import pandas as pd

df = pd.DataFrame({
    'col1': ['a', 'b', 'a', 'c'],
    'col2': [1, 2, 2, 3]
})

print(df.apply(lambda x: x.unique()))

col1    [a, b, c]
col2    [1, 2, 3]
dtype: object

import pandas as pd

df = pd.DataFrame({
    'col1': ['a', 'b', 'a', 'c'],
    'col2': [1, 2, 2, 3]
})

print(df.nunique())

col1    3
col2    3
dtype: int64

import pandas as pd

df = pd.DataFrame({
    'col1': ['a', 'b', 'a', 'c'],
    'col2': [1, 2, 2, 3]
})

unique_values = set().union(*[df[col].unique() for col in df.columns])
print(unique_values)

{1, 2, 3, 'a', 'b', 'c'}