尝试添加到字典时，Python输出类型“ Nonetype”

Question

Here is the code(Running Python 2.7.6):

currency_pairs = {'PPC': 10}
print currency_pairs
currency_pairs = currency_pairs.update({'NMC': 50})
print type (currency_pairs)

Output:

{'PPC': 10}
<type 'NoneType'>

Why won't Python add to the dictionary? I don't understand this. Any thoughts?

Answer 1

The method update just updates the dictionary but don't return anything (in other words, returns None ) In the line

currency_pairs = currency_pairs.update({'NMC': 50})

you are assigning None to currecy_pairs . The method itself, will modify the dictionary, so you should call it like this:

currency_pairs = {'PPC': 10}
print currency_pairs
currency_pairs.update({'NMC': 50})
print currency_pairs

Output:

{'PPC': 10}
{'PPC': 10, 'NMC': 50}