update method of python dictionary did not work

Question

I have two dictionaries.

a = {"ab":3, "bd":4}
b = {"cd":3, "ed":5}` 

I want to combine them to {'bd': 4, 'ab': 3, 'ed': 5, 'cd': 3}.

As this says, a.update(b) can complete it. But when I try, I get:

type(a.update(b)) #--> type 'NoneType'

Would anyone like to explain it to me why I cannot gain a dict type?

I also tried this, and it did well:

type(dict(a,**b)) #-->type 'dict'

What is the difference between these two methods and why did the first one not work?

Answer 1

The update method updates a dict in-place. It returns None, just like list.extend does. To see the result, look at the dict that you updated.

>>> a = {"ab":3, "bd":4}
>>> b = {"cd":3, "ed":5}
>>> update_result = a.update(b)
>>> print(update_result)
None
>>> print(a)
{'ed': 5, 'ab': 3, 'bd': 4, 'cd': 3}

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If you want the result to be a third, separate dictionary, you shouldn't be using update. Use something like dict(a, **b) instead, which, as you've already noticed, constructs a new dict from the two components, rather than updating one of the existing ones.