Summary
This chapter covers using built-in methods to extract the set of unique values from a data column and to count how many distinct entries it contains.

General domain of usage
Data analysis

Data often gets **duplicated** in DataFrames. For instance, in the `countries` DataFrame, the `'continent'` column has repeated entries. There's a method that retrieves an **array of distinct values** from a specific DataFrame column.

import pandas as pd 

country_data = {'country' : ['Thailand', 'Philippines', 'Monaco', 'Malta', 'Sweden', 'Paraguay', 'Latvia'],
          'continent' : ['Asia', 'Asia', 'Europe', 'Europe', 'Europe', 'South America', 'Europe'],
          'capital':['Bangkok', 'Manila', 'Monaco', 'Valletta', 'Stockholm', 'Asuncion', 'Riga']}
countries = pd.DataFrame(country_data)
print(countries)

Next, apply the `unique()` method to the `'continent'` and `'country'` columns:


import pandas as pd

country_data = {'country' : ['Thailand', 'Philippines', 'Monaco', 'Malta', 'Sweden', 'Paraguay', 'Latvia'],
          'continent' : ['Asia', 'Asia', 'Europe', 'Europe', 'Europe', 'South America', 'Europe'],
          'capital':['Bangkok', 'Manila', 'Monaco', 'Valletta', 'Stockholm', 'Asuncion', 'Riga']}
countries = pd.DataFrame(country_data)
unique_countries = countries['country'].unique()
unique_continents = countries['continent'].unique()
print(unique_countries)
print(unique_continents)

Sometimes, instead of retrieving all unique values, you might just want to know **how many distinct values** exist in a column. In such cases, you can use the **`nunique()`** method. It returns the **count** of unique entries in a column — not the values themselves.

import pandas as pd

country_data = {'country' : ['Thailand', 'Philippines', 'Monaco', 'Malta', 'Sweden', 'Paraguay', 'Latvia'],
          'continent' : ['Asia', 'Asia', 'Europe', 'Europe', 'Europe', 'South America', 'Europe'],
          'capital':['Bangkok', 'Manila', 'Monaco', 'Valletta', 'Stockholm', 'Asuncion', 'Riga']}
countries = pd.DataFrame(country_data)
print(countries['continent'].nunique())

import unittest
import importlib
import numpy as np


# Helper function for dynamic test names and assertions
def _dynamic_test(test_case, condition, success_message, failure_message):
    if condition:
        test_case._testMethodName = success_message
        test_case.assertTrue(True, success_message)
    else:
        test_case._testMethodName = failure_message
        test_case.fail(failure_message)


class TestUserCode(unittest.TestCase):

    def test_unique_years_is_declared(self):
        import user_code
        _dynamic_test(
            self,
            hasattr(user_code, 'unique_years'),
            "The `unique_years` variable is declared.",
            "Expected `unique_years` to be declared."
        )

    def test_unique_years_contains_correct_values(self):
        import user_code
        expected_value = sorted([2017, 2016, 2019])
        try:
            if isinstance(user_code.unique_years, np.ndarray):
                actual_value = sorted(user_code.unique_years)
                condition = actual_value == expected_value
                failure_message = f"Expected `unique_years` to contain `{expected_value}`, but got `{actual_value}`."
            else:
                condition = False
                failure_message = f"`unique_years` is not a `numpy.ndarray`."

        except AttributeError:
            condition = False
            failure_message = "The `unique_years` variable is not declared."

        _dynamic_test(
            self,
            condition,
            f"`unique_years` contains correct values.",
            failure_message
        )

    def test_unique_fueltype_is_declared(self):
        import user_code
        _dynamic_test(
            self,
            hasattr(user_code, 'unique_fueltype'),
            "The `unique_fueltype` variable is declared.",
            "Expected `unique_fueltype` to be declared."
        )

    def test_unique_fueltype_contains_correct_values(self):
        import user_code
        expected_value = sorted(["petrol", "diesel"])
        try:
            if isinstance(user_code.unique_fueltype, np.ndarray):
                actual_value = sorted(user_code.unique_fueltype)
                condition = actual_value == expected_value
                failure_message = f"Expected `unique_fueltype` to contain `{expected_value}`, but got `{actual_value}`."
            else:
                condition = False
                failure_message = f"`unique_fueltype` is not a `numpy.ndarray`."
        except AttributeError:
            condition = False
            failure_message = "The `unique_fueltype` variable is not declared."

        _dynamic_test(
            self,
            condition,
            f"`unique_fueltype` contains correct values.",
            failure_message
        )

    def test_count_unique_fueltypes_is_declared(self):
        import user_code
        _dynamic_test(
            self,
            hasattr(user_code, 'count_unique_fueltypes'),
            "The `count_unique_fueltypes` variable is declared.",
            "Expected `count_unique_fueltypes` to be declared."
        )

    def test_count_unique_fueltypes_is_correct(self):
        import user_code
        expected_value = 2
        try:
            actual_value = user_code.count_unique_fueltypes
            condition = actual_value == expected_value
            failure_message = f"Expected `count_unique_fueltypes` to be `{expected_value}`, but got `{actual_value}`."
        except AttributeError:
            condition = False
            failure_message = "The `count_unique_fueltypes` variable is not declared."

        _dynamic_test(
            self,
            condition,
            f"`count_unique_fueltypes` is correct.",
            failure_message
        )


if __name__ == '__main__':
    unittest.main()

test_main.py

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Unique Values

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