Python Append to a list returns none [duplicate]

问题

This question already has answers here:

Why does append() always return None in Python? [duplicate] (4 answers)

Closed 4 years ago.

I know different versions of this question has been answered for example [here].1 But I just couldn't seem to get it working on my example. I am trying to make a copy of ListA called ListB and then add an additional item to ListB i.e. 'New_Field_Name' in which later on I use as column headers for a dataframe.

Here is my code:

ListA = ['BUSINESSUNIT_NAME' ,'ISONAME', 'Planning_Channel', 'Planning_Partner', 'Is_Tracked', 'Week', 'Period_Number']
print type(ListA)

Output:

ListB = list(ListA)
print '######', ListB, '#####'

Output: ###### ['BUSINESSUNIT_NAME', 'ISONAME', 'Planning_Channel', 'Planning_Partner', 'Is_Tracked', 'Week', 'Period_Number'] #####

ListB = ListB.append('New_Field_Name')
print type(ListB)
print ListB

Output: None

回答1:

append does not return anything, so you cannot say

ListB = ListB.append('New_Field_Name')

It modifies the list in place, so you just need to say

ListB.append('New_Field_Name')

Or you could say

ListB += ['New_Field_Name']

来源：https://stackoverflow.com/questions/28110161/python-append-to-a-list-returns-none