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Nested dictionary that acts as defaultdict when setting items but not when getting items

I want to implement a dict-like data structure that has the following properties:
from collections import UserDict
class TestDict(UserDict):
pass
test_dict = TestDict()
# Create empty dictionaries at 'level_1' and 'level_2' and insert 'Hello' at the 'level_3' key.
test_dict['level_1']['level_2']['level_3'] = 'Hello'
>>> test_dict
{
'level_1': {
'level_2': {
'level_3': 'Hello'
}
}
}
# However, this should not return an empty dictionary but raise a KeyError.
>>> test_dict['unknown_key']
KeyError: 'unknown_key'
The problem, to my knowledge, is that python does not know whether __getitem__ is being called in the context of setting an item, i.e. the first example, or in the context of getting and item, the second example.
I have already seen Python `defaultdict`: Use default when setting, but not when getting, but I do not think that this question is a duplicate, or that it answers my question.
Please let me know if you have any ideas.
Thanks in advance.
EDIT:
It is possible to achieve something similar using:
def set_nested_item(dict_in: Union[dict, TestDict], value, keys):
for i, key in enumerate(keys):
is_last = i == (len(keys) - 1)
if is_last:
dict_in[key] = value
else:
if key not in dict_in:
dict_in[key] = {}
else:
if not isinstance(dict_in[key], (dict, TestDict)):
dict_in[key] = {}
dict_in[key] = set_nested_item(dict_in[key], value, keys[(i + 1):])
return dict_in
class TestDict(UserDict):
def __init__(self):
super().__init__()
def __setitem__(self, key, value):
if isinstance(key, list):
self.update(set_nested_item(self, value, key))
else:
super().__setitem__(key, value)
test_dict[['level_1', 'level_2', 'level_3']] = 'Hello'
>>> test_dict
{
'level_1': {
'level_2': {
'level_3': 'Hello'
}
}
}

It's impossible.
test_dict['level_1']['level_2']['level_3'] = 'Hello'
is semantically equivalent to:
temp1 = test_dict['level_1'] # Should this line fail?
temp1['level_2']['level_3'] = 'Hello'
But... if determined to implement it anyway, you could inspect the Python stack to grab/parse the calling line of code, and then vary the behaviour depending on whether the calling line of code contains an assignment! Unfortunately, sometimes the calling code isn't available in the stack trace (e.g. when called interactively), in which case you need to work with Python bytecode.
import dis
import inspect
from collections import UserDict
def get_opcodes(code_object, lineno):
"""Utility function to extract Python VM opcodes for line of code"""
line_ops = []
instructions = dis.get_instructions(code_object).__iter__()
for instruction in instructions:
if instruction.starts_line == lineno:
# found start of our line
line_ops.append(instruction.opcode)
break
for instruction in instructions:
if not instruction.starts_line:
line_ops.append(instruction.opcode)
else:
# start of next line
break
return line_ops
class TestDict(UserDict):
def __getitem__(self, key):
try:
return super().__getitem__(key)
except KeyError:
# inspect the stack to get calling line of code
frame = inspect.stack()[1].frame
opcodes = get_opcodes(frame.f_code, frame.f_lineno)
# STORE_SUBSCR is Python opcode for TOS1[TOS] = TOS2
if dis.opmap['STORE_SUBSCR'] in opcodes:
# calling line of code contains a dict/array assignment
default = TestDict()
super().__setitem__(key, default)
return default
else:
raise
test_dict = TestDict()
test_dict['level_1']['level_2']['level_3'] = 'Hello'
print(test_dict)
# {'level_1': {'level_2': {'level_3': 'Hello'}}}
test_dict['unknown_key']
# KeyError: 'unknown_key'
The above is just a partial solution. It can still be fooled if there are other dictionary/array assignments on the same line, e.g. other['key'] = test_dict['unknown_key']. A more complete solution would need to actually parse the line of code to figure out where the variable occurs in the assignment.

