add test for pair path

tests/tests_integration/test_get_rates.py

@@ -143,3 +143,60 @@ def test_get_rates_dry_run_always_returns_42_as_rates():
             assert (
                 row["to_currency"] in some_random_currencies
             ), f"Unexpected to_currency {row['to_currency']}"
+
+
+def test_get_rates_dry_run__with_pairs_always_returns_42_as_rates():
+    """
+    Same as the test above, but relying on the pairs argument instead
+    of the currencies one.
+    """
+
+    some_random_date = datetime.datetime(
+        year=random.choice(range(2010, 2020)),
+        month=random.choice(range(1, 13)),
+        day=random.choice(range(1, 29)),
+    ).date()
+
+    some_pairs = ["USDEUR", "USDGBP", "GBPEUR"]
+
+    runner = CliRunner()
+
+    with runner.isolated_filesystem():
+        run_result = runner.invoke(
+            get_rates,
+            [
+                "--start-date",
+                some_random_date.strftime("%Y-%m-%d"),
+                "--end-date",
+                (some_random_date + datetime.timedelta(days=3)).strftime("%Y-%m-%d"),
+                "--pairs",
+                ",".join(some_pairs),
+                "--output",
+                "test_output.csv",
+            ],
+        )
+
+        assert run_result.exit_code == 0
+
+        with open("test_output.csv", newline="") as csv_file:
+            reader = csv.DictReader(csv_file)
+            rows = list(reader)
+
+        # Ensure that the output contains the correct number of rows
+        expected_num_rows = 36
+        assert (
+            len(rows) == expected_num_rows
+        ), f"Expected {expected_num_rows} rows, but got {len(rows)}"
+
+        # Check that all rows have the expected rate of 42, 1/42 or 1 and the correct dates
+        for row in rows:
+            assert row["rate"] in (
+                "42.00000000",
+                "0.02380952",
+                "0.00000000",
+                "1.00000000",
+            ), f"Expected rate to be 42, 1/42 or 1, but got {row['rate']}"
+            assert row["rate_date"] in [
+                (some_random_date + datetime.timedelta(days=i)).strftime("%Y-%m-%d")
+                for i in range(4)
+            ], f"Unexpected rate_date {row['rate_date']}"