interpreter: add fallback argument to subproject.get_variable()

docs/markdown/Reference-manual.md

@@ -2288,10 +2288,14 @@ opaque object representing it.
 - `found()` *(added 0.48.0)* which returns whether the subproject was
   successfully setup
 
-- `get_variable(name)` fetches the specified variable from inside the
+- `get_variable(name, fallback)` fetches the specified variable from inside the
   subproject. This is useful to, for instance, get a [declared
   dependency](#declare_dependency) from the [subproject](Subprojects.md).
 
+  If the variable does not exist, the variable `fallback` is returned.
+  If a fallback is not specified, then attempting to read a non-existing
+  variable will cause a fatal error.
+
 ### `run result` object
 
 This object encapsulates the result of trying to compile and run a

docs/markdown/snippets/add_fallback_argument_to_subproject_get_variable.md

@@ -0,0 +1,13 @@
+---
+short-description: Add fallback argument to subproject.get_variable()
+...
+
+## subproject.get_variable() now accepts a `fallback` argument
+
+Similar to `get_variable`, a fallback argument can now be passed to
+`subproject.get_variable()`, it will be returned if the requested
+variable name did not exist.
+
+``` meson
+var = subproject.get_variable('does-not-exist', 'fallback-value')
+```

mesonbuild/interpreter.py

@@ -917,18 +917,25 @@ class SubprojectHolder(InterpreterObject, ObjectHolder):
         return self.held_object is not None
 
     @permittedKwargs({})
+    @noArgsFlattening
     def get_variable_method(self, args, kwargs):
-        if len(args) != 1:
-            raise InterpreterException('Get_variable takes one argument.')
+        if len(args) < 1 or len(args) > 2:
+            raise InterpreterException('Get_variable takes one or two arguments.')
         if not self.found():
             raise InterpreterException('Subproject "%s/%s" disabled can\'t get_variable on it.' % (
                 self.subproject_dir, self.name))
         varname = args[0]
         if not isinstance(varname, str):
-            raise InterpreterException('Get_variable takes a string argument.')
-        if varname not in self.held_object.variables:
-            raise InvalidArguments('Requested variable "{0}" not found.'.format(varname))
-        return self.held_object.variables[varname]
+            raise InterpreterException('Get_variable first argument must be a string.')
+        try:
+            return self.held_object.variables[varname]
+        except KeyError:
+            pass
+
+        if len(args) == 2:
+            return args[1]
+
+        raise InvalidArguments('Requested variable "{0}" not found.'.format(varname))
 
 header_permitted_kwargs = set([
     'required',

test cases/common/46 subproject/meson.build

@@ -21,3 +21,8 @@ e = executable('user', 'user.c', include_directories : inc, link_with : lib, ins
 test('subdirtest', e)
 
 meson.install_dependency_manifest('share/sublib/sublib.depmf')
+
+unknown_var = sub.get_variable('does-not-exist', [])
+if unknown_var != []
+  error ('unexpetced fallback value for subproject.get_variable()')
+endif