The subprocess documentation currently just says that Popen.stdin et all are "file objects", which is linked to the glossary entry. This isn't very helpful, as it doesn't tell whether the streams are bytes or text streams.

Suggested patch:

diff --git a/Doc/library/subprocess.rst b/Doc/library/subprocess.rst
--- a/Doc/library/subprocess.rst
+++ b/Doc/library/subprocess.rst
@@ -677,21 +677,29 @@
 
 .. attribute:: Popen.stdin
 
-   If the *stdin* argument was :data:`PIPE`, this attribute is a :term:`file
-   object` that provides input to the child process.  Otherwise, it is ``None``.
+   If the *stdin* argument was :data:`PIPE`, this attribute is a
+   :class:`io.BufferedWriter` (if the *universal_newlines* argument
+   was False) or :class:`io.TextIOWrapper` (if the
+   *universal_newlines* argument was True) object that provides input
+   to the child process. Otherwise, it is ``None``.
 
 
 .. attribute:: Popen.stdout
 
-   If the *stdout* argument was :data:`PIPE`, this attribute is a :term:`file
-   object` that provides output from the child process.  Otherwise, it is ``None``.
+   If the *stdout* argument was :data:`PIPE`, this attribute is a
+   :class:`io.BufferedReader` (if the *universal_newlines* argument
+   was False) or :class:`io.TextIOWrapper` (if the
+   *universal_newlines* argument was True) object that provides output
+   from the child process. Otherwise, it is ``None``.
 
 
 .. attribute:: Popen.stderr
 
-   If the *stderr* argument was :data:`PIPE`, this attribute is a :term:`file
-   object` that provides error output from the child process.  Otherwise, it is
-   ``None``.
+   If the *stderr* argument was :data:`PIPE`, this attribute is a
+   :class:`io.BufferedReader` (if the *universal_newlines* argument
+   was False) or :class:`io.TextIOWrapper` (if the
+   *universal_newlines* argument was True) object that provides output
+   from the child process. Otherwise, it is ``None``.
 
 
 .. attribute:: Popen.pid

I think the specific classes are an implementation detail, but mentioning if the file object is a bytes or text stream seems reasonable.
(Also it's better if you attach patches, instead of pasting them in the message.)

I think it would also be rather important to know if the streams are buffered or not.

patch attached.

*ping*

Is this suitable for inclusion? Or do I need to do anything else?

The patch specifies the stream types are either BufferedReader/Writer, TextIOWrapper, or None. However they can also be plain FileIO in my experience (Python 3.3 with bufsize=0).

Maybe it would be simpler to defer to the documentation for open(), which already mentions the three possible layers of file wrapping. Although, looking at the code just now, the TextIOWrapper is added separately by the “subprocess” module, and uses write_through=True.

Thanks for looking at this Martin! I have attached an updated patch that includes a reference to open and slightly changed language.


But please, let's not have the best be the enemy of the good here. There will probably always be room for further improvement, but if the patch as-is improves over the status quo - can't it just be applied? It's not as if this is the last and only chance to make changes...

New changeset 946257d3c648 by Benjamin Peterson in branch '3.3':
describe type of Popen streams (closes #17814)
http://hg.python.org/cpython/rev/946257d3c648

New changeset 1a7d416c57ff by Benjamin Peterson in branch 'default':
merge 3.3 (#17814)
http://hg.python.org/cpython/rev/1a7d416c57ff