It seems that you are running w3af using Python3, which is not
officially supported by the w3af team. To force w3af to be run using python2.7 run it as follows
(depending on your OS):
*python2.7 w3af console python2 w3af_console
To make this change permanent modify the shebang line in the w3af console, w3af gul and w3af apk scripts.
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