A minor mistake?

[sunsongxp@…]

​https://docs.djangoproject.com/en/1.4/topics/signing/

In this page, which is about signing, enumerates several examples about how to use it. When it comes to that handling Exception, to be specific,

value += 'm'
try:

... original = signer.unsign(value)
... except signing.BadSignature:
... print "Tampering detected!"

I found that these lines failed, the error shows signing.BadSignature is not defined, I tested a while, and then I knew,

from django.core import signing

is required. The example miss that. I don't know if that count as a mistake. Besides, the information about previous line is so scarce in reference, do I need to check the source code?