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Django - differences between using request.GET and request.POST

From all the HTML books i've read, I know that POST should be used when changing, adding or removing from the database and when handling sensitive information like passwords. GET should be used when you just want to search through a database without making any changes. With that said, I am reading a book on Django and up until now, to handle forms, we did it like this:

def RegistrationFormView(request):
    form = RegistrationForm()
        if request.method == "POST": #if the user has clicked the 'submit' button on the form and is sending data
            form = RegistrationForm(request.POST)

which makes sense. The book goes on to teach how to create a search page which searches through the database. For this, we use GET, which makes sense. This is the form:

class SearchForm(forms.Form):
    query = forms.CharField(
    label='Enter a keyword to search for',
    widget=forms.TextInput(attrs={'size': 32})
)

But this is the view (and this is what confused me):

def search_page(request):
    form = SearchForm()
    bookmarks = []
    show_results = False #Only show results if the user has searched something
    if request.GET.has_key('query'): #check if the user submitted GET data
        show_results = True #return results since the user has submitted GET data
        query = request.GET['query'].strip() 
        if query:
            form = SearchForm({'query' : query})

I want to clarify four things here.

1) Would it be exactly the same if I just did

if request.method == "GET":

instead of 

 if request.GET.has_key('query'):

2) in the line

if request.GET.has_key('query'):

according to the Djangobook, it says "has_key Returns True or False, designating whether request.GET or request.POST has the given key." Now, what exactly is a 'key'? Is a key a field in the form, and 

if request.GET.has_key('query'):

checks to see if the user has filled out the formField which is called 'query'?

3) Am I allowed to call form.is_valid() when the method is GET? Because what I was thinking was doing

form = SearchForm(request.GET)
if form.is_valid():
    query = form.cleaned_data['query']

Is that allowed?

4) why does the book do

if query:

after 

query = request.GET['query'].strip()

? Doesn't the line

if request.GET.has_key('query'):

already verify that the 'query' field is filled in?

Solution

    1. No. if request.method == "GET": is in no way equivalent to if request.GET.has_key('query'):

    2. request.GET and request.POST are dictionary subclasses and has_key is part of the built-in dictionary interface http://docs.python.org/2/library/stdtypes.html#dict.has_key however it is deprecated in favor of 'query' in request.GET.

    3. Forms do not care about the request method or that there is a request at all. Forms validate dictionaries whatever the source might be.

    4. In the case of ?query= or ?query=%20 the key query would evaluate to '' and ' ' which would both be False after running through strip(). if request.GET.has_key('query'): only checks that the key is present and does not look at the value.