Route warnings and error messages to stderr, not stdout.

cibuildwheel/logger.py

@@ -116,26 +116,18 @@ class Logger:
         self.error(error)
 
     def warning(self, message: str) -> None:
-        print()
-
         if self.fold_mode == 'github':
-            print(f'::warning::{message}')
+            print(f'::warning::{message}\n', file=sys.stderr)
         else:
             c = self.colors
-            print(f'{c.yellow}Warning{c.end} {message}')
-
-        print()
+            print(f'{c.yellow}Warning{c.end}: {message}\n', file=sys.stderr)
 
     def error(self, error: Union[BaseException, str]) -> None:
-        print()
-
         if self.fold_mode == 'github':
-            print(f'::error::{error}')
+            print(f'::error::{error}\n', file=sys.stderr)
         else:
             c = self.colors
-            print(f'{c.bright_red}Error{c.end} {error}')
-
-        print()
+            print(f'{c.bright_red}Error{c.end}: {error}\n', file=sys.stderr)
 
     def _start_fold_group(self, name: str) -> None:
         self._end_fold_group()

test/utils.py

@@ -37,10 +37,7 @@ def cibuildwheel_get_build_identifiers(project_path, env=None):
         env=env,
     )
 
-    lines = cmd_output.strip().split('\n')
-
-    # ignore warning lines in output
-    return [l for l in lines if l != '' and not l.startswith('Warning ')]
+    return cmd_output.strip().split('\n')
 
 
 def cibuildwheel_run(project_path, package_dir='.', env=None, add_env=None, output_dir=None):