validate user input on templates

Hi django experts,

just want to know your opinion on how to validate the user input
(example: the user input field should be integer, but the user
accidentally inputs any character), here is my code:

FYI; the numberofapples field is IntegerField

here is my sample code:

def apples_edit(request):
if request.POST['id']:
id = request.POST['id']
numberofapples = request.POST['numberofapples']
conn = psycopg2.connect("dbname=my_db user=user password=pass
host=localhost")
cur = conn.cursor()

try:
if isDigit(playerid):
pass
else:
template_name = 'err_template.html'
t = loader.get_template(template_name)
c = RequestContext(request, {"err_msg":"number of
apples should be integer, Please correct your input"})
return HttpResponse(t.render(c))
except ValueError:
template_name = 'err_template.html'
t = loader.get_template(template_name)
c = RequestContext(request, {"err_msg":"number of apples
should be integer, Please correct your input"})
return HttpResponse(t.render(c))

sql = """ UPDATE my_db SET "numberofapples"='%s' where "id"='%s'
""" % (numberofapples,id)
cur.execute(sql)
conn.commit()
conn.close()

----------------------
My problem is that, the error template does not appear after I type in
any character for numberofapples field.
What would be the correct code for this?

Thanks.

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