I need help with the syntax to replace the diagonal elements of a matrix with zero in Python. Can anyone please provide code examples or guidance on how to achieve this? Thank you.

```
import pandas as pd
import numpy as np
# create a sample DataFrame
df = pd.DataFrame({'a': [1, 2, 3], 'b': [4, 5, 6], 'c': [7, 8, 9]})
# convert the DataFrame to a NumPy array
arr = df.values.reshape((3, 2))
# replace both diagonals of the array with zeros
np.fill_diagonal(arr, 0)
np.fill_diagonal(np.fliplr(arr), 0)
# convert the array back to a DataFrame
df = pd.DataFrame(arr, columns=df.columns, index=df.index)
df
ValueError: cannot reshape array of size 9 into shape (3,2)
```