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](https://stackoverflow.com/questions/577234/python-extend-for-a-dictionary) 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?