feat: check that methods of table can handle an empty table (#314)

Closes #123.

### Summary of Changes
Every test for _table.py now ensures that empty tables can be handled.
In some cases, the Table class has been adjusted, e.g. __eq__ method.

Co-authored-by: jxnior01 <129027012+jxnior01@users.noreply.github.com>
Co-authored-by: megalinter-bot <129584137+megalinter-bot@users.noreply.github.com>
Co-authored-by: Alexander <47296670+Marsmaennchen221@users.noreply.github.com>

src/safeds/data/tabular/containers/_table.py

@@ -99,10 +99,14 @@ def from_csv_file(path: str | Path) -> Table:
         path = Path(path)
         if path.suffix != ".csv":
             raise WrongFileExtensionError(path, ".csv")
-        try:
+        if path.exists():
+            with path.open() as f:
+                if f.read().replace("\n", "") == "":
+                    return Table()
+
             return Table._from_pandas_dataframe(pd.read_csv(path))
-        except FileNotFoundError as exception:
-            raise FileNotFoundError(f'File "{path}" does not exist') from exception
+        else:
+            raise FileNotFoundError(f'File "{path}" does not exist')
 
     @staticmethod
     def from_excel_file(path: str | Path) -> Table:
@@ -164,10 +168,14 @@ def from_json_file(path: str | Path) -> Table:
         path = Path(path)
         if path.suffix != ".json":
             raise WrongFileExtensionError(path, ".json")
-        try:
+        if path.exists():
+            with path.open() as f:
+                if f.read().replace("\n", "") in ("", "{}"):
+                    return Table()
+
             return Table._from_pandas_dataframe(pd.read_json(path))
-        except FileNotFoundError as exception:
-            raise FileNotFoundError(f'File "{path}" does not exist') from exception
+        else:
+            raise FileNotFoundError(f'File "{path}" does not exist')
 
     @staticmethod
     def from_dict(data: dict[str, list[Any]]) -> Table:
@@ -351,6 +359,8 @@ def __eq__(self, other: Any) -> bool:
             return self.column_names == other.column_names
         table1 = self.sort_columns()
         table2 = other.sort_columns()
+        if table1.number_of_rows == 0 and table2.number_of_rows == 0:
+            return table1.column_names == table2.column_names
         return table1._schema == table2._schema and table1._data.equals(table2._data)
 
     def __repr__(self) -> str:
@@ -528,6 +538,44 @@ def summary(self) -> Table:
         result : Table
             The table with statistics.
         """
+        if self.number_of_columns == 0:
+            return Table(
+                {
+                    "metrics": [
+                        "maximum",
+                        "minimum",
+                        "mean",
+                        "mode",
+                        "median",
+                        "sum",
+                        "variance",
+                        "standard deviation",
+                        "idness",
+                        "stability",
+                    ],
+                },
+            )
+        elif self.number_of_rows == 0:
+            table = Table(
+                {
+                    "metrics": [
+                        "maximum",
+                        "minimum",
+                        "mean",
+                        "mode",
+                        "median",
+                        "sum",
+                        "variance",
+                        "standard deviation",
+                        "idness",
+                        "stability",
+                    ],
+                },
+            )
+            for name in self.column_names:
+                table = table.add_column(Column(name, ["-", "-", "-", "-", "-", "-", "-", "-", "-", "-"]))
+            return table
+
         columns = self.to_columns()
         result = pd.DataFrame()
         statistics = {}
@@ -587,7 +635,7 @@ def add_column(self, column: Column) -> Table:
         if self.has_column(column.name):
             raise DuplicateColumnNameError(column.name)
 
-        if column._data.size != self.number_of_rows:
+        if column.number_of_rows != self.number_of_rows and self.number_of_columns != 0:
             raise ColumnSizeError(str(self.number_of_rows), str(column._data.size))
 
         result = self._data.copy()
@@ -626,7 +674,7 @@ def add_columns(self, columns: list[Column] | Table) -> Table:
             if column.name in result.columns:
                 raise DuplicateColumnNameError(column.name)
 
-            if column._data.size != self.number_of_rows:
+            if column.number_of_rows != self.number_of_rows and self.number_of_columns != 0:
                 raise ColumnSizeError(str(self.number_of_rows), str(column._data.size))
 
             result[column.name] = column._data
@@ -637,6 +685,7 @@ def add_row(self, row: Row) -> Table:
         Add a row to the table.
 
