Add Python < 3.3 support for pyabc.i

pyabc.i for abstract base classes now supports versions of Python
prior to 3.3 by using the collection module for these older versions.
Python-3.3 and later continue to use the collections.abc module.
The -py3 option no longer has any effect on the %pythonabc feature.

CHANGES.current

@@ -7,6 +7,12 @@ the issue number to the end of the URL: https://github.com/swig/swig/issues/
 Version 4.1.0 (in progress)
 ===========================
 
+2022-03-21: wsfulton
+            [Python] #1779 pyabc.i for abstract base classes now supports versions of
+            Python prior to 3.3 by using the collection module for these older versions.
+            Python-3.3 and later continue to use the collections.abc module.
+            The -py3 option no longer has any effect on the %pythonabc feature.
+
 2022-03-21: jschueller, jim-easterbrook, wsfulton
             [Python] #2137 C++ static member functions no longer generate a "flattened"
             name in the Python module. For example:

Doc/Manual/Python.html

@@ -7036,10 +7036,11 @@ modify the buffer.
 
 
 <p>
-By including <tt>pyabc.i</tt> and using the <tt>-py3</tt> command
+By including <tt>pyabc.i</tt> in your interface file,
-line option when calling SWIG, the proxy classes of the STL containers
+the proxy classes of the STL containers
 will automatically gain an appropriate abstract base class from the
-<tt>collections.abc</tt> module. For
+<tt>collections.abc</tt> module for Python 3.3 and later, otherwise from the
+<tt>collections</tt> module. For
 example, the following SWIG interface:
 </p>
 
@@ -7056,8 +7057,10 @@ namespace std {
 
 <p>
 will generate a Python proxy class <tt>Mapii</tt> inheriting from
-<tt>collections.abc.MutableMap</tt> and a proxy class <tt>IntList</tt>
+<tt>collections.abc.MutableMap</tt> for Python 3.3 and later, or <tt>collections.MutableMap</tt>
-inheriting from <tt>collections.abc.MutableSequence</tt>.
+for earlier versions and a proxy class <tt>IntList</tt>
+inheriting from <tt>collections.abc.MutableSequence</tt> for Python 3.3 or later,
+or <tt>collections.MutableSequence</tt> for earlier versions.
 </p>
 
 <p>
@@ -7066,7 +7069,9 @@ used to define an abstract base class for your own C++ class:
 </p>
 
 <div class="code"><pre>
-%pythonabc(MySet, collections.abc.MutableSet);
+%pythonabc(MySet, collections.abc.MutableSet); # Python 3.3 and later
+%pythonabc(MySet, collections.MutableSet);     # Prior to Python 3.3
+%pythonabc(MySet, "collections.abc.MutableSet if _swig_python_version_info &gt;= (3, 3) else collections.MutableSet"); # All Python versions
 </pre></div>
 
 <p>
@@ -7080,6 +7085,8 @@ For details of abstract base class, please see
 of the classes in the <tt>collections</tt> module in Python 3.7.
 The <tt>collections.abc</tt> module was introduced in Python 3.3 and hence this feature
 requires Python 3.3 or later.
+SWIG-4.1.0 introduced the flexibility of using
+either the <tt>collections.abc</tt> module for Python 3.3 and later or the <tt>collections</tt> module for earlier Python versions.
 </p>
 
 <H3><a name="Python_nn77">33.12.4 Byte string output conversion</a></H3>

Examples/test-suite/python/python_abstractbase_runme.py

@@ -1,31 +1,36 @@
 import sys
 
 # collections.abc requires Python 3.3+
-if sys.version_info[0:2] < (3, 3):
-    exit(0)
 
 from python_abstractbase import *
+if sys.version_info[0:2] >= (3, 3):
     import collections.abc
+else:
+    import collections
 
 # This is expected to fail with -builtin option
 # Builtin types can't inherit from pure-python abstract bases
 if is_python_builtin():
     exit(0)
 
-# Python abc is only turned on when -py3 option is passed to SWIG
-if not is_swig_py3:
-    exit(0)
-
 def check_issubclass(derived, base):
     if not issubclass(derived, base):
         raise RuntimeError("{} is not a subclass of {}".format(derived, base))
 
+if sys.version_info[0:2] >= (3, 3):
     check_issubclass(Mapii, collections.abc.MutableMapping)
     check_issubclass(Multimapii, collections.abc.MutableMapping)
     check_issubclass(IntSet, collections.abc.MutableSet)
     check_issubclass(IntMultiset, collections.abc.MutableSet)
     check_issubclass(IntVector, collections.abc.MutableSequence)
     check_issubclass(IntList, collections.abc.MutableSequence)
+else:
+    check_issubclass(Mapii, collections.MutableMapping)
+    check_issubclass(Multimapii, collections.MutableMapping)
+    check_issubclass(IntSet, collections.MutableSet)
+    check_issubclass(IntMultiset, collections.MutableSet)
+    check_issubclass(IntVector, collections.MutableSequence)
+    check_issubclass(IntList, collections.MutableSequence)
 
 mapii = Mapii()
 multimapii = Multimapii()

Examples/test-suite/python_abstractbase.i

@@ -24,9 +24,3 @@ bool is_python_builtin() { return true; }
 bool is_python_builtin() { return false; }
 #endif
 %}
-
-#ifdef SWIGPYTHON_PY3 // set when using -py3
-#define is_swig_py3 1
-#else
-#define is_swig_py3 0
-#endif

Lib/python/pyabc.i

@@ -1,10 +1,14 @@
 %define %pythonabc(Type, Abc)
-  %feature("python:abc", #Abc) Type;
+  %feature("python:abc", Abc) Type;
 %enddef
-%pythoncode %{import collections.abc%}
+%pythoncode %{if _swig_python_version_info[0:2] >= (3, 3):
-%pythonabc(std::vector, collections.abc.MutableSequence);
+    import collections.abc
-%pythonabc(std::list, collections.abc.MutableSequence);
+else:
-%pythonabc(std::map, collections.abc.MutableMapping);
+    import collections
-%pythonabc(std::multimap, collections.abc.MutableMapping);
+%}
-%pythonabc(std::set, collections.abc.MutableSet);
+%pythonabc(std::vector, "collections.abc.MutableSequence if _swig_python_version_info >= (3, 3) else collections.MutableSequence");
-%pythonabc(std::multiset, collections.abc.MutableSet);
+%pythonabc(std::list, "collections.abc.MutableSequence if _swig_python_version_info >= (3, 3) else collections.MutableSequence");
+%pythonabc(std::map, "collections.abc.MutableMapping if _swig_python_version_info >= (3, 3) else collections.MutableMapping");
+%pythonabc(std::multimap, "collections.abc.MutableMapping if _swig_python_version_info >= (3, 3) else collections.MutableMapping");
+%pythonabc(std::set, "collections.abc.MutableSet if _swig_python_version_info >= (3, 3) else collections.MutableSet");
+%pythonabc(std::multiset, "collections.abc.MutableSet if _swig_python_version_info >= (3, 3) else collections.MutableSet");

Source/Modules/python.cxx

@@ -4480,10 +4480,9 @@ public:
 
       /* dealing with abstract base class */
       String *abcs = Getattr(n, "feature:python:abc");
-      if (py3 && abcs) {
+      if (abcs) {
-	if (Len(base_class)) {
+	if (Len(base_class) > 0)
 	  Printv(base_class, ", ", NIL);
-	}
 	Printv(base_class, abcs, NIL);
       }