diff --git a/bypy/linux/launcher.c b/bypy/linux/launcher.c
index 5e7ef57def..fc9cbbf7c6 100644
--- a/bypy/linux/launcher.c
+++ b/bypy/linux/launcher.c
@@ -5,6 +5,7 @@
  * Distributed under terms of the GPL3 license.
  */
 
+#define PY_SSIZE_T_CLEAN
 #include <stdio.h>
 #include <string.h>
 #include <errno.h>
diff --git a/bypy/macos/launcher.c b/bypy/macos/launcher.c
index 165b11c303..a03b778d00 100644
--- a/bypy/macos/launcher.c
+++ b/bypy/macos/launcher.c
@@ -1,3 +1,4 @@
+#define PY_SSIZE_T_CLEAN
 #include "util.h"
 #include <stdlib.h>
 #include <libproc.h>
diff --git a/bypy/run-python.h b/bypy/run-python.h
index cae241b539..6d55b3c972 100644
--- a/bypy/run-python.h
+++ b/bypy/run-python.h
@@ -5,7 +5,7 @@
  */
 
 #pragma once
-
+#define PY_SSIZE_T_CLEAN
 #include <stdio.h>
 #include <stdbool.h>
 #include <time.h>
diff --git a/bypy/windows/util.c b/bypy/windows/util.c
index 8ecff90909..70f2abbe43 100644
--- a/bypy/windows/util.c
+++ b/bypy/windows/util.c
@@ -3,6 +3,7 @@
  */
 
 #define UNICODE
+#define PY_SSIZE_T_CLEAN
 
 #define _WIN32_WINNT 0x0502
 #define WINDOWS_LEAN_AND_MEAN
diff --git a/src/calibre/devices/libusb/libusb.c b/src/calibre/devices/libusb/libusb.c
index bd159ec73d..7d1f2c2274 100644
--- a/src/calibre/devices/libusb/libusb.c
+++ b/src/calibre/devices/libusb/libusb.c
@@ -6,6 +6,7 @@
  */
 
 #define UNICODE
+#define PY_SSIZE_T_CLEAN
 
 #include <Python.h>
 #ifdef __FreeBSD__
diff --git a/src/calibre/devices/mtp/unix/libmtp.c b/src/calibre/devices/mtp/unix/libmtp.c
index df653c67bd..faed4424ea 100644
--- a/src/calibre/devices/mtp/unix/libmtp.c
+++ b/src/calibre/devices/mtp/unix/libmtp.c
@@ -6,6 +6,7 @@
  */
 
 
+#define PY_SSIZE_T_CLEAN
 #define UNICODE
 #include <Python.h>
 #include <sys/types.h>
diff --git a/src/calibre/devices/mtp/windows/global.h b/src/calibre/devices/mtp/windows/global.h
index 30ed76f342..9660b21c40 100644
--- a/src/calibre/devices/mtp/windows/global.h
+++ b/src/calibre/devices/mtp/windows/global.h
@@ -7,6 +7,7 @@
 
 #pragma once
 #define UNICODE
+#define PY_SSIZE_T_CLEAN
 #include <Windows.h>
 #include <atlbase.h>
 #include <Python.h>
diff --git a/src/calibre/devices/usbobserver/usbobserver.c b/src/calibre/devices/usbobserver/usbobserver.c
index 89f23c4a59..716ed61745 100644
--- a/src/calibre/devices/usbobserver/usbobserver.c
+++ b/src/calibre/devices/usbobserver/usbobserver.c
@@ -20,6 +20,8 @@
  */
 
 #define _DARWIN_USE_64_BIT_INODE
+#define PY_SSIZE_T_CLEAN
+
 #include <Python.h>
 
 #include <stdio.h>
diff --git a/src/calibre/gui2/tts/nsss.m b/src/calibre/gui2/tts/nsss.m
index 805b8a6c78..ee19fe5614 100644
--- a/src/calibre/gui2/tts/nsss.m
+++ b/src/calibre/gui2/tts/nsss.m
@@ -4,6 +4,7 @@
  *
  * Distributed under terms of the GPL3 license.
  */
+#define PY_SSIZE_T_CLEAN
 
 #include <Python.h>
 #import <AppKit/AppKit.h>
diff --git a/src/calibre/gui2/tweak_book/diff/_patiencediff_c.c b/src/calibre/gui2/tweak_book/diff/_patiencediff_c.c
index aa64c71efa..7ce4d98fce 100644
--- a/src/calibre/gui2/tweak_book/diff/_patiencediff_c.c
+++ b/src/calibre/gui2/tweak_book/diff/_patiencediff_c.c
@@ -22,6 +22,7 @@
    Copyright (C) 2005 Bram Cohen, Copyright (C) 2005, 2006 Canonical Ltd
 */
 
+#define PY_SSIZE_T_CLEAN
 
 #include <Python.h>
 #include <stdlib.h>
diff --git a/src/calibre/library/sqlite_custom.c b/src/calibre/library/sqlite_custom.c
index 9ed40a2b61..e4383129b0 100644
--- a/src/calibre/library/sqlite_custom.c
+++ b/src/calibre/library/sqlite_custom.c
@@ -1,4 +1,5 @@
 #define UNICODE
+#define PY_SSIZE_T_CLEAN
 #include <Python.h>
 
 
diff --git a/src/calibre/srv/html_as_json.cpp b/src/calibre/srv/html_as_json.cpp
index 9cfaa0f17b..1ffd133a94 100644
--- a/src/calibre/srv/html_as_json.cpp
+++ b/src/calibre/srv/html_as_json.cpp
@@ -4,6 +4,7 @@
  *
  * Distributed under terms of the GPL3 license.
  */
+#define PY_SSIZE_T_CLEAN
 
