Avoid double negatives

A double negative is a sentence or phrase that contains two negative words, such as:

  • not, including contractions like can't, won't
  • no

Readers find double negatives hard to parse. ("Wait, do two negatives make a positive or is the author of the error message using two negatives to emphasize something I shouldn't do?")

Some double negatives in error messages are blatant:

Not recommended

You cannot not invoke this flag.


You must invoke this flag.

Other double negatives are more subtle. For example, the words prevents and forbidding in the following error message are both negatives, leading to a confusing message:

Not recommended

The universal read permission on pathname prevents the operating system from forbidding access.


The universal read permission on pathname enables anyone to read this file. Giving access to everyone is a security flaw. See hyperlink for details on how to restrict readers.

Similarly, avoid exceptions to exceptions.

Not recommended

The App Engine service account must have permissions on the image, except the Storage Object Viewer role, unless the Storage Object Admin role is available.


The App Engine service account must have one of the following roles:
  • Storage Object Admin
  • Storage Object Creator

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