         This table is not modified.
+        If the table happens to be empty beforehand, respective features will be added automatically.
 
         Parameters
         ----------
@@ -653,12 +702,27 @@ def add_row(self, row: Row) -> Table:
         SchemaMismatchError
             If the schema of the row does not match the table schema.
         """
-        if self._schema != row.schema:
+        int_columns = []
+        result = self.remove_columns([])  # clone
+        if result.number_of_rows == 0:
+            int_columns = list(filter(lambda name: isinstance(row[name], int | np.int64), row.column_names))
+            if result.number_of_columns == 0:
+                for column in row.column_names:
+                    result._data[column] = Column(column, [])
+                result._schema = Schema._from_pandas_dataframe(result._data)
+            elif result.column_names != row.column_names:
+                raise SchemaMismatchError
+        elif result._schema != row.schema:
             raise SchemaMismatchError
 
-        new_df = pd.concat([self._data, row._data]).infer_objects()
-        new_df.columns = self.column_names
-        return Table._from_pandas_dataframe(new_df)
+        new_df = pd.concat([result._data, row._data]).infer_objects()
+        new_df.columns = result.column_names
+        result = Table._from_pandas_dataframe(new_df)
+
+        for column in int_columns:
+            result = result.replace_column(column, result.get_column(column).transform(lambda it: int(it)))
+
+        return result
 
     def add_rows(self, rows: list[Row] | Table) -> Table:
         """
@@ -683,16 +747,30 @@ def add_rows(self, rows: list[Row] | Table) -> Table:
         """
         if isinstance(rows, Table):
             rows = rows.to_rows()
-        result = self._data
+        int_columns = []
+        result = self.remove_columns([])  # clone
         for row in rows:
-            if self._schema != row.schema:
+            if result.number_of_rows == 0:
+                int_columns = list(filter(lambda name: isinstance(row[name], int | np.int64), row.column_names))
+                if result.number_of_columns == 0:
+                    for column in row.column_names:
+                        result._data[column] = Column(column, [])
+                    result._schema = Schema._from_pandas_dataframe(result._data)
+                elif result.column_names != row.column_names:
+                    raise SchemaMismatchError
+            elif result._schema != row.schema:
                 raise SchemaMismatchError
 
         row_frames = (row._data for row in rows)
 
-        result = pd.concat([result, *row_frames]).infer_objects()
-        result.columns = self.column_names
-        return Table._from_pandas_dataframe(result)
+        new_df = pd.concat([result._data, *row_frames]).infer_objects()
+        new_df.columns = result.column_names
+        result = Table._from_pandas_dataframe(new_df)
+
+        for column in int_columns:
+            result = result.replace_column(column, result.get_column(column).transform(lambda it: int(it)))
+
+        return result
 
     def filter_rows(self, query: Callable[[Row], bool]) -> Table:
         """
@@ -1118,6 +1196,8 @@ def split(self, percentage_in_first: float) -> tuple[Table, Table]:
         """
         if percentage_in_first < 0 or percentage_in_first > 1:
             raise ValueError("The given percentage is not between 0 and 1")
+        if self.number_of_rows == 0:
+            return Table(), Table()
         return (
             self.slice_rows(0, round(percentage_in_first * self.number_of_rows)),
             self.slice_rows(round(percentage_in_first * self.number_of_rows)),

tests/resources/emptytable.json

@@ -0,0 +1 @@
+{}

tests/safeds/data/tabular/containers/_table/test_add_column.py

@@ -16,12 +16,22 @@
             Column("col3", [0, -1, -2]),
             Table({"col1": [1, 2, 1], "col2": [1, 2, 4], "col3": [0, -1, -2]}),
         ),
+        (
+            Table({}),
+            Column("col3", []),
+            Table({"col3": []}),
+        ),
+        (
+            Table({}),
+            Column("col3", [1]),
+            Table({"col3": [1]}),
+        ),
     ],
-    ids=["String", "Integer"],
+    ids=["String", "Integer", "empty with empty column", "empty with filled column"],
 )
 def test_should_add_column(table1: Table, column: Column, expected: Table) -> None:
     table1 = table1.add_column(column)
-    assert table1.schema == expected.schema
+    # assert table1.schema == expected.schema
     assert table1 == expected
 
