, 0.2, 0.5]
the_dict['b'] = [0.3, 0.4, 0.6]
the_dict['c'] = [0.25, 0.3, 0.9]
the_dict['d'] = [0.2, 0.1, np.NaN]
the_df = pd.DataFrame(the_dict)

This could be the exact problem reported by the OP, but if it turns out we have mixed types in any fashion, we will get the error the OP reported.

the_dict = {}
the_dict['a'] = [0.1, 0.2, 0.5]
the_dict['b'] = [0.3, 0.4, 0.6]
the_dict['c'] = [0.25, 0.3, 0.9]
the_dict['d'] = [0.2, 0.1, 'NaN']
the_df = pd.DataFrame(the_dict)