Handle %s strings with length and alignment

src/flynt/transform/percent_transformer.py

@@ -11,7 +11,7 @@
 FORMAT_GROUP = f"[hlL]?[{FORMATS}]"
 FORMAT_GROUP_MATCH = f"[hlL]?([{FORMATS}])"
 
-PREFIX_GROUP = "[0-9]*[.]?[0-9]*"
+PREFIX_GROUP = "[+-]?[0-9]*[.]?[0-9]*"
 
 ANY_DICT = re.compile(r"(?<!%)%\([^)]+?\)")
 DICT_PATTERN = re.compile(rf"(%\([^)]+\){PREFIX_GROUP}{FORMAT_GROUP})")
@@ -53,6 +53,32 @@ def formatted_value(
         fmt_prefix = fmt_prefix.replace(".", "0")
 
     if fmt_spec in conversion_methods:
+        if fmt_spec == "s" and fmt_prefix:
+            # Strings are right aligned in percent fmt by default, and indicate
+            # left alignment through a negative prefix.
+            #
+            # fstrings left align by default, and separate signs from alignment
+            #
+            # Python even accepts float values here, for both percent fmt
+            # and fstrings
+            #
+            # In order to not have to figure out what sort of number we are
+            # dealing with, just look at the leading - sign (if there is one)
+            # and remove it for the conversion
+            if aggressive and fmt_prefix.startswith("-"):
+                # Left alignment
+                return ast_formatted_value(
+                    val,
+                    fmt_str=f"{fmt_prefix[1:]}",
+                    conversion=conversion_methods[fmt_spec],
+                )
+            if aggressive and not fmt_prefix.startswith("-"):
+                # Right alignment
+                return ast_formatted_value(
+                    val,
+                    fmt_str=f">{fmt_prefix}",
+                    conversion=conversion_methods[fmt_spec],
+                )
         if not aggressive and fmt_prefix:
             raise ConversionRefused(
                 "Default text alignment has changed between percent fmt and fstrings. "

test/test_edits.py

@@ -40,6 +40,24 @@ def test_call(state: State):
 
 def test_string_specific_len(state: State):
     s_in = """'%5s' % CLASS_NAMES[labels[j]]"""
+    s_expected = """f'{CLASS_NAMES[labels[j]]:>5}'"""
+
+    state.aggressive = True
+    s_out, count = code_editor.fstringify_code_by_line(s_in, state)
+    assert s_out == s_expected
+
+
+def test_string_specific_len_right_aligned(state: State):
+    s_in = """'%5s' % CLASS_NAMES[labels[j]]"""
+    s_expected = """f'{CLASS_NAMES[labels[j]]:>5}'"""
+
+    state.aggressive = True
+    s_out, count = code_editor.fstringify_code_by_line(s_in, state)
+    assert s_out == s_expected
+
+
+def test_string_specific_len_left_aligned(state: State):
+    s_in = """'%-5s' % CLASS_NAMES[labels[j]]"""
     s_expected = """f'{CLASS_NAMES[labels[j]]:5}'"""
 
     state.aggressive = True