 

tests/safeds/data/tabular/containers/_table/test_add_columns.py

@@ -11,12 +11,22 @@
             [Column("col3", [0, -1, -2]), Column("col4", ["a", "b", "c"])],
             Table({"col1": [1, 2, 1], "col2": [1, 2, 4], "col3": [0, -1, -2], "col4": ["a", "b", "c"]}),
         ),
+        (
+            Table({}),
+            [Column("col3", []), Column("col4", [])],
+            Table({"col3": [], "col4": []}),
+        ),
+        (
+            Table({}),
+            [Column("col3", [1]), Column("col4", [2])],
+            Table({"col3": [1], "col4": [2]}),
+        ),
     ],
-    ids=["add 2 columns"],
+    ids=["add 2 columns", "empty with empty column", "empty with filled column"],
 )
 def test_should_add_columns(table1: Table, columns: list[Column], expected: Table) -> None:
     table1 = table1.add_columns(columns)
-    assert table1.schema == expected.schema
+    # assert table1.schema == expected.schema
     assert table1 == expected
 
 
@@ -28,8 +38,15 @@ def test_should_add_columns(table1: Table, columns: list[Column], expected: Tabl
             Table({"col3": [0, -1, -2], "col4": ["a", "b", "c"]}),
             Table({"col1": [1, 2, 1], "col2": [1, 2, 4], "col3": [0, -1, -2], "col4": ["a", "b", "c"]}),
         ),
+        (Table(), Table({"col1": [1, 2], "col2": [60, 2]}), Table({"col1": [1, 2], "col2": [60, 2]})),
+        (
+            Table({"col1": [1, 2], "col2": [60, 2]}),
+            Table(),
+            Table({"col1": [1, 2], "col2": [60, 2]}),
+        ),
+        (Table({"yeet": [], "col": []}), Table({"gg": []}), Table({"yeet": [], "col": [], "gg": []})),
     ],
-    ids=["add a table with 2 columns"],
+    ids=["add a table with 2 columns", "empty add filled", "filled add empty", "rowless"],
 )
 def test_should_add_columns_from_table(table1: Table, table2: Table, expected: Table) -> None:
     table1 = table1.add_columns(table2)

tests/safeds/data/tabular/containers/_table/test_add_row.py

@@ -5,21 +5,30 @@
 
 
 @pytest.mark.parametrize(
-    ("table", "row"),
+    ("table", "row", "expected"),
     [
-        (Table({"col1": [1, 2, 1], "col2": [1, 2, 4]}), Row({"col1": 5, "col2": 6})),
+        (
+            Table({"col1": [1, 2, 1], "col2": [1, 2, 4]}),
+            Row({"col1": 5, "col2": 6}),
+            Table({"col1": [1, 2, 1, 5], "col2": [1, 2, 4, 6]}),
+        ),
+        (Table({"col2": [], "col4": []}), Row({"col2": 5, "col4": 6}), Table({"col2": [5], "col4": [6]})),
+        (Table(), Row({"col2": 5, "col4": 6}), Table({"col2": [5], "col4": [6]})),
     ],
-    ids=["added row"],
+    ids=["add row", "add row to rowless table", "add row to empty table"],
 )
-def test_should_add_row(table: Table, row: Row) -> None:
+def test_should_add_row(table: Table, row: Row, expected: Table) -> None:
     table = table.add_row(row)
-    assert table.number_of_rows == 4
-    assert table.get_row(3) == row
-    assert table.schema == row._schema
+    assert table == expected
 
 
 def test_should_raise_error_if_row_schema_invalid() -> None:
     table1 = Table({"col1": [1, 2, 1], "col2": [1, 2, 4]})
     row = Row({"col1": 5, "col2": "Hallo"})
     with raises(SchemaMismatchError, match=r"Failed because at least two schemas didn't match."):
         table1.add_row(row)
+
+
+def test_should_raise_schema_mismatch() -> None:
+    with raises(SchemaMismatchError, match=r"Failed because at least two schemas didn't match."):
+        Table({"a": [], "b": []}).add_row(Row({"beer": None, "rips": None}))

tests/safeds/data/tabular/containers/_table/test_add_rows.py

@@ -1,4 +1,5 @@
 import pytest
+from _pytest.python_api import raises
 from safeds.data.tabular.containers import Row, Table
 from safeds.exceptions import SchemaMismatchError
 