Error: 'return' outside of function

Error: 'return' outside of function
Why??
def get_all_students(self):
database = open(self.database_path, 'r')
list_of_students = list()
for idx, l in enumerate(database.readlines()):
params = list()
params.append(idx)
params += l.split(self.data_delimiter)
student = Student(*self.item_un_escape(params))
list_of_students.append(student)
return list_of_students
Edit: I'll post the rest of the class, I'm pretty sure it's correctly indented though. Can't seem to find the error, I can bet it's probably something silly. Apologies, first time coding in python but this error has been annoying me for like an hour or so! haha
Edit2: It's throwing an error on params.append(idx) saying unexpected indentation?
from student import Student
database_path = 'C:/Users/Alan/Desktop/flask/flask/app/database'
class Database(object):
data_delimiter = ','
#staticmethod
def escape_new_lines(value):
if type(value) == str:
value = value.replace('\n', '#n-nl#')
value = value.replace('\r', '#r-nl#')
return value
#staticmethod
def un_escape_new_lines(value):
if type(value) == str:
value = value.replace('#n-nl#', '\n')
value = value.replace('#r-nl#', '\r')
return value
def __init__(self, database_path=database_path):
self.database_path = database_path
def get_all_students(self):
database = open(self.database_path, 'r')
list_of_students = list()
for idx, l in enumerate(database.readlines()):
params = list()
params.append(idx)
params += l.split(self.data_delimiter)
student = Student(*self.item_un_escape(params))
list_of_students.append(student)
return list_of_students

The code you have posted is fine. Your likely error is from missing an indent before a return statement. EDIT: As #Anonymous has noted in the comments, another possibility as that you're mixing tab indents with space indents. Check your indentations.

I got aTypeError: 'NoneType' object is not iterable

in fucntion getLink(urls), I have return (cloud,parent,children)
in main function, I have (cloud,parent,children) = getLink(urls) and I got error of this line: TypeError: 'NoneType' object is not iterable
parent and children are all list of http links. since, it is not able to paste them here, parent is a list contains about 30 links; children is a list contains about 30 items, each item is about 10-100 links which is divide by ",".
cloud is a list contain about 100 words, like that: ['official store', 'Java Applets Centre', 'About Google', 'Web History'.....]
I didnot know why I get an error. Is there anything wrong in passing parameter? Or because the list take too much space?
#crawler url: read webpage and return a list of url and a list of its name
def crawler(url):
try:
m = urllib.request.urlopen(url)
msg = m.read()
....
return (list(set(list(links))),list(set(list(titles))) )
except Exception:
print("url wrong!")
#this is the function has gone wrong: it throw an exception here, also the error I mentioned, also it will end while before len(parent) reach 100.
def getLink(urls):
try:
newUrl=[]
parent = []
children =[]
cloud =[]
i=0
while len(parent)<=100:
url = urls[i]
if url in parent:
i += 1
continue
(links, titles) = crawler(url)
parent.append(url)
children.append(",".join(links))
cloud = cloud + titles
newUrl= newUrl+links
print ("links: ",links)
i += 1
if i == len(urls):
urls = list(set(newUrl))
newUrl = []
i = 0
return (cloud,parent,children)
except Exception:
print("can not get links")
def readfile(file):
#not related, this function will return a list of url
def main():
file='sampleinput.txt'
urls=readfile(file)
(cloud,parent,children) = getLink(urls)
if __name__=='__main__':
main()

There might be a way that your function ends without reaching the explicit return statement.
Look at the following example code.
def get_values(x):
if x:
return 'foo', 'bar'
x, y = get_values(1)
x, y = get_values(0)
When the function is called with 0 as parameter the return is skipped and the function will return None.
You could add an explicit return as the last line of your function. In the example given in this answer it would look like this.
def get_values(x):
if x:
return 'foo', 'bar'
return None, None
Update after seing the code
When the exception is triggered in get_link you just print something and return from the function. You have no return statement, so Python will return None. The calling function now tries to expand None into three values and that fails.
Change your exception handling to return a tuple with three values like you do it when everything is fine. Using None for each value is a good idea for it shows you, that something went wrong. Additionally I wouldn't print anything in the function. Don't mix business logic and input/output.
except Exception:
return None, None, None
Then in your main function use the following:
cloud, parent, children = getLink(urls)
if cloud is None:
print("can not get links")
else:
# do some more work

