Add support for Python variable annotations as a feature.

Both function annotations and variable annotations are turned on using the
"python:annotations" feature. Example:

  %feature("python:annotations", "c");

  struct V {
    float val;
  };

The generated code contains a variable annotation containing the C float type:

  class V(object):
      val: "float" = property(_example.V_val_get, _example.V_val_set)
      ...

Python 3.5 and earlier do not support variable annotations, so variable
annotations can be turned off with a "python:annotations:novar" feature flag.
Example turning on function annotations but not variable annotations globally:

  %feature("python:annotations", "c");
  %feature("python:annotations:novar");

or via the command line:

  -features python:annotations=c,python:annotations:novar

Closes #1951

CHANGES.current

@@ -7,6 +7,37 @@ the issue number to the end of the URL: https://github.com/swig/swig/issues/
 Version 4.1.0 (in progress)
 ===========================
 
+2022-03-02: geographika, wsfulton
+            [Python] #1951 Add Python variable annotations support.
+
+            Both function annotations and variable annotations are turned on using the
+            "python:annotations" feature. Example:
+
+              %feature("python:annotations", "c");
+
+              struct V {
+                float val;
+              };
+
+            The generated code contains a variable annotation containing the C float type:
+
+              class V(object):
+                  val: "float" = property(_example.V_val_get, _example.V_val_set)
+                  ...
+
+            Python 3.5 and earlier do not support variable annotations, so variable
+            annotations can be turned off with a "python:annotations:novar" feature flag.
+            Example turning on function annotations but not variable annotations globally:
+
+              %feature("python:annotations", "c");
+              %feature("python:annotations:novar");
+
+            or via the command line:
+
+              -features python:annotations=c,python:annotations:novar
+
+            *** POTENTIAL INCOMPATIBILITY ***
+
 2022-03-02: olly
 	    #891 Give error for typemap argument without a value.  Previously
 	    SWIG segfaulted.
@@ -23,6 +54,8 @@ Version 4.1.0 (in progress)
 
               -features python:annotations=c
 
+            Also see entry dated 2022-03-02, regarding variable annotations.
+
             *** POTENTIAL INCOMPATIBILITY ***
 
 2022-02-26: wsfulton

Doc/Manual/Contents.html

@@ -1478,7 +1478,7 @@
 </ul>
 <li><a href="Python.html#Python_python3support">Python 3 Support</a>
 <ul>
-<li><a href="Python.html#Python_annotations">Function annotations</a>
+<li><a href="Python.html#Python_annotations">Python function annotations and variable annotations</a>
 <ul>
 <li><a href="Python.html#Python_annotations_c">C/C++ annotation types</a>
 </ul>

Doc/Manual/Python.html

@@ -133,7 +133,7 @@
 </ul>
 <li><a href="#Python_python3support">Python 3 Support</a>
 <ul>
-<li><a href="#Python_annotations">Function annotations</a>
+<li><a href="#Python_annotations">Python function annotations and variable annotations</a>
 <ul>
 <li><a href="#Python_annotations_c">C/C++ annotation types</a>
 </ul>
@@ -2536,7 +2536,7 @@ assert(issubclass(B.Derived, A.Base))
 </li>
 
 
-<li><p><a href="#Python_annotations">Python function annotations</a> are not supported.
+<li><p><a href="#Python_annotations">Python annotations</a> are not supported.
 </p>
 </li>
 
@@ -3998,7 +3998,7 @@ Also included in the table for comparison is using the <tt>-builtin</tt> option
 
 <p>
 Although the <tt>-fastproxy</tt> option results in faster code over the default, the generated proxy code is not as user-friendly
-as docstring/doxygen comments, <a href="#Python_annotations">Python function annotations</a> and functions with default values are not visible in the generated Python proxy class.
+as docstring/doxygen comments, <a href="#Python_annotations">Python annotations</a> and functions with default values are not visible in the generated Python proxy class.
 The <tt>-olddefs</tt> option can rectify this.
 </p>
 
@@ -6762,13 +6762,15 @@ The following are Python 3 new features that are currently supported by
 SWIG.
 </p>
 
-<H3><a name="Python_annotations">33.12.1 Function annotations</a></H3>
+<H3><a name="Python_annotations">33.12.1 Python function annotations and variable annotations</a></H3>
 
 
 <p>
 Python 3 supports function annotations as defined in
 <a href="https://www.python.org/dev/peps/pep-3107/">PEP 3107</a>.
-Note that there is currently no annotations support for the <tt>-builtin</tt> nor
+Python 3.6 and later additionally support variable annotations as defined in
+<a href="https://www.python.org/dev/peps/pep-526/">PEP 526</a>.
+Note that currently there is no annotations support in SWIG for the <tt>-builtin</tt> nor
 the <tt>-fastproxy</tt> option.
 Annotations are added via the <tt>python:annotations</tt>
 <a href="Customization.html#Customization_features">%feature directives</a>.
@@ -6779,7 +6781,7 @@ SWIG currently supports one type of function annotation.
 
