Strange error involving “AttributeError: 'NoneType' object has no attribute 'append' ”

Question

so i am trying to parse out a text file by converting it to a list and splitting each item in the list at the space.

i have created a test variable to run this part of the code by itself. my code in the spyder editor:

test = ['NC_009142.1_03_012_002_001 560', 'NC_017586.1_13_009_003_001 555', 'NC_016111.1_13_010_003_001 555']
ListOfLinesParsed = test

PN_List = []
counter_iterative = 0
while counter_iterative < len(ListOfLinesParsed):
    PN_List = PN_List.append(ListOfLinesParsed[counter_iterative].split()[0])
    counter_iterative += 1

print PN_List

Which returns an error:

runfile(r'/home/jake/.spyder2/.temp.py', wdir=r'/home/jake/.spyder2')

Traceback (most recent call last):

  File "<stdin>", line 1, in <module>

  File "/usr/lib/python2.7/dist-
packages/spyderlib/widgets/externalshell/sitecustomize.py", line 493, in runfile

    execfile(filename, namespace)
  File "/home/jake/.spyder2/.temp.py", line 7, in <module>

    PN_List = PN_List.append(ListOfLinesParsed[counter_iterative].split()[0])

AttributeError: 'NoneType' object has no attribute 'append'

BUT if i enter the commands directly into the terminal i get no error:

testL = []

testL.append(test[0].split()[0])

testL

['NC_009142.1_03_012_002_001']

testL.append(test[1].split()[0])

testL

['NC_009142.1_03_012_002_001', 'NC_017586.1_13_009_003_001']

testL.append(test[2].split()[0])

testL

['NC_009142.1_03_012_002_001', 'NC_017586.1_13_009_003_001', 'NC_016111.1_13_010_003_001']

Shouldn't the 2 things be EXACTLY the same? i don't understand why the one in my script is acting any differently than the terminal commands.

Answer 1

The line

PN_List = PN_List.append(ListOfLinesParsed[counter_iterative].split()[0])

is the problem.

list.append is an in-place operation, which returns None , but alters the original list itself. If you assign PN_List to the result, it becomes None . If you don't, then your program will run smoothly. This is why when you try appending things without an assignment, you get the expected answer.