Django - view didn't return an HttpResponse object

I'm facing this exception error and I'm puzzled by it, as this method worked in similar system, appreciate any help or pointers. Many Thanks!
Exception Value: The view Project.qna.views.add_vote didn't return an HttpResponse object.
def add_vote(request):
if request.method == "POST":
q_id = request.POST['vote_form_q_id']
a_id = request.POST['vote_form_a_id']
vote_value = request.POST['vote_form_value']
ok = False
vote_num = None
name = None
if q_id:
try:
question = Question.objects.get(id=q_id)
question.num_vote += int(vote_value)
question.save()
vote_num = question.num_vote
name = 'Question_'+str(q_id)
ok = True
except Question.DoesNotExist:
pass
elif a_id:
try:
answer = Answer.objects.get(id=a_id)
answer.num_vote += int(vote_value)
answer.save()
vote_num = answer.num_vote
name = 'Answer_'+str(a_id)
ok = True
except Answer.DoesNotExist:
pass
if ok and request.is_ajax:
result = simplejson.dumps({
"vote_num": vote_num,
}, cls=LazyEncoder)
response = HttpResponse(result, mimetype='application/javascript')
response.set_cookie(name, datetime.now)
return response

Fix your indention please, also you seem to have a lot of workarounds that could be simplified.
Every django view should return a HttpResponse object, you seem to have a lot of places where this would not be the case. To narrow down your problem change every pass to a print statement to see where your code actually fails. It would be quite helpful if you could present your POST data.

Well it's hard to tell without seeing what kind of request you are making to the view. But are you sending a POST request? Because you don't handle GET requests in any way. Also the indentation is wrong. But that might just be cutting and pasting gone awry.

This is untested, but it's a cleaner and more robust design, which I believe fits in with your logic and highlights the points where returning an HttpResponse is necessary:
def add_vote(request):
if not (request.method == 'POST' and request.is_ajax):
return # Some suitable response here
try:
vote_value = int(request.POST.get('vote_form_value',''))
except ValueError as e:
pass # Some suitable response here
def saveobj(model, key, val): # helper function to reduce code repetition
item = model.objects.get(id=key)
item.num_vote += val
item.save()
return item.num_vote, '%s_%s' % (model.__class__.__name__, key)
for model, key in [(Question, 'vote_form_q_id'), (Answer, 'vote_form_a_id')]):
try:
new_vote_value, name = saveobj(model, request.POST[key], vote_value)
break
except (KeyError, ObjectDoesNotExist) as e:
continue # or error out
else:
pass # neither question or answer found - so suitable response here
# return ajax response here....

Global variables in Ironpython

I'm having terrible trouble trying to understand ironpython scoping rules.
With the following script:
global data
// function for call xml-rpc
def CallListDatabases(self):
global synC, synCtx, result, data
self.synCtx = synC.Current
service = XmlRpcService("http://localhost:8000/rpc")
req = XmlRpcRequest(service, 'vocab_list')
req.XmlRpcCallCompleteHandler += self.req_XmlRpcCallCompleteHandler
result = req.Execute(self)
//if call xml-rpc complete then use working rpc
def req_XmlRpcCallCompleteHandler (self, response, userState):
global synCtx, synC, data
word = []
f = response.TryCast(clr.GetClrType(Fault))
if f != None:
self.synCtx.Post(self.SetCallResult, f)
if f.FaultCode == -1:
pass
else:
self.synCtx.Post(self.SetCallResult, response)
// show result with rpc complete
def SetCallResult(self, userState):
global data, result
if userState.GetType() == clr.GetClrType(Fault):
f = userState
if f != None:
print str(f.FaultString)
return
response = userState
result = response.TryCast(clr.GetClrType(Array[str]))
data = result //I want to use value it
print "value: "+data //show value
Problem
print "value: "+data
value: [] <<<======== Not value

First of all, you don't seem to ever be calling any of the functions you have defined. If you are calling the functions, it appears that the return value of response.TryCast(clr.GetClrType(Array[str])) is an empty list. Have you tried printing the value of result within SetCallResult()? I'd bet that it's [].