python · vstinner · Jun 10, 2019
diff --git a/Doc/library/exceptions.rst b/Doc/library/exceptions.rst
@@ -683,6 +683,14 @@ The following exceptions are used as warning categories; see the
    Base class for warnings generated by user code.
 
 
+.. exception:: EncodingWarning
+
+   Base class for warnings about encodings when those warnings are intended for
+   other Python developers.
+
+   .. versionadded:: 3.9
+
+
 .. exception:: DeprecationWarning
 
    Base class for warnings about deprecated features when those warnings are

diff --git a/Include/pyerrors.h b/Include/pyerrors.h
@@ -139,6 +139,7 @@ PyAPI_DATA(PyObject *) PyExc_WindowsError;
 /* Predefined warning categories */
 PyAPI_DATA(PyObject *) PyExc_Warning;
 PyAPI_DATA(PyObject *) PyExc_UserWarning;
+PyAPI_DATA(PyObject *) PyExc_EncodingWarning;
 PyAPI_DATA(PyObject *) PyExc_DeprecationWarning;
 PyAPI_DATA(PyObject *) PyExc_PendingDeprecationWarning;
 PyAPI_DATA(PyObject *) PyExc_SyntaxWarning;

diff --git a/Lib/_pyio.py b/Lib/_pyio.py
@@ -1986,6 +1986,12 @@ def __init__(self, buffer, encoding=None, errors=None, newline=None,
                  line_buffering=False, write_through=False):
         self._check_newline(newline)
         if encoding is None:
+            import warnings
+            stack_level = 2
+            if sys._getframe(1).f_code.co_name == "open":
+                # open(encoding=None): report the caller frame
+                stack_level += 1
+            warnings.warn("encoding=None", EncodingWarning, stack_level)
             try:
                 encoding = os.device_encoding(buffer.fileno())
             except (AttributeError, UnsupportedOperation):

diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
@@ -3642,6 +3642,11 @@ def test_issue25862(self):
         t.write('x')
         t.tell()
 
+    def test_encoding_warning(self):
+        with support.check_warnings(('encoding=None', EncodingWarning)):
+            t = self.TextIOWrapper(self.BytesIO(b'test'), encoding=None)
+            t.close()
+
 
 class MemviewBytesIO(io.BytesIO):
     '''A BytesIO object whose read method returns memoryviews

diff --git a/Misc/NEWS.d/next/Core and Builtins/2019-06-10-12-50-34.bpo-37214.5J2UJ3.rst b/Misc/NEWS.d/next/Core and Builtins/2019-06-10-12-50-34.bpo-37214.5J2UJ3.rst
@@ -0,0 +1,3 @@
+Add a new :exc:`EncodingWarning` warning category. :func:`open`
+(:func:`io.open`) now emits an :exc:`EncodingWarning` if called with
+``encoding=None``.
diff --git a/Modules/_io/textio.c b/Modules/_io/textio.c
@@ -1109,6 +1109,10 @@ _io_TextIOWrapper___init___impl(textio *self, PyObject *buffer,
         }
     }
     if (encoding == NULL && self->encoding == NULL) {
+        if (PyErr_WarnEx(PyExc_EncodingWarning, "encoding=None", 0)) {
+            goto error;
+        }
+
         PyObject *locale_module = _PyIO_get_locale_module(state);
         if (locale_module == NULL)
             goto catch_ImportError;

diff --git a/Objects/exceptions.c b/Objects/exceptions.c
@@ -2375,6 +2375,13 @@ SimpleExtendsException(PyExc_Warning, UserWarning,
                        "Base class for warnings generated by user code.");
 
 
+/*
+ *    EncodingWarning extends Warning
+ */
+SimpleExtendsException(PyExc_Warning, EncodingWarning,
+                       "Base class for warnings about encodings.");
+
+
 /*
  *    DeprecationWarning extends Warning
  */
@@ -2560,6 +2567,7 @@ _PyExc_Init(void)
     PRE_INIT(BufferError);
     PRE_INIT(Warning);
     PRE_INIT(UserWarning);
+    PRE_INIT(EncodingWarning);
     PRE_INIT(DeprecationWarning);
     PRE_INIT(PendingDeprecationWarning);
     PRE_INIT(SyntaxWarning);
@@ -2701,6 +2709,7 @@ _PyBuiltins_AddExceptions(PyObject *bltinmod)
     POST_INIT(BufferError);
     POST_INIT(Warning);
     POST_INIT(UserWarning);
+    POST_INIT(EncodingWarning);
     POST_INIT(DeprecationWarning);
     POST_INIT(PendingDeprecationWarning);
     POST_INIT(SyntaxWarning);