 
 <p>
-The <tt>%feature("python:annotations", "c")</tt> directive generates function annotations
+The <tt>%feature("python:annotations", "c")</tt> directive generates annotations
 containing C/C++ types. For example:
 </p>
 
@@ -6789,16 +6791,16 @@ int *global_ints(int &amp;ri);
 </pre></div>
 
 <p>
-The generated code then contains function annotations containing the C types:
+The generated code then contains function annotations containing the C++ types:
 </p>
 
 <div class="targetlang"><pre>
 def global_ints(ri: "int &amp;") -&gt; "int *":
-    return _python_annotations_c.global_ints(ri)
+    return _example.global_ints(ri)
 </pre></div>
 
 <p>
-There are some limitations with annotations support, for example, overloaded functions use
+There are some limitations with function annotations support, for example, overloaded functions use
 <tt>*args</tt> or <tt>**kwargs</tt> when keyword arguments are enabled.
 The parameter names and types are then not shown. For example, with input:
 </p>
@@ -6815,13 +6817,65 @@ Only the return type is annotated.
 
 <div class="targetlang"><pre>
 def global_overloaded(*args) -&gt; "int *":
-    return _python_annotations_c.global_overloaded(*args)
+    return _example.global_overloaded(*args)
 </pre></div>
 
+<p>
+Below is an example demonstrating variable annotations.
+</p>
+
+<div class="code"><pre>
+%feature("python:annotations", "c");
+
+struct V {
+  float val;
+};
+</pre></div>
+
+<p>
+The generated code contains a variable annotation containing the C <tt>float</tt> type:
+</p>
+
+<div class="targetlang"><pre>
+class V(object):
+    val: "float" = property(_example.V_val_get, _example.V_val_set)
+    ...
+</pre></div>
+
+<p>
+Variable annotations are only supported from Python 3.6. If you need to support earlier versions of Python, you'll need to turn variable annotations off via the <tt>python:annotations:novar</tt> feature flag.
+It is quite easy to support function annotations but turn off variable annotations. The next example shows how to do this for all variables.
+</p>
+
+<div class="code"><pre>
+%feature("python:annotations", "c");  // Turn on function annotations and variable annotations globally
+%feature("python:annotations:novar"); // Turn off variable annotations globally
+
+struct V {
+  float val;
+  void vv(float *v) const;
+};
+</pre></div>
+
+<p>
+The resulting code will work with versions older than Python 3.6 as the variable annotations are turned off:
+</p>
+
+<div class="targetlang"><pre>
+class V(object):
+    val = property(_example.V_val_get, _example.V_val_set)
+
+    def vv(self, v: "float *") -&gt; "void":
+        return _example.V_vv(self, v)
+    ...
+</pre></div>
+
+
 <p>
 <b>Compatibility Note:</b> SWIG-4.1.0 changed the way that function annotations are generated.
-Prior versions required the <tt>-py3</tt> option which enabled function annotation support
+Prior versions required the <tt>-py3</tt> option to generate function annotation support
 containing C/C++ types instead of supporting <tt>%feature("python:annotations", "c")</tt>.
+Variable annotations were also added in SWIG-4.1.0.
 </p>
 
 <H3><a name="Python_nn75">33.12.2 Buffer interface</a></H3>

Examples/test-suite/python/Makefile.in

@@ -49,6 +49,7 @@ CPP_TEST_CASES += \
 	primitive_types \
 	python_abstractbase \
 	python_annotations_c \
+	python_annotations_variable_c \
 	python_append \
 	python_builtin \
 	python_destructor_exception \

