Use hash rather than backslash for escaping

cibuildwheel/util.py

@@ -68,7 +68,7 @@ def format_safe(template: str, **kwargs: Any) -> str:
     for key, value in kwargs.items():
         find_pattern = re.compile(
             fr"""
-                (?<!\\)  # don't match if preceded by a backslash
+                (?<!\#)  # don't match if preceded by a hash
                 {{  # literal open curly bracket
                 {re.escape(key)}  # the field name
                 }}  # literal close curly bracket
@@ -85,7 +85,7 @@ def format_safe(template: str, **kwargs: Any) -> str:
         )
 
         # transform escaped sequences into their literal equivalents
-        result = result.replace(f"\\{{{key}}}", f"{{{key}}}")
+        result = result.replace(f"#{{{key}}}", f"{{{key}}}")
 
     return result
 

unit_test/utils_test.py

@@ -3,8 +3,8 @@ from cibuildwheel.util import format_safe, prepare_command
 
 def test_format_safe():
     assert format_safe("{wheel}", wheel="filename.whl") == "filename.whl"
-    assert format_safe("command \\{wheel}", wheel="filename.whl") == "command {wheel}"
+    assert format_safe("command #{wheel}", wheel="filename.whl") == "command {wheel}"
-    assert format_safe("{command \\{wheel}}", wheel="filename.whl") == "{command {wheel}}"
+    assert format_safe("{command #{wheel}}", wheel="filename.whl") == "{command {wheel}}"
 
     # check unmatched brackets
     assert format_safe("{command {wheel}", wheel="filename.whl") == "{command filename.whl"
@@ -15,6 +15,10 @@ def test_format_safe():
         == "find . -name  * -exec ls -a {} \\;"
     )
 
+    assert format_safe("{param} {param}", param="1") == "1 1"
+    assert format_safe("# {param} {param}", param="1") == "# 1 1"
+    assert format_safe("#{not_a_param} {param}", param="1") == "#{not_a_param} 1"
+
 
 def test_prepare_command():
     assert prepare_command("python -m {project}", project="project") == "python -m project"