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

AttributeError: 'NoneType' object has no attribute 'x' means the variable before the dot is None, so the attribute x does not exist. The bug is almost never on the failing line: find where the value became None. Typical sources are a function without return, an in-place method whose result was assigned, or a lookup with no match.

Reproduce it

python3 -c "None.foo"

Traceback (most recent call last):
  File "<string>", line 1, in <module>
AttributeError: 'NoneType' object has no attribute 'foo'

The three usual sources:

python3 - <<'PY'
import re

def load():
    data = [3, 1, 2]

items = [3, 1, 2].sort()
m = re.match(r'\d+', 'abc')
for name, value in [('function', load()), ('sort', items), ('match', m)]:
    try:
        value.append(1)
    except AttributeError as e:
        print(name, '->', e)
PY

function -> 'NoneType' object has no attribute 'append'
sort -> 'NoneType' object has no attribute 'append'
match -> 'NoneType' object has no attribute 'append'

Fix it

python3 - <<'PY'
import re

def load():
    return [3, 1, 2]

items = sorted(load())
m = re.match(r'\d+', 'abc')
print(items)
print(m.group() if m is not None else 'no match')
PY

[1, 2, 3]
no match

list.sort() sorts in place and returns None; use sorted() when you need the new list. dict.get(key) returns None for a missing key; pass a default, d.get(key, default).