Checksum warning now prompts for override.

lib/spack/llnl/util/tty/__init__.py

@@ -114,6 +114,33 @@ def get_number(prompt, **kwargs):
     return number
 
 
+def get_yes_or_no(prompt, **kwargs):
+    default_value = kwargs.get('default', None)
+
+    if default_value is None:
+        prompt += ' [y/n] '
+    elif default_value is True:
+        prompt += ' [Y/n] '
+    elif default_value is False:
+        prompt += ' [y/N] '
+    else:
+        raise ValueError("default for get_yes_no() must be True, False, or None.")
+
+    result = None
+    while result is None:
+        ans = raw_input(prompt).lower()
+        if not ans:
+            result = default_value
+            if result is None:
+                print "Please enter yes or no."
+        else:
+            if ans == 'y' or ans == 'yes':
+                result = True
+            elif ans == 'n' or ans == 'no':
+                result = False
+    return result
+
+
 def hline(label=None, **kwargs):
     """Draw an optionally colored or labeled horizontal line.
        Options: