Avoid KeyErrors in os._Environ.get() and __contains__()

Implement get() and __contains__() on os._Environ directly, which cuts out the
cost of raising and catching a KeyError when the keys do not exist.

Author: alexmalyshev

Lib/os.py

@@ -720,6 +720,11 @@ def get_exec_path(env=None):
 # Change environ to automatically call putenv() and unsetenv()
 from _collections_abc import MutableMapping, Mapping
 
+# Sentinel used for seeing if a value is found within the internal _Environ
+# dictionary.
+_environ_missing = object()
+
+
 class _Environ(MutableMapping):
     def __init__(self, data, encodekey, decodekey, encodevalue, decodevalue):
         self.encodekey = encodekey
@@ -728,6 +733,9 @@ def __init__(self, data, encodekey, decodekey, encodevalue, decodevalue):
         self.decodevalue = decodevalue
         self._data = data
 
+    def __contains__(self, key):
+        return self.encodekey(key) in self._data
+
     def __getitem__(self, key):
         try:
             value = self._data[self.encodekey(key)]
@@ -770,6 +778,10 @@ def __repr__(self):
     def copy(self):
         return dict(self)
 
+    def get(self, key, default = None):
+        val = self._data.get(self.encodekey(key), _environ_missing)
+        return default if val is _environ_missing else self.decodevalue(val)
+
     def setdefault(self, key, value):
         if key not in self:
             self[key] = value