Keep the error message simple.

Author: giosiragusa

Doc/library/io.rst

@@ -66,8 +66,8 @@ In-memory text streams are also available as :class:`StringIO` objects::
 
 .. note::
 
-   When working with a non-blocking stream, be aware that operations on text I/O objects
-   might raise a :exc:`BlockingIOError` if the stream cannot perform a read operation
+   When working with a non-blocking stream, be aware that read operations on text I/O objects
+   might raise a :exc:`BlockingIOError` if the stream cannot perform the operation
    immediately.
 
 

Lib/_pyio.py

@@ -2525,8 +2525,7 @@ def read(self, size=None):
         if size < 0:
             chunk = self.buffer.read()
             if chunk is None:
-                raise BlockingIOError("Unexpected None encountered. This may be due to non-blocking I/O or an issue "
-                                      "with the underlying I/O implementation.")
+                raise BlockingIOError("Read returned None.")
             # Read everything.
             result = (self._get_decoded_chars() +
                       decoder.decode(chunk, final=True))

Modules/_io/textio.c

@@ -1993,8 +1993,7 @@ _io_TextIOWrapper_read_impl(textio *self, Py_ssize_t n)
             goto fail;
 
         if (bytes == Py_None){
-            PyErr_SetString(PyExc_BlockingIOError, "Unexpected None encountered. This may be due to non-blocking I/O "
-                                                   "or an issue with the underlying I/O implementation.");
+            PyErr_SetString(PyExc_BlockingIOError, "Read returned None.");
             return NULL;
 
         }