Django document said the QueryDict is only immutable, but why I can edit it sometimes

Django document said the QueryDict is only immutable, but why I can
edit it sometimes

The codes is almost the same, for example following,

The first kind of condition, the item I want to edit is not a field of
form.

I add a Captcha when user register or login, the querydict is mutable
when registering

if request.method == 'POST':
data = request.POST
data['captcha_text'] = Captcha(request).get()

But the same code when do login, the querydict is immutable, it is
must be create a copy

if request.method == "POST":
data = request.POST.copy()
data['captcha_text'] = Captcha(request).get()

The second kind of condition, the item I want to edit is a field of
form, but in fact I think it has no relation with form.

user = request.user
if request.method == "POST":
data = request.POST
data['title'] = data['title'].strip()
now = datetime.datetime.now()
topic = Topic(id = _md5_key(now, user.username), profile =
user.get_profile(), \

It is ok, but when I do this in other view, then django tell me the
querydict is immutable.

if request.method == 'POST':
data = request.POST
print data, '\n\n'
data['name'] = data['name'].strip()
print data, '\n\n'

What's difference ?

Thanks

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