diff --git a/Doc/library/os.rst b/Doc/library/os.rst
index 67fe36b..fe9531b 100644
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -1251,27 +1251,27 @@
       :exc:`InterruptedError` exception (see :pep:`475` for the rationale).
 
 
-.. function:: sendfile(out, in, offset, count)
-              sendfile(out, in, offset, count, [headers], [trailers], flags=0)
+.. function:: sendfile(out_fd, in_fd, offset, count)
+              sendfile(out_fd, in_fd, offset, count, [headers], [trailers], flags=0)
 
-   Copy *count* bytes from file descriptor *in* to file descriptor *out*
+   Copy *count* bytes from file descriptor *in_fd* to file descriptor *out_fd*
    starting at *offset*.
-   Return the number of bytes sent. When EOF is reached return 0.
+   Return the number of bytes sent. When EOF is reached return ``0``.
 
    The first function notation is supported by all platforms that define
    :func:`sendfile`.
 
    On Linux, if *offset* is given as ``None``, the bytes are read from the
-   current position of *in* and the position of *in* is updated.
+   current position of *in_fd* and the position of *in_fd* is updated.
 
    The second case may be used on Mac OS X and FreeBSD where *headers* and
    *trailers* are arbitrary sequences of buffers that are written before and
-   after the data from *in* is written. It returns the same as the first case.
+   after the data from *in_fd* is written. It returns the same as the first case.
 
-   On Mac OS X and FreeBSD, a value of 0 for *count* specifies to send until
-   the end of *in* is reached.
+   On Mac OS X and FreeBSD, a value of ``0`` for *count* specifies to send until
+   the end of *in_fd* is reached.
 
-   All platforms support sockets as *out* file descriptor, and some platforms
+   All platforms support sockets as *out_fd* file descriptor, and some platforms
    allow other types (e.g. regular file, pipe) as well.
 
    Cross-platform applications should not use *headers*, *trailers* and *flags*
@@ -1286,6 +1286,9 @@
 
    .. versionadded:: 3.3
 
+   .. versionchanged:: 3.9
+      Parameters *out* and *in* was renamed to *out_fd* and *in_fd*.
+
 
 .. function:: set_blocking(fd, blocking)
 
diff --git a/Lib/test/test_os.py b/Lib/test/test_os.py
index 4a076e3..d2bd9c2 100644
--- a/Lib/test/test_os.py
+++ b/Lib/test/test_os.py
@@ -3110,11 +3110,12 @@
 
     def test_keywords(self):
         # Keyword arguments should be supported
-        os.sendfile(out=self.sockno, offset=0, count=4096,
-            **{'in': self.fileno})
+        os.sendfile(out_fd=self.sockno, in_fd=self.fileno,
+                    offset=0, count=4096)
         if self.SUPPORT_HEADERS_TRAILERS:
-            os.sendfile(self.sockno, self.fileno, offset=0, count=4096,
-                headers=(), trailers=(), flags=0)
+            os.sendfile(out_fd=self.sockno, in_fd=self.fileno,
+                        offset=0, count=4096,
+                        headers=(), trailers=(), flags=0)
 
     # --- headers / trailers tests
 
diff --git a/Misc/NEWS.d/next/Library/2019-10-13-11-00-03.bpo-38378.yYNpSm.rst b/Misc/NEWS.d/next/Library/2019-10-13-11-00-03.bpo-38378.yYNpSm.rst
new file mode 100644
index 0000000..1bc5282
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2019-10-13-11-00-03.bpo-38378.yYNpSm.rst
@@ -0,0 +1,2 @@
+Parameters *out* and *in* of :func:`os.sendfile` was renamed to *out_fd* and
+*in_fd*.
diff --git a/Modules/clinic/posixmodule.c.h b/Modules/clinic/posixmodule.c.h
index 00b05f7..561cbb0 100644
--- a/Modules/clinic/posixmodule.c.h
+++ b/Modules/clinic/posixmodule.c.h
@@ -4986,7 +4986,7 @@
 #if defined(__APPLE__)
 
