NameError: name 'foo' is not defined

NameError: name 'foo' is not defined means Python looked for foo in the local, enclosing, global and built-in scopes and found nothing. The cause is almost always a typo, a missing import, or a name that is used before it is assigned or outside the function that defines it. The quoted text in the message is the exact name Python could not find.

Reproduce it

cat > app.py <<'EOF'
print(foo)
EOF
python app.py

Traceback (most recent call last):
  File "/tmp/work/app.py", line 1, in <module>
    print(foo)
          ^^^
NameError: name 'foo' is not defined

Missing import

cat > app.py <<'EOF'
print(os.getcwd())
EOF
python app.py

Traceback (most recent call last):
  File "/tmp/work/app.py", line 1, in <module>
    print(os.getcwd())
          ^^
NameError: name 'os' is not defined. Did you forget to import 'os'?

Fix

cat > fix.py <<'EOF'
import os

def make():
    value = 42
    return value

result = make()
print(result)
print(type(os.getcwd()).__name__)
EOF
python fix.py

42
str

Read the hint at the end of the message

Since Python 3.10 the interpreter compares the missing name with the names that do exist and adds a suggestion such as Did you mean: 'foo'?. Python 3.12 extended this: inside a method it suggests self.x when the instance has that attribute, and for a standard library module used without import it asks Did you forget to import 'os'?. Older versions print only the bare message.

cat > typo.py <<'EOF'
foo = 1
def show():
    print(Foo)
show()
EOF
python typo.py

Traceback (most recent call last):
  File "/tmp/work/typo.py", line 4, in <module>
    show()
    ~~~~^^
  File "/tmp/work/typo.py", line 3, in show
    print(Foo)
          ^^^
NameError: name 'Foo' is not defined. Did you mean: 'foo'?

How to recognise the cause

Prevent it

Run a linter such as pyflakes, Ruff or pylint: they report undefined names without running the code, including in branches your tests do not reach. Give every variable a value before conditional code assigns it, and import modules at the top of the file.

A variable assigned inside a function is not visible outside it: return it, as make() does above. If Python reports UnboundLocalError instead, the name is assigned somewhere in the same function; see the related page.

FAQ