 #include <Python.h>
 #include <algorithm>
diff --git a/src/calibre/utils/certgen.c b/src/calibre/utils/certgen.c
index 7955a0f0f8..72a6728182 100644
--- a/src/calibre/utils/certgen.c
+++ b/src/calibre/utils/certgen.c
@@ -6,6 +6,7 @@
  */
 
 #define UNICODE
+#define PY_SSIZE_T_CLEAN
 #include <Python.h>
 
 #include <openssl/rand.h>
diff --git a/src/calibre/utils/cocoa.m b/src/calibre/utils/cocoa.m
index f2ef2670de..888e0f95a2 100644
--- a/src/calibre/utils/cocoa.m
+++ b/src/calibre/utils/cocoa.m
@@ -5,6 +5,7 @@
  * Distributed under terms of the GPL3 license.
  */
 
+#define PY_SSIZE_T_CLEAN
 #import <AppKit/AppKit.h>
 #import <AppKit/NSWindow.h>
 #import <Availability.h>
diff --git a/src/calibre/utils/hyphenation/hyphen.c b/src/calibre/utils/hyphenation/hyphen.c
index 2dbecd9265..d91ad33dc8 100644
--- a/src/calibre/utils/hyphenation/hyphen.c
+++ b/src/calibre/utils/hyphenation/hyphen.c
@@ -5,6 +5,7 @@
  * Distributed under terms of the GPL3 license.
  */
 
+#define PY_SSIZE_T_CLEAN
 #include <Python.h>
 #include <hyphen.h>
 
diff --git a/src/calibre/utils/imageops/imageops.h b/src/calibre/utils/imageops/imageops.h
index 208cb6ef03..9a5578230e 100644
--- a/src/calibre/utils/imageops/imageops.h
+++ b/src/calibre/utils/imageops/imageops.h
@@ -6,6 +6,7 @@
  */
 
 #pragma once
+#define PY_SSIZE_T_CLEAN
 
 #include <Python.h>
 #include <QImage>
diff --git a/src/calibre/utils/lzx/compressor.c b/src/calibre/utils/lzx/compressor.c
index c9fba107d1..6f43bbc3c9 100644
--- a/src/calibre/utils/lzx/compressor.c
+++ b/src/calibre/utils/lzx/compressor.c
@@ -3,6 +3,7 @@
  *
  * Python/C implementation of an LZX compressor type.
  */
+#define PY_SSIZE_T_CLEAN
 
 #include <Python.h>
 #include <structmember.h>
diff --git a/src/calibre/utils/lzx/lzxmodule.c b/src/calibre/utils/lzx/lzxmodule.c
index 6b8910e563..bb18277ccf 100644
--- a/src/calibre/utils/lzx/lzxmodule.c
+++ b/src/calibre/utils/lzx/lzxmodule.c
@@ -3,6 +3,7 @@
  *
  * Python module C glue code.
  */
+#define PY_SSIZE_T_CLEAN
 
 #include <Python.h>
 
diff --git a/src/calibre/utils/msdes/msdesmodule.c b/src/calibre/utils/msdes/msdesmodule.c
index dd9145ac95..a69ff19ea2 100644
--- a/src/calibre/utils/msdes/msdesmodule.c
+++ b/src/calibre/utils/msdes/msdesmodule.c
@@ -4,7 +4,7 @@
  * Python module C glue code.
  */
 
-
+#define PY_SSIZE_T_CLEAN
 #include <Python.h>
 
 #include <d3des.h>
diff --git a/src/calibre/utils/speedup.c b/src/calibre/utils/speedup.c
index ff5c55a690..ef6d87c752 100644
--- a/src/calibre/utils/speedup.c
+++ b/src/calibre/utils/speedup.c
@@ -1,4 +1,6 @@
 #define UNICODE
+#define PY_SSIZE_T_CLEAN
+
 #include <Python.h>
 #include <datetime.h>
 #include <errno.h>
diff --git a/src/calibre/utils/spell/hunspell_wrapper.cpp b/src/calibre/utils/spell/hunspell_wrapper.cpp
index 371c65e963..0a672fd6d6 100644
--- a/src/calibre/utils/spell/hunspell_wrapper.cpp
+++ b/src/calibre/utils/spell/hunspell_wrapper.cpp
@@ -6,7 +6,7 @@
  * Distributed under terms of the GPL3 license.
  */
 
-#define PY_SSIZE_T_CLEAN 1
+#define PY_SSIZE_T_CLEAN
 #include <Python.h>
 #include <new>
 #include <string>
diff --git a/src/unicode_names/data-types.h b/src/unicode_names/data-types.h
index 264ef25aff..cf743c6b67 100644
--- a/src/unicode_names/data-types.h
+++ b/src/unicode_names/data-types.h
@@ -5,6 +5,7 @@
  */
 
 #pragma once
+#define PY_SSIZE_T_CLEAN
 
 #include <Python.h>
 #include <stdint.h>