fix: pass through unexpected format strings

cibuildwheel/util.py

@@ -12,7 +12,7 @@
 from enum import Enum
 from pathlib import Path
 from time import sleep
-from typing import Dict, Iterator, List, Optional
+from typing import Dict, Iterator, List, Optional, TypeVar
 
 import bracex
 import certifi
@@ -48,14 +48,36 @@
 )
 
 
+T = TypeVar("T", bound="LimitedExpandStr")
+
+
+class LimitedExpandStr:
+    def __init__(self, inner: PathOrStr) -> None:
+        self.inner = str(inner)
+
+    def __format__(self, fmt: str) -> str:
+        return str(self.__class__(self.inner + (f":{fmt}" if fmt else "")))
+
+    def __getitem__(self: T, item: int) -> T:
+        return self.__class__(f"{self.inner}[{item}]")
+
+    def __str__(self) -> str:
+        return f"{{{self.inner}}}"
+
+
+class SafeDict(Dict[str, PathOrStr]):
+    def __missing__(self, key: str) -> LimitedExpandStr:
+        return LimitedExpandStr(key)
+
+
 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 command.format_map(SafeDict(python="python", pip="pip", **kwargs))
 
 
 def get_build_verbosity_extra_flags(level: int) -> List[str]:

unit_test/utils_test.py

@@ -0,0 +1,12 @@
+from cibuildwheel.util import prepare_command
+
+
+def test_prepare_command():
+    assert prepare_command("python -m {project}", project="project") == "python -m project"
+    assert prepare_command("python -m {something}", project="project") == "python -m {something}"
+    assert (
+        prepare_command(
+            "{a,b}{b:.2e}{{c}}{d%s}{e:3}{f[0]}", a=42, b=3.14159  # type:ignore[arg-type]
+        )
+        == "{a,b}3.14e+00{c}{d%s}{e:3}{f[0]}"
+    )