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Wed Jun 23 18:53:58 EDT 2004 · https://mail.python.org/pipermail/python-dev/2004-June/045527.html

What is the motivation for "safedict"?  I can imagine two uses.  One
seems like it could lead to some kind of security problem.

The "harmless" (?) use would be in debugging, so that the program would
continue when a key was missing, but the programmer could see after the
fact what that key was.

The harmful case would be one where the string is substituted in several
stages.  Just like % substitutions, $-substitutions are not safe for
repeated expansion.

Here's an example:
    def something(user_controlled_string):
        mypassword = "drowssap"
        bar = "1/8 x 1 inch aluminum bar"
        s = dstring("${foo} is {$bar}")
        s = s % safedict({'foo': user_controlled_string})
        s = s % nsdict()
        print s

The malicious user supplies user_controlled_string:
    http://python.example.com/something?user_controlled_string=%24mypassword
and gets back
    drowssap is 1/8 x 1 inch aluminum bar

Jeff
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[Python-Dev] PEP 292 for Python 2.4