Cannot concatenate object of type ‘‘; only series and dataframe objs are valid

When you encounter the error message “cannot concatenate object of type ‘numpy.ndarray’; only series and dataframe objs are valid,” it means that you are trying to perform concatenation using a NumPy ndarray object, which is not supported in this context.

Concatenation often refers to combining two or more data structures, such as arrays or dataframes, along a particular axis. To perform concatenation, you need to use pandas series or dataframe objects, as they provide built-in concatenation functions.

Here’s a simple example demonstrating the error and its solution using pandas:

import pandas as pd
import numpy as np

# Creating NumPy ndarrays
array1 = np.array([1,2,3])
array2 = np.array([4,5,6])

# Trying to concatenate ndarrays directly
concatenated_array = np.concatenate((array1, array2)) # This will result in the mentioned error

# Converting ndarrays to pandas series and concatenating
series1 = pd.Series(array1)
series2 = pd.Series(array2)
concatenated_series = pd.concat([series1, series2]) # This will work

print(concatenated_series)
  

In the example above, we first try to concatenate the NumPy ndarrays (array1 and array2) directly using the np.concatenate function which results in the mentioned error. To resolve this, we convert the ndarrays into pandas series (series1 and series2) and then use the pd.concat function to concatenate them along the axis (in this case, the default axis 0). Finally, we print the concatenated series, which combines the values from both original arrays.