Examples/test-suite/python/python_annotations_variable_c_runme.py

@@ -0,0 +1,24 @@
+import sys
+
+# Variable annotations for properties is only supported in python-3.6 and later (PEP 526)
+if sys.version_info[0:2] >= (3, 6):
+    from python_annotations_variable_c import *
+
+    # No SWIG __annotations__ support with -builtin or -fastproxy
+    annotations_supported = not(is_python_builtin() or is_python_fastproxy())
+
+    if annotations_supported:
+        ts = TemplateShort()
+        anno = ts.__annotations__
+        if anno != {'member_variable': 'int'}:
+            raise RuntimeError("annotations mismatch: {}".format(anno))
+
+        ts = StructWithVar()
+        anno = ts.__annotations__
+        if anno != {'member_variable': 'int'}:
+            raise RuntimeError("annotations mismatch: {}".format(anno))
+
+        ts = StructWithVarNotAnnotated()
+        if getattr(ts, "__annotations__", None) != None:
+            anno = ts.__annotations__
+            raise RuntimeError("annotations mismatch: {}".format(anno))

Examples/test-suite/python_annotations_variable_c.i

@@ -0,0 +1,44 @@
+%module python_annotations_variable_c
+
+// Tests Python variable annotations, containing C/C++ types, in C++ member variables wrappers.
+// Member variables are wrapped using Python properties.
+// This is in a separate test to python_annotations_c.i (which tests function annotations) so that runtime testing
+// of variable annotations can be done which requires Python 3.6 and later. A syntax error occurs in earlier
+// versions of Python when importing code containing variable annotations.
+
+%feature("python:annotations", "c");
+%feature("python:annotations:novar") member_variable_not_annotated;
+
+
+%inline %{
+namespace Space {
+template<class T>
+struct Template {
+  int member_variable;
+  int member_variable_not_annotated;
+};
+struct StructWithVar{
+  int member_variable;
+};
+struct StructWithVarNotAnnotated {
+  int member_variable_not_annotated;
+};
+short global_variable;
+}
+%}
+%template(TemplateShort) Space::Template<short>;
+
+%inline %{
+#ifdef SWIGPYTHON_BUILTIN
+int is_python_builtin() { return 1; }
+#else
+int is_python_builtin() { return 0; }
+#endif
+
+#if defined SWIGPYTHON_FASTPROXY
+int is_python_fastproxy() { return 1; }
+#else
+int is_python_fastproxy() { return 0; }
+#endif
+%}
+

Source/Modules/python.cxx

@@ -2395,20 +2395,20 @@ public:
   }
 
   /* ------------------------------------------------------------
-   * returnPropertyAnnotation()
+   * variableAnnotation()
    *
-   * Helper function for constructing a property annotation
+   * Helper function for constructing a variable annotation
-   * return a empty string for Python 2.x
    * ------------------------------------------------------------ */
 
-  String *returnPropertyAnnotation(Node *n) {
+  String *variableAnnotation(Node *n) {
-    String *ret = 0;
+    String *type = Getattr(n, "type");
-     ret = Getattr(n, "type");
+    if (type)
-      if (ret) {
+      type = SwigType_str(type, 0);
-          ret = SwigType_str(ret, 0);
+    bool anno = Equal(Getattr(n, "feature:python:annotations"), "c") ? true : false;
-      }
+    anno = GetFlag(n, "feature:python:annotations:novar") ? false : anno;
-    return (ret && py3) ? NewStringf(": \"%s\"", ret)
+    String *annotation = (type && anno) ? NewStringf(": \"%s\"", type) : NewString("");
-	: NewString("");
+    Delete(type);
+    return annotation;
   }
 
   /* ------------------------------------------------------------
@@ -5059,12 +5059,14 @@ public:
       String *setname = Swig_name_set(NSPACE_TODO, mname);
       String *getname = Swig_name_get(NSPACE_TODO, mname);
       int assignable = is_assignable(n);
-      Printv(f_shadow, tab4, symname, returnPropertyAnnotation(n), " = property(", module, ".", getname, NIL);
+      String *variable_annotation = variableAnnotation(n);
+      Printv(f_shadow, tab4, symname, variable_annotation, " = property(", module, ".", getname, NIL);
       if (assignable)
 	Printv(f_shadow, ", ", module, ".", setname, NIL);
       if (have_docstring(n))
 	Printv(f_shadow, ", doc=", docstring(n, AUTODOC_VAR, tab4), NIL);
       Printv(f_shadow, ")\n", NIL);
+      Delete(variable_annotation);
       Delete(mname);
       Delete(setname);
       Delete(getname);