Issue #18076: Introduce imoportlib.util.decode_source().

The helper function makes it easier to implement
imoprtlib.abc.InspectLoader.get_source() by making that function
require just the raw bytes for source code and handling all other
details.

Doc/library/importlib.rst

@@ -916,6 +916,14 @@ an :term:`importer`.
 
    .. versionadded:: 3.4
 
+.. function:: decode_source(source_bytes)
+
+   Decode the given bytes representing source code and return it as a string
+   with universal newlines (as required by
+   :meth:`importlib.abc.InspectLoader.get_source`).
+
+   .. versionadded:: 3.4
+
 .. function:: resolve_name(name, package)
 
    Resolve a relative module name to an absolute one.

Lib/importlib/_bootstrap.py

@@ -723,6 +723,18 @@ def _code_to_bytecode(code, mtime=0, source_size=0):
     return data
 
 
+def decode_source(source_bytes):
+    """Decode bytes representing source code and return the string.
+
+    Universal newline support is used in the decoding.
+    """
+    import tokenize  # To avoid bootstrap issues.
+    source_bytes_readline = _io.BytesIO(source_bytes).readline
+    encoding = tokenize.detect_encoding(source_bytes_readline)
+    newline_decoder = _io.IncrementalNewlineDecoder(None, True)
+    return newline_decoder.decode(source_bytes.decode(encoding[0]))
+
+
 # Loaders #####################################################################
 
 class BuiltinImporter:
@@ -965,11 +977,7 @@ class SourceLoader(_LoaderBasics):
         except OSError as exc:
             raise ImportError("source not available through get_data()",
                               name=fullname) from exc
-        import tokenize
-        readsource = _io.BytesIO(source_bytes).readline
-        encoding = tokenize.detect_encoding(readsource)
-        newline_decoder = _io.IncrementalNewlineDecoder(None, True)
-        return newline_decoder.decode(source_bytes.decode(encoding[0]))
+        return decode_source(source_bytes)
 
     def source_to_code(self, data, path, *, _optimize=-1):
         """Return the code object compiled from source.

Lib/importlib/util.py

@@ -2,6 +2,7 @@
 
 from ._bootstrap import MAGIC_NUMBER
 from ._bootstrap import cache_from_source
+from ._bootstrap import decode_source
 from ._bootstrap import module_to_load
 from ._bootstrap import set_loader
 from ._bootstrap import set_package

Lib/test/test_importlib/test_util.py

@@ -9,6 +9,27 @@ import unittest
 import warnings
 
 
+class DecodeSourceBytesTests(unittest.TestCase):
+
+    source = "string ='ü'"
+
+    def test_ut8_default(self):
+        source_bytes = self.source.encode('utf-8')
+        self.assertEqual(util.decode_source(source_bytes), self.source)
+
+    def test_specified_encoding(self):
+        source = '# coding=latin-1\n' + self.source
+        source_bytes = source.encode('latin-1')
+        assert source_bytes != source.encode('utf-8')
+        self.assertEqual(util.decode_source(source_bytes), source)
+
+    def test_universal_newlines(self):
+        source = '\r\n'.join([self.source, self.source])
+        source_bytes = source.encode('utf-8')
+        self.assertEqual(util.decode_source(source_bytes),
+                         '\n'.join([self.source, self.source]))
+
+
 class ModuleToLoadTests(unittest.TestCase):
 
     module_name = 'ModuleManagerTest_module'

Misc/NEWS

@@ -123,6 +123,8 @@ Core and Builtins
 Library
 -------
 
+- Issue #18076: Introduce importlib.util.decode_source().
+
 - importlib.abc.SourceLoader.get_source() no longer changes SyntaxError or
   UnicodeDecodeError into ImportError.