example_test.py

import pandas as pd
import pytest


def func(expected_columns):

    df = pd.DataFrame({
        "A": [1, 2, 3],
        "B": [1, 2, 3],
        "C": [1, 2, 3]
    })

    diff_columns = set(df.columns.tolist()).symmetric_difference(
        expected_columns
    )
    if len(diff_columns):
        raise ValueError(
            f"Discrepency regarding the columns. The following columns are "
            f"either missing or not expected: {diff_columns}."
        )

    return df


def test_func_error():
    expected_columns = set([
        "A", "B", "C", "D",
    ])
    err_msg = "Discrepency regarding the columns."
    with pytest.raises(ValueError, match=err_msg):
        func(expected_columns)


def test_func():
    expected_columns = set([
        "A", "B", "C"
    ])
    df = func(expected_columns)

    assert set(df.columns.tolist()) == expected_columns