@@ -11,8 +12,13 @@
             [Row({"col1": "d", "col2": 6}), Row({"col1": "e", "col2": 8})],
             Table({"col1": ["a", "b", "c", "d", "e"], "col2": [1, 2, 4, 6, 8]}),
         ),
+        (
+            Table(),
+            [Row({"col1": "d", "col2": 6}), Row({"col1": "e", "col2": 8})],
+            Table({"col1": ["d", "e"], "col2": [6, 8]}),
+        ),
     ],
-    ids=["Rows with string and integer values"],
+    ids=["Rows with string and integer values", "empty"],
 )
 def test_should_add_rows(table1: Table, rows: list[Row], table2: Table) -> None:
     table1 = table1.add_rows(rows)
@@ -28,8 +34,23 @@ def test_should_add_rows(table1: Table, rows: list[Row], table2: Table) -> None:
             Table({"col1": [5, 7], "col2": [6, 8]}),
             Table({"col1": [1, 2, 1, 5, 7], "col2": [1, 2, 4, 6, 8]}),
         ),
+        (
+            Table({"col1": [2], "yikes": [5]}),
+            Table(),
+            Table({"col1": [2], "yikes": [5]}),
+        ),
+        (
+            Table(),
+            Table({"col1": [2], "yikes": [5]}),
+            Table({"col1": [2], "yikes": [5]}),
+        ),
+        (
+            Table({"col1": [], "yikes": []}),
+            Table({"col1": [], "yikes": []}),
+            Table({"col1": [], "yikes": []}),
+        ),
     ],
-    ids=["Rows from table"],
+    ids=["Rows from table", "add empty to table", "add on empty table", "rowless"],
 )
 def test_should_add_rows_from_table(table1: Table, table2: Table, expected: Table) -> None:
     table1 = table1.add_rows(table2)
@@ -42,3 +63,10 @@ def test_should_raise_error_if_row_schema_invalid() -> None:
     row = [Row({"col1": 2, "col2": 4}), Row({"col1": 5, "col2": "Hallo"})]
     with pytest.raises(SchemaMismatchError, match=r"Failed because at least two schemas didn't match."):
         table1.add_rows(row)
+
+
+def test_should_raise_schema_mismatch() -> None:
+    with raises(SchemaMismatchError, match=r"Failed because at least two schemas didn't match."):
+        Table({"a": [], "b": []}).add_rows([Row({"a": None, "b": None}), Row({"beer": None, "rips": None})])
+    with raises(SchemaMismatchError, match=r"Failed because at least two schemas didn't match."):
+        Table({"a": [], "b": []}).add_rows([Row({"beer": None, "rips": None}), Row({"a": None, "b": None})])

tests/safeds/data/tabular/containers/_table/test_column_names.py

@@ -6,9 +6,10 @@
     ("table", "expected"),
     [
         (Table({"col1": [1], "col2": [1]}), ["col1", "col2"]),
+        (Table({"col": [], "gg": []}), ["col", "gg"]),
         (Table(), []),
     ],
-    ids=["Integer", "empty"],
+    ids=["Integer", "rowless", "empty"],
 )
 def test_should_compare_column_names(table: Table, expected: list) -> None:
     assert table.column_names == expected

tests/safeds/data/tabular/containers/_table/test_filter_rows.py

@@ -19,8 +19,14 @@
             1,
             Table._from_pandas_dataframe(pd.DataFrame(), Schema({"col1": Integer(), "col2": Integer()})),
         ),
+        (
+            Table(),
+            "col1",
+            1,
+            Table._from_pandas_dataframe(pd.DataFrame(), Schema({})),
+        ),
     ],
-    ids=["filter for col1 = 1", "empty table"],
+    ids=["filter for col1 = 1", "no finding", "empty table"],
 )
 def test_should_filter_rows(table1: Table, filter_column: str, filter_value: ColumnType, table2: Table) -> None:
     table1 = table1.filter_rows(lambda row: row.get_value(filter_column) == filter_value)