interpreter: Implement array.get(index, fallback)

If the index is out of range, the fallback value is used. Includes
a test.

Closes #1337

2 files changed, 20 insertions, 4 deletions

diff --git a/mesonbuild/interpreterbase.py b/mesonbuild/interpreterbase.py
index 837a4f8..12e4fc4 100644
--- a/mesonbuild/interpreterbase.py
+++ b/mesonbuild/interpreterbase.py
@@ -501,12 +501,24 @@ class InterpreterBase:
             return len(obj)
         elif method_name == 'get':
             index = args[0]
+            fallback = None
+            if len(args) == 2:
+                fallback = args[1]
+            elif len(args) > 2:
+                m = 'Array method \'get()\' only takes two arguments: the ' \
+                    'index and an optional fallback value if the index is ' \
+                    'out of range.'
+                raise InvalidArguments(m)
             if not isinstance(index, int):
                 raise InvalidArguments('Array index must be a number.')
             if index < -len(obj) or index >= len(obj):
-                raise InvalidArguments('Array index %s is out of bounds for array of size %d.' % (index, len(obj)))
+                if fallback is None:
+                    m = 'Array index {!r} is out of bounds for array of size {!r}.'
+                    raise InvalidArguments(m.format(index, len(obj)))
+                return fallback
             return obj[index]
-        raise InterpreterException('Arrays do not have a method called "%s".' % method_name)
+        m = 'Arrays do not have a method called {!r}.'
+        raise InterpreterException(m.format(method_name))
 
     def reduce_arguments(self, args):
         assert(isinstance(args, mparser.ArgumentNode))
diff --git a/test cases/common/67 foreach/meson.build b/test cases/common/67 foreach/meson.build
index 0e81d3d..e633de8 100644
--- a/test cases/common/67 foreach/meson.build
+++ b/test cases/common/67 foreach/meson.build
@@ -1,8 +1,12 @@
 project('foreach', 'c')
 
 tests = [['test1', 'prog1', 'prog1.c'],
-  ['test2', 'prog2', 'prog2.c'],
-  ['test3', 'prog3', 'prog3.c']]
+  ['test2', 'prog2', 'prog2.c', 'fallback'],
+  ['test3', 'prog3', 'prog3.c', 'urgh']]
+
+assert(tests[0].get(3, 'fallbck') == 'fallbck', 'array #1 fallback did not match')
+assert(tests[1].get(3, 'failbk') == 'fallback', 'array #2 value did not match')
+assert(tests[2].get(3, 'urgh') == 'urgh', 'array #3 value did not match')
 
 foreach i : tests
   test(i.get(0), executable(i.get(1), i.get(2), install : true))