Render desc as a template again

docs/howto/tqdm-style.rst

@@ -9,8 +9,8 @@ here.
 .. demo:: howto/tqdm-style
 
 ``desc`` and ``total`` work on any ``ProgressBar``: ``desc`` becomes the
-prefix (rendered as ``f'{desc}: '``, as literal text, so braces in a file
-name stay braces), and ``total`` becomes ``max_value`` when ``max_value``
+prefix (rendered as ``f'{desc}: '``, a format template like ``prefix``,
+so double a literal brace), and ``total`` becomes ``max_value`` when ``max_value``
 isn't given explicitly. ``unit=``/``unit_scale=``
 don't render anything by themselves -- they only show up through a
 widget that reads them, such as ``UnitProgress()`` -- so pass a widget

progressbar/bar.py

@@ -915,10 +915,10 @@ class ProgressBar(
         line_offset: The number of lines to offset the progressbar from
             your current line. This is useful if you have other output or
             multiple progressbars.
-        desc: tqdm-style description, rendered literally as
-            `f'{desc}: '` in front of the bar. Unlike `prefix` it is
-            never a format template: braces in a file name stay braces.
-            Ignored if `prefix` is also given.
+        desc: tqdm-style alias for `prefix`, rendered as `f'{desc}: '`
+            in front of the bar. Like `prefix` it is a format template,
+            so `desc='{value}'` shows the value and a literal brace
+            must be doubled. Ignored if `prefix` is also given.
         total: tqdm-style alias for `max_value`. Ignored if `max_value` is
             also given.
         unit: The unit label used by unit-aware widgets. Defaults to
@@ -1002,14 +1002,9 @@ def __init__(
         if max_value is None and total is not None:
             # tqdm-style alias for `max_value`
             max_value = total
-        self._literal_prefix: str | None = None
         if prefix is None and desc is not None:
-            # tqdm-style alias for `prefix`: `desc` is literal text, as in
-            # tqdm and as the fast path renders it. Remember the exact
-            # string, so `_init_prefix` escapes its braces and a prefix
-            # assigned later is a template again.
+            # tqdm-style alias for `prefix`, a template like `prefix`
             prefix = f'{desc}: '
-            self._literal_prefix = prefix
 
         if max_value and min_value > typing.cast(NumberT, max_value):
             raise ValueError(
@@ -1823,12 +1818,8 @@ def _init_prefix(self) -> None:
         if self.prefix:
             widgets = _load_widgets()
 
-            template: str = self.prefix
-            if template is getattr(self, '_literal_prefix', None):
-                # A desc is literal text: escape its braces for the label
-                template = template.replace('{', '{{').replace('}', '}}')
             self.widgets.insert(
-                0, widgets.FormatLabel(template, new_style=True)
+                0, widgets.FormatLabel(self.prefix, new_style=True)
             )
             self.prefix = None
 

tests/test_format_security.py

@@ -24,16 +24,20 @@ def _render(**kwargs: object) -> str:
     return fd.getvalue()
 
 
-def test_desc_attribute_walk_renders_literally() -> None:
+def test_desc_attribute_walk_is_refused() -> None:
     desc: str = (
         '{variables.__class__.__getattr__.__globals__[os].environ[HOME]}'
     )
-    output: str = _render(desc=desc)
-    assert desc + ': ' in output
+    with pytest.raises(ValueError, match='__class__'):
+        _render(desc=desc)
 
 
-def test_desc_with_braces_renders_literally() -> None:
-    output: str = _render(desc='report{2024}.txt')
+def test_desc_is_a_template_like_prefix() -> None:
+    # 4.6.0 rendered desc through the prefix label, so a field in it
+    # shows the bar's data and a literal brace has to be doubled.
+    output: str = _render(desc='{value} done')
+    assert '10 done: ' in output  # the finishing redraw
+    output = _render(desc='report{{2024}}.txt')
     assert 'report{2024}.txt: ' in output
 
 
@@ -239,30 +243,24 @@ def warn() -> None:
     assert len(messages) == 18
 
 
-def test_desc_prefix_is_a_format_label_that_renders_literally() -> None:
+def test_desc_prefix_is_a_format_label_with_variables() -> None:
     fd: io.StringIO = io.StringIO()
     bar: progressbar.ProgressBar = progressbar.ProgressBar(
         fd=fd,
         max_value=10,
-        desc='{files}',
+        desc='{variables.files}',
+        variables={'files': 'spam'},
         widgets=[progressbar.SimpleProgress()],
     )
-    assert bar.prefix == '{files}: '  # read back unescaped, as in 4.6.0
+    assert bar.prefix == '{variables.files}: '  # as in 4.6.0
     bar.start()
     assert isinstance(bar.widgets[0], progressbar.FormatLabel)  # as in 4.6.0
     bar.update(4, force=True)
-    assert '{files}: 4 of 10' in fd.getvalue()
+    assert 'spam: 4 of 10' in fd.getvalue()
 
 
-def test_desc_with_braces_renders_literally_on_the_fast_path() -> None:
-    fd: io.StringIO = io.StringIO()
-    bar: progressbar.ProgressBar = progressbar.ProgressBar(
-        fd=fd, max_value=10, desc='{files}'
-    )
-    bar.start()
-    bar.update(4, force=True)
-    assert '{files}: ' in fd.getvalue()
-    assert '{{files}}' not in fd.getvalue()
+def test_desc_renders_literally_on_the_fast_path() -> None:
+    # The fast formatter never rendered templates, in 4.6.0 either.
     fast: progressbar.FastProgressBar = progressbar.FastProgressBar(
         fd=io.StringIO(), max_value=10, desc='{files}'
     )