On 2011-11-12 16:24, lina wrote:
Thanks, ^_^, now better.
No, I'm afraid you are still not understanding.
I checked, the sublist (list) here can't be as a key of the results (dict).
"result" isn't a dictionary. It started as an empty list and later
becomes a null object ("NoneType").
You must not forget that you are inside a for-loop. Simplified your
situation is like this:
>>> result = []
>>> for i in range(1,10):
... print("Iteration {0}, result = {1}".format(i, result))
... result = result.append(i)
...
Iteration 1, result = []
Iteration 2, result = None
Traceback (most recent call last):
File "<stdin>", line 3, in <module>
AttributeError: 'NoneType' object has no attribute 'append'
As you see the error happens in the *second* iteration, because result
is no list any more.
Dave gave you already the explanation: functions and method always
return a value in Python. If the don't have a return statement they
return "None".
Another simple example:
>>> a = print("Test")
Test
"print" is a function which prints out the text you passed to it and you
usually aren't interested in its return value. But every function/method
in Python returns something. You save this value in "a"
>>> print(a)
None
As you see the return value of "print" is "None".
>>> a.append(x)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'NoneType' object has no attribute 'append'
Same error as above, because "NoneType" objects (null objects) don't
have a method "append".
I also think you mix two different ways to add an element to a list:
result.append(x)
is equivalent to
result = result + [x] (that's what you will use in other languages)
HTH, Andreas
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