 PyDoc_STRVAR(os__fcopyfile__doc__,
-"_fcopyfile($module, infd, outfd, flags, /)\n"
+"_fcopyfile($module, in_fd, out_fd, flags, /)\n"
 "--\n"
 "\n"
 "Efficiently copy content or metadata of 2 regular file descriptors (macOS).");
@@ -4995,14 +4995,14 @@
     {"_fcopyfile", (PyCFunction)(void(*)(void))os__fcopyfile, METH_FASTCALL, os__fcopyfile__doc__},
 
 static PyObject *
-os__fcopyfile_impl(PyObject *module, int infd, int outfd, int flags);
+os__fcopyfile_impl(PyObject *module, int in_fd, int out_fd, int flags);
 
 static PyObject *
 os__fcopyfile(PyObject *module, PyObject *const *args, Py_ssize_t nargs)
 {
     PyObject *return_value = NULL;
-    int infd;
-    int outfd;
+    int in_fd;
+    int out_fd;
     int flags;
 
     if (!_PyArg_CheckPositional("_fcopyfile", nargs, 3, 3)) {
@@ -5013,8 +5013,8 @@
                         "integer argument expected, got float" );
         goto exit;
     }
-    infd = _PyLong_AsInt(args[0]);
-    if (infd == -1 && PyErr_Occurred()) {
+    in_fd = _PyLong_AsInt(args[0]);
+    if (in_fd == -1 && PyErr_Occurred()) {
         goto exit;
     }
     if (PyFloat_Check(args[1])) {
@@ -5022,8 +5022,8 @@
                         "integer argument expected, got float" );
         goto exit;
     }
-    outfd = _PyLong_AsInt(args[1]);
-    if (outfd == -1 && PyErr_Occurred()) {
+    out_fd = _PyLong_AsInt(args[1]);
+    if (out_fd == -1 && PyErr_Occurred()) {
         goto exit;
     }
     if (PyFloat_Check(args[2])) {
@@ -5035,7 +5035,7 @@
     if (flags == -1 && PyErr_Occurred()) {
         goto exit;
     }
-    return_value = os__fcopyfile_impl(module, infd, outfd, flags);
+    return_value = os__fcopyfile_impl(module, in_fd, out_fd, flags);
 
 exit:
     return return_value;
@@ -8731,4 +8731,4 @@
 #ifndef OS__REMOVE_DLL_DIRECTORY_METHODDEF
     #define OS__REMOVE_DLL_DIRECTORY_METHODDEF
 #endif /* !defined(OS__REMOVE_DLL_DIRECTORY_METHODDEF) */
-/*[clinic end generated code: output=799c75140d84ace5 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=fe7897441fed5402 input=a9049054013a1b77]*/
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index aeb0e9d..3c4e254 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -8993,10 +8993,10 @@
 
 #ifdef HAVE_SENDFILE
 PyDoc_STRVAR(posix_sendfile__doc__,
-"sendfile(out, in, offset, count) -> byteswritten\n\
-sendfile(out, in, offset, count[, headers][, trailers], flags=0)\n\
+"sendfile(out_fd, in_fd, offset, count) -> byteswritten\n\
+sendfile(out_fd, in_fd, offset, count[, headers][, trailers], flags=0)\n\
             -> byteswritten\n\
-Copy count bytes from file descriptor in to file descriptor out.");
+Copy count bytes from file descriptor in_fd to file descriptor out_fd.");
 
 /* AC 3.5: don't bother converting, has optional group*/
 static PyObject *
@@ -9016,8 +9016,7 @@
     off_t sbytes;
     struct sf_hdtr sf;
     int flags = 0;
-    /* Beware that "in" clashes with Python's own "in" operator keyword */
-    static char *keywords[] = {"out", "in",
+    static char *keywords[] = {"out_fd", "in_fd",
                                 "offset", "count",
                                 "headers", "trailers", "flags", NULL};
 
@@ -9133,7 +9132,7 @@
 #else
     Py_ssize_t count;
     PyObject *offobj;
-    static char *keywords[] = {"out", "in",
+    static char *keywords[] = {"out_fd", "in_fd",
                                 "offset", "count", NULL};
     if (!PyArg_ParseTupleAndKeywords(args, kwdict, "iiOn:sendfile",
             keywords, &out, &in, &offobj, &count))
@@ -9170,8 +9169,8 @@
 /*[clinic input]
 os._fcopyfile
 
-    infd: int
-    outfd: int
+    in_fd: int
+    out_fd: int
     flags: int
     /
 
@@ -9179,13 +9178,13 @@
 [clinic start generated code]*/
 
 static PyObject *
-os__fcopyfile_impl(PyObject *module, int infd, int outfd, int flags)
-/*[clinic end generated code: output=8e8885c721ec38e3 input=69e0770e600cb44f]*/
+os__fcopyfile_impl(PyObject *module, int in_fd, int out_fd, int flags)
+/*[clinic end generated code: output=c9d1a35a992e401b input=1e34638a86948795]*/
 {
     int ret;
 
     Py_BEGIN_ALLOW_THREADS
-    ret = fcopyfile(infd, outfd, NULL, flags);
+    ret = fcopyfile(in_fd, out_fd, NULL, flags);
     Py_END_ALLOW_THREADS
     if (ret < 0)
         return posix_error();