Why does dict(k=4, z=2).update(dict(l=1)) return None in Python?

Question

Why does dict(k=4, z=2).update(dict(l=1)) return None? It seems as if it should return dict(k=4, z=2, l=1)? I'm using Python 2.7 should that matter.

Answer 1

The .update() method alters the dictionary in place and returns None. The dictionary itself is altered, no altered dictionary needs to be returned.

Assign the dictionary first:

a_dict = dict(k=4, z=2)
a_dict.update(dict(l=1))
print a_dict

This is clearly documented, see the dict.update() method documentation:

Update the dictionary with the key/value pairs from other, overwriting existing keys. Return None.

Answer 2

dict.update() method does update in place. It does not return the modified dict, but None.

The doc says it in first line:

Update the dictionary with the key/value pairs from other, overwriting existing keys. Return None.

Answer 3

For completion's sake, if you do want to return a modified version of the dictionary, without modifying the original you can do it like this:

original_dict = {'a': 'b', 'c': 'd'}
new_dict = dict(original_dict.items() + {'c': 'f', 'g': 'h'}.items())

Which gives you the following:

new_dict == {'a': 'b', 'c': 'f', 'g': 'h'}
original_dict == {'a': 'b', 'c': 'd'}