Merge pull request #889 from pypa/fmtpass-alt

Alternative string-based approach to format string passthrough

cibuildwheel/util.py

@@ -48,14 +48,56 @@ MUSLLINUX_ARCHS = (
 )
 
 
+def format_safe(template: str, **kwargs: Any) -> str:
+    """
+    Works similarly to `template.format(**kwargs)`, except that unmatched
+    fields in `template` are passed through untouched.
+
+    >>> format_safe('{a} {b}', a='123')
+    '123 {b}'
+    >>> format_safe('{a} {b[4]:3f}', a='123')
+    '123 {b[4]:3f}'
+
+    To avoid variable expansion, precede with a single backslash e.g.
+    >>> format_safe('\\{a} {b}', a='123')
+    '{a} {b}'
+    """
+
+    result = template
+
+    for key, value in kwargs.items():
+        find_pattern = re.compile(
+            fr"""
+                (?<!\#)  # don't match if preceded by a hash
+                {{  # literal open curly bracket
+                {re.escape(key)}  # the field name
+                }}  # literal close curly bracket
+            """,
+            re.VERBOSE,
+        )
+
+        # we use a lambda for repl to prevent re.sub interpreting backslashes
+        # in repl as escape sequences
+        result = re.sub(
+            pattern=find_pattern,
+            repl=lambda _: str(value),
+            string=result,
+        )
+
+        # transform escaped sequences into their literal equivalents
+        result = result.replace(f"#{{{key}}}", f"{{{key}}}")
+
+    return result
+
+
 def prepare_command(command: str, **kwargs: PathOrStr) -> str:
     """
     Preprocesses a command by expanding variables like {python}.
 
     For example, used in the test_command option to specify the path to the
-    project's root.
+    project's root. Unmatched syntax will mostly be allowed through.
     """
-    return command.format(python="python", pip="pip", **kwargs)
+    return format_safe(command, python="python", pip="pip", **kwargs)
 
 
 def get_build_verbosity_extra_flags(level: int) -> List[str]: