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:

lib/spack/spack/package.py

@@ -613,11 +613,14 @@ def do_fetch(self):
             raise ValueError("Can only fetch concrete packages.")
 
         if spack.do_checksum and not self.version in self.versions:
-            raise FetchError(
-                "Cannot fetch %s safely; there is no checksum on file for version %s."
-                % (self.name, self.version),
-                "Add a checksum to the package file, or use --no-checksum to "
-                "skip this check.")
+            tty.warn("There is no checksum on file to fetch %s safely."
+                     % self.spec.format('$_$@'))
+            ignore = tty.get_yes_or_no("  Fetch anyway?", default=False)
+            msg = "Add a checksum or use --no-checksum to skip this check."
+            if ignore:
+                tty.msg("Fetching with no checksum.", msg)
+            else:
+                raise FetchError("Will not fetch %s." % self.spec.format('$_$@'), msg)
 
         self.stage.fetch()