Repository: jazzband/tablib (python)

Issue:
Output formatter corrupts original data
version: 3.5.0
python 3.11

I'm trying to export the same dataset several times in different file formats. I've found that specifying a formatter for a column data alters the underlying data. The issue occurs if exporting the same format too.

```python
import tablib

dataset = tablib.Dataset()
dataset.headers = ["text"]
dataset.append(["aAbBcC"])
dataset.add_formatter("text", lambda s: s[2:])

print(dataset.export("csv"))
print(dataset.export("csv"))
```
Output:
```
text
bBcC

text
cC
```


Pull request:
Fixes #578 - Do not mutate data in place when applying formatters

Patch:
diff --git a/src/tablib/core.py b/src/tablib/core.py
index 956458b..70e26f0 100644
--- a/src/tablib/core.py
+++ b/src/tablib/core.py
@@ -73,6 +73,9 @@ class Row:
     def insert(self, index, value):
         self._row.insert(index, value)
 
+    def copy(self):
+        return Row(self._row.copy(), self.tags.copy())
+
     def __contains__(self, item):
         return item in self._row
 
@@ -270,27 +273,25 @@ class Dataset:
 
         _data = list(self._data)
 
-        # Execute formatters
-        if self._formatters:
-            for row_i, row in enumerate(_data):
+        def format_row(row):
+            # Execute formatters
+            if self._formatters:
+                row = row.copy()  # To not mutate internal data structure
                 for col, callback in self._formatters:
-                    try:
-                        if col is None:
-                            for j, c in enumerate(row):
-                                _data[row_i][j] = callback(c)
-                        else:
-                            _data[row_i][col] = callback(row[col])
-                    except IndexError:
-                        raise InvalidDatasetIndex
+                    if col is None:
+                        # Apply formatter to all cells
+                        row = [callback(cell) for cell in row]
+                    else:
+                        row[col] = callback(row[col])
+            return list(row)
 
         if self.headers:
             if dicts:
-                data = [dict(list(zip(self.headers, data_row))) for data_row in _data]
+                data = [dict(list(zip(self.headers, format_row(row)))) for row in _data]
             else:
-                data = [list(self.headers)] + list(_data)
+                data = [list(self.headers)] + [format_row(row) for row in _data]
         else:
-            data = [list(row) for row in _data]
-
+            data = [format_row(row) for row in _data]
         return data
 
     def _get_headers(self):
@@ -622,7 +623,8 @@ class Dataset:
     def add_formatter(self, col, handler):
         """Adds a formatter to the :class:`Dataset`.
 
-        :param col: column to. Accepts index int or header str.
+        :param col: column to. Accepts index int, header str, or None to apply
+                    the formatter to all columns.
         :param handler: reference to callback function to execute against
                         each cell value.
         """
@@ -633,7 +635,7 @@ class Dataset:
             else:
                 raise KeyError
 
-        if not col > self.width:
+        if col is None or col <= self.width:
             self._formatters.append((col, handler))
         else:
             raise InvalidDatasetIndex


Test patch:
diff --git a/tests/test_tablib.py b/tests/test_tablib.py
index ee34186..ee562b9 100755
--- a/tests/test_tablib.py
+++ b/tests/test_tablib.py
@@ -564,12 +564,34 @@ class TablibTestCase(BaseTestCase):
         """Confirm formatters are being triggered."""
 
         def _formatter(cell_value):
-            return str(cell_value).upper()
+            return str(cell_value)[1:]
 
         self.founders.add_formatter('last_name', _formatter)
 
-        for name in [r['last_name'] for r in self.founders.dict]:
-            self.assertTrue(name.isupper())
+        expected = [
+            {'first_name': 'John', 'last_name': 'dams', 'gpa': 90},
+            {'first_name': 'George', 'last_name': 'ashington', 'gpa': 67},
+            {'first_name': 'Thomas', 'last_name': 'efferson', 'gpa': 50},
+        ]
+        self.assertEqual(self.founders.dict, expected)
+        # Test once more as the result should be the same
+        self.assertEqual(self.founders.dict, expected)
+
+    def test_formatters_all_cols(self):
+        """
+        Passing None as first add_formatter param apply formatter to all columns.
+        """
+
+        def _formatter(cell_value):
+            return str(cell_value).upper()
+
+        self.founders.add_formatter(None, _formatter)
+
+        self.assertEqual(self.founders.dict, [
+            {'first_name': 'JOHN', 'last_name': 'ADAMS', 'gpa': '90'},
+            {'first_name': 'GEORGE', 'last_name': 'WASHINGTON', 'gpa': '67'},
+            {'first_name': 'THOMAS', 'last_name': 'JEFFERSON', 'gpa': '50'},
+        ])
 
     def test_unicode_renders_markdown_table(self):
         # add another entry to test right field width for