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class HumidityServer(CoAP):
 def __init__(self, host, port, noOfSensors=10, multicast=False):
 CoAP.__init__(self, (host, port), multicast)

 for num in range(noOfSensors):
 self.add_resource('humidity'+num+'/', HumidityResource(num))

This excerpt is part of a program that generates:

Traceback (most recent call last):
 File "humidityserver.py", line 10, in <module>
 class HumidityServer(CoAP):
 File "humidityserver.py", line 14, in HumidityServer
 for num in range(noOfSensors):
NameError: name 'noOfSensors' is not defined

Why does this happen even though I've defined a default value for the variable?

You are mixing tabs and spaces in your code; this is your original code as pasted into the question:

The solid grey lines are tabs, the dots are spaces.

Note how the for loop is indented to 8 spaces, buth def __init__ is indented by one tab? Python expands tabs to eight spaces, not four, so to Python your code looks like this instead:

Now you can see that the for loop is outside the __init__ method, and the noOfSensors variable from the __init__ function signature is not defined there.

Don't mix tabs and spaces in indentation, stick to just tabs or just spaces. The PEP 8 Python style guide strongly advises you to use only spaces for indentation. Your editor can easily be configured to insert spaces whenever you use the TAB key, for example.

I copied and run the code, it was not because of the mixing tabs and spaces issues answered by @Martijn. I have just ran into a similar issue while creating a small game based on classes.

Even though I have assigned a default value into the variable but it stuck and gave me error:

NameError: name 'mental' is not defined #where mental is the variable

I research a bit and saw somebody talking about instances. Then I tried to create an instance and make the instance execute the function, meanwhile I define a function to what I intended to execute. And it works out. Below is my example on the fix:

class People(object):
 def __init__(self, vital, mental):
 self.vital = vital
 self.mental = mental

class Andy(People):
 print "My name is Andy... I am not the killer! Trust me..."
 chat1 = raw_input(">")
 if chat1 == ('i believe you' or 'yes i believe' or 'believe' or 'i trust you' or 'yes i trust you'):
 self.mental += -1
 print "(checking) option 1"
 elif chat1 == ('you are killer' or 'you are the one' or 'really?' or 'i doubt' or 'i don't believe' or 'i don't trust you'):
 self.mental += 1
 print "(checking) option 2"
 else:
 print "Pass to else"
 print self.mental
 print self.vital

andy = Andy(1, 5)

The solution I found was:

class People(object):
 def __init__(self, vital, mental):
 self.vital = vital
 self.mental = mental

class Andy(People):
 def play(self):
 print "My name is Andy... I am not the killer! Trust me..."
 chat1 = raw_input(">")
 if chat1 == ('i believe you' or 'yes i believe' or 'believe' or 'i trust you' or 'yes i trust you'):
 self.mental += -1
 print "(checking) option 1"
 elif chat1 == ('you are killer' or 'you are the one' or 'really?' or 'i doubt' or 'i don't believe' or 'i don't trust you'):
 self.mental += 1
 print "(checking) option 2"
 else:
 print "Pass to else"
 print self.mental
 print self.vital

andy = Andy(1, 5)
andy.play()

Maybe there are other solutions to your question, but I am quite new in programming, there are stuff in your code I don't understand. But regarding to the error you get I think its because of the 'self' has to be an instance you set for it to run through the class. Please correct me if I got the concept wrong.

it's not instanced Yet so the defalut value executed when class is Created man