_yaml.py: Implement `get()` for `ChainMap`

Since the core Python `ChainMap.get()` implements with:

    self[key] if key in self else default

The double-chain-lookup is expensive.  This simple change solves that
for our ChainMap derived structure.  As such it improves matters
for #466 somewhat.

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>

1 files changed, 6 insertions, 0 deletions

diff --git a/buildstream/_yaml.py b/buildstream/_yaml.py
index ca12acae9..f44572ca5 100644
--- a/buildstream/_yaml.py
+++ b/buildstream/_yaml.py
@@ -1049,6 +1049,12 @@ class ChainMap(collections.ChainMap):
         for key in clearable:
             del self[key]
 
+    def get(self, key, default=None):
+        try:
+            return self[key]
+        except KeyError:
+            return default
+
 
 def node_chain_copy(source):
     copy = ChainMap({}, source)