PEP 513: Cover unicode ABI compatibility

pep-0513.txt

@@ -151,8 +151,12 @@ included in the following list: ::
     libgthread-2.0.so.0
     libglib-2.0.so.0
 
-and (b), work on a stock CentOS 5.11 [6]_ system that contains the system
-package manager's provided versions of these libraries.
+and, (b) work on a stock CentOS 5.11 [6]_ system that contains the system
+package manager's provided versions of these libraries. In addition,
+for wheels targeting CPython 3.2 and earlier (including all 2.x
+versions), there is an extra requirement that (c) the wheel be
+built against a version of CPython compiled with 4-byte unicode
+support (i.e. one where ``sys.maxunicode > 0xFFFF``).
 
 Because CentOS 5 is only available for x86_64 and i386 architectures,
 these are the only architectures currently supported by the ``manylinux1``
@@ -344,9 +348,11 @@ suggest that installation tools should error on the side of assuming
 that a system *is* compatible, unless there is specific reason to
 think otherwise.
 
-We know of three main sources of potential incompatibility that are likely to
-arise in practice:
+We know of four main sources of potential incompatibility that are
+likely to arise in practice:
 
+* "Narrow" unicode builds of Python 3.2 and earlier (including all
+  versions of Python 2)
 * Eventually, in the future, there may exist distributions that break
   compatibility with this profile (e.g., if one of the libraries in
   the profile changes its ABI in a backwards-incompatible way)
@@ -354,20 +360,22 @@ arise in practice:
 * A linux distribution that does not use ``glibc`` (e.g. Alpine Linux, which is
   based on musl ``libc``, or Android)
 
-Therefore, we propose a two-pronged approach. To catch the first
-case, we standardize a mechanism for a Python distributor to signal
-that a particular Python install definitely is or is not compatible
-with ``manylinux1``: this is done by installing a module named
-``_manylinux``, and setting its ``manylinux1_compatible``
-attribute. We do not propose adding any such module to the standard
-library -- this is merely a well-known name by which distributors and
-installation tools can rendezvous. However, if a distributor does add
-this module, *they should add it to the standard library* rather than
-to a ``site-packages/`` directory, because the standard library is
+Checking for unicode configuration compatibility is straightforward,
+but the other cases are more subtle. We propose a two-pronged
+approach. To handle potential future incompatibilities, we standardize
+a mechanism for a Python distributor to signal that a particular
+Python install definitely is or is not compatible with ``manylinux1``:
+this is done by installing a module named ``_manylinux``, and setting
+its ``manylinux1_compatible`` attribute. We do not propose adding any
+such module to the standard library -- this is merely a well-known
+name by which distributors and installation tools can
+rendezvous. However, if a distributor does add this module, *they
+should add it to the standard library* rather than to a
+``site-packages/`` directory, because the standard library is
 inherited by virtualenvs (which we want), and ``site-packages/`` in
 general is not.
 
-Then, to handle the latter two cases for existing Python
+Then, to handle the last two cases for existing Python
 distributions, we suggest a simple and reliable method to check for
 the presence and version of ``glibc`` (basically using it as a "clock"
 for the overall age of the distribution).
@@ -380,6 +388,11 @@ Specifically, the algorithm we propose is::
         if get_platform() not in ["linux_x86_64", "linux_i386"]:
             return False
 
+        # "wide" Unicode mode is mandatory (always true on CPython 3.3+)
+        import sys
+        if sys.maxunicode <= 0xFFFF:
+            return False
+
         # Check for presence of _manylinux module
         try:
             import _manylinux