checking for missing and invalid inputs so farwe've been triggering the cgi script by selecting language name from the pulldown list in the main html page in this contextwe can be fairly sure that the script will receive valid inputs noticethoughthat there is nothing to prevent client from passing the requested language name at the end of the cgi script' url as an explicit query parameterinstead of using the html page form for instancea url of the following kind typed into browser' address field or submitted with the module urllib requestyields the same "pythonresponse page shown in figure - howeverbecause it' always possible for user to bypass the html file and use an explicit urla user could invoke our script with an unknown language nameone that is not in the html file' pull-down list (and so not in our script' tablein factthe script might be triggered with no language input at all if someone explicitly submits its url with no language parameter (or no parameter valueat the end such an erroneous url could be entered into browser' address field or be sent by another script using the urllib request module techniques described earlier in this for instancevalid requests work normally server-side scripting