#22719: Proposal: Delayed Form binding (with working example code)
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Reporter: bernhard.hermann@… | Owner: nobody
Type: New feature | Status: new
Component: Forms | Version: 1.6
Severity: Normal | Keywords: bound unbound bind
Triage Stage: Unreviewed | Has patch: 0
Easy pickings: 0 | UI/UX: 0
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Hello everybody!
I would like to propose adding a mechanism to "bind" Form-objects in a way
other than at instantiation (e.g. {{{ Form(request.POST)}}}).
This idea was inspired by the following assertion by Paul Graham:
"When I see patterns in my programs, I consider it a sign of trouble.
The shape of a program should reflect only the problem it needs to
solve. Any other regularity in the code is a sign, to me at least,
that I'm using abstractions that aren't powerful enough-- often that
I'm generating by hand the expansions of some macro that I need to
write." [ http://www.paulgraham.com/icad.html ]
Let me give a quick example from the Django documentation's "Working with
forms":
{{{#!python
def contact(request):
if request.method == 'POST':
form = ContactForm(request.POST)
if form.is_valid():
pass
else:
form = ContactForm()
return render(request, 'contact.html', {'form': form,})
}}}
[ highlighted: http://pastebin.com/MAKUSrmY ]
This works well when I don't need to supply any keyword arguments. As soon
as I want to do that, the code looks like this:
{{{#!python
def contact(request):
if request.method == 'POST':
form = ContactForm(request.POST, prefix='something-%s' %
(some_id,), label_suffix=': ')
if form.is_valid():
pass
else:
form = ContactForm(prefix='something-%s' % (some_id,),
label_suffix=': ')
return render(request, 'contact.html', {'form': form,})
}}}
[ highlighted: http://pastebin.com/9FKS9ExU ]
The really bad (IMHO: wrong) thing about this is that the code
{{{ prefix='something-%s' % (some_id,), label_suffix=': '}}}
must be duplicated.
I am therefore using this class to bind the Form late:
{{{#!python
class FlexiForm(forms.Form):
def set_fields(self):
for (f_name, attributes) in self._thefields.items():
for (att_name, att_value) in attributes.items():
setattr(self._theinstance.fields[f_name], att_name,
att_value)
def __init__(self, form, args=None, kwargs=None, fields=None):
self._theform = form
self._theargs = args or []
self._thekwargs = kwargs or {}
self._thefields = fields or {}
self.bind()
# !: make it possible to use kwargs here
def bind(self, *args):
self._theinstance = self._theform(*(list(args) + self._theargs),
**self._thekwargs)
self.set_fields()
return self
def __getattr__(self, name):
return getattr(self._theinstance, name)
def __setattr__(self, name, value):
inst = self
if name not in ['_theform', '_theargs', '_thekwargs',
'_thefields', '_theinstance']:
inst = self.__theinstance
super(FlexiForm, inst).__setattr__(name, value)
}}}
[ highlighted: http://pastebin.com/wzY4qWUJ ]
(This is the actual verbatim class from my code. If you spot any errors,
please tell!)
Melding this with the above example, we get:
{{{#!python
def contact(request):
form = FlexiForm(ContactForm, kwargs={'prefix': 'something-%s' %
(some_id,), 'label_suffix': ': ',})
if request.method == 'POST':
form.bind(request.POST) # substitutes internal ContactForm() with
ContactForm(request.POST)
if form.is_valid():
pass
else:
pass # Form already instantiated!
return render(request, 'contact.html', {
'form': form,
})
}}}
[ highlighted: http://pastebin.com/zNA9MzRJ ]
'''Advantage: keyword (and other) arguments that are vitally important to
many of my forms don't need to be duplicated in different locations.
Probability of mistakes/bugs is lowered, maintainability and readability
improved.'''
I am sure there is a more elegant/correct way to do this.
I suspect it should be added to Django's Form class.
I am looking forward to your ...
[x] criticism
[x] reasons why this shouldn't be done
[x] promises that it will be implemented soon
[x] mindless ranting
best regards,
Bernhard
--
Ticket URL: <https://code.djangoproject.com/ticket/22719>
Django <https://code.djangoproject.com/>
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