How to count the number of unique values in a DataFrame column?

In pandas, to count the number of unique values in a DataFrame column, you can use the nunique() method. For example:

import pandas as pd

df = pd.DataFrame({
    'col': [1, 2, 2, 3, 4, 4, 4]
})

unique_count = df['col'].nunique()
print(unique_count)  # Will output: 4

This method returns the number of unique elements in the specified column.