Summary  
This chapter explains how to use the pandas DataFrame.insert() method to add a new column at a specified position within a DataFrame by providing its index, name, and values.

General domain of usage  
Data analysis

Next, explore another way to add a column to a `DataFrame` using the `insert()` method. This method allows you to **specify the position** of the new column within the `DataFrame`.


```python
df.insert(column_index, 'column_name', [value_1, value_2, value_3])
```

- `df`: the name of the existing DataFrame;
- `insert()`: the method used for adding new columns;
- `column_index`: the position where the new column will be inserted (keep in mind that indexing starts at **0**);
- `column_name`: the name for the new column;
- `[value_1, value_2, value_3]`: the values that will populate the new column.

Focus on the `countries` DataFrame and add a new column named `'population'`, which represents the populations of countries, right after the first column (`'country'`).


import pandas as pd
countries_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(countries_data)

countries.insert(1, 'population', [61399000, 75967000, 39244, 380200, 10380491, 5496000, 2424200])
print(countries)

import unittest
import importlib


# Helper function to dynamically generate 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_audi_cars_is_declared(self):
        import user_code
        _dynamic_test(
            self,
            hasattr(user_code, 'audi_cars'),
            "The `audi_cars` variable is declared.",
            "Expected `audi_cars` variable to be declared."
        )

    def test_audi_cars_is_dataframe(self):
        import user_code
        import pandas as pd
        try:
            condition = isinstance(user_code.audi_cars, pd.DataFrame)
            failure_message = f"Expected `audi_cars` to be a `DataFrame`, but got `{type(user_code.audi_cars).__name__}`."
        except AttributeError:
            condition = False
            failure_message = "The `audi_cars` variable is not declared."

        _dynamic_test(
            self,
            condition,
            "The variable `audi_cars` is a `DataFrame`.",
            failure_message
        )

    def test_audi_cars_columns_correct(self):
        import user_code
        try:
            expected_columns = ['model', 'year', 'price', 'fueltype']
            actual_columns = list(user_code.audi_cars.columns)
            condition = actual_columns == expected_columns
            failure_message = f"Expected `audi_cars` columns to be `{expected_columns}`, but got `{actual_columns}`."

        except TypeError:
            condition = False
            failure_message = f"`audi_cars` is not a `DataFrame`."

        except AttributeError:
            condition = False

            if not getattr(user_code, 'audi_cars', None):
                failure_message = "The `audi_cars` variable is not declared."
            else:
                failure_message = f"`audi_cars` is not a `DataFrame`."

        _dynamic_test(
            self,
            condition,
            f"`audi_cars` has the correct columns.",
            failure_message
        )

    def test_price_column_correct_values(self):
        import user_code
        expected_values = [12500, 16500, 16800, 17300, 13900]
        try:
            actual_values = user_code.audi_cars['price'].tolist()
            condition = actual_values == expected_values
            failure_message = f"Expected `price` column values to be `{expected_values}`, but got `{actual_values}`."

        except TypeError:
            condition = False
            failure_message = f"`audi_cars` is not a `DataFrame`."

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

        except KeyError:
            condition = False
            failure_message = "The `price` column is missing in the `audi_cars` `DataFrame`."

        _dynamic_test(
            self,
            condition,
            f"The `price` column contains the correct values.",
            failure_message
        )

    def test_insert_method_used(self):
        with open('user_code.py', 'r') as file:
            user_code_source = file.read()
        condition = ".insert(" in user_code_source
        failure_message = "Expected the `.insert()` method to be used to add the `'price'` column."

        _dynamic_test(
            self,
            condition,
            "The `.insert()` method is used to add the `'price'` column.",
            failure_message
        )


# Run the tests when executing the script
if __name__ == '__main__':
    unittest.main()

test_main.py

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Inserting a New Column

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