At the moment we have to manage, and keep track of, and keep in sync, two
different files: the HTML input form and the processing script. However taking advantage of the
ability of programs to do selection we can combine these two into a
single file that can select the appropriate response based on what is passed
to the script. If the FieldStorage instance has content then it was the result of
a user filling out the form and pressing Submit, and we should process the data. On the other hand
if the FieldStorage instance is empty it means no form has been filled out
(perhaps because they are coming directly to this URL from a link or a search
result) and we should present the form to the user. In code this looks like
this:
# mi2km_v3.py
#
# Demonstrates unified cgi script that detects whether request
# is:
# 1) from a completed form, or
# 2) a first time visit.
import cgi
# We read the templates in from external files because
# that makes them easier to create. You, or your web designer,
# can create complete HTML pages in your editor of choice
# and then you just replace the values to be interpolated for
# with appropriate format codes.
output_template = open("mi2km_output_v3.html","r").read()
input_template = open("mi2km_input_v3.html","r").read()
# Parse the CGI request into dictionary-like FieldStorage.
form = cgi.FieldStorage()
if 'miles' in form:
# If they filled in the miles field, convert miles to km,
miles = float( form['miles'].value )
km = miles * 1.609
# and display the output.
print 'Content-type: text/html\n'
print output_template % (miles, km)
elif form.has_key('km'):
# If they filled in the km field, convert km to miles,
km = float( form['km'].value )
miles = km / 1.609
# and display the output.
print 'Content-type: text/html\n'
print output_template % (miles, km)
else:
# They filled in neither field so give them an input form to fill out.
print 'Content-type: text/html\n'
print input_templateI have used two different ways of telling if a form field was filled out so you could become aware of both.
First, if 'miles' in form: and second, elif form.has_key('km'): Both work, and I usually
prefer the first for its better readability.
If you look carefully at mi2km_output_v3.html you will see that one common hack is required. % signs in CSS need to escaped, i.e. replaced with %% so the Python string interpolation mechanism won't interpret them as formatting fields. This is usually avoided by importing the CSS file instead of inserting the styles into the HTML file (which is best practice, but I wanted to avoid a veritable explosion in the number of files we had to keep track of).