NameError: name is not defined

What you see

print(sys.version)

Traceback (most recent call last):
  File "/home/me/project/main.py", line 1, in <module>
    print(sys.version)
          ^^^
NameError: name 'sys' is not defined. Did you forget to import 'sys'?

What it means

Python looked for the name in the function, in any enclosing function, in the module and in the builtins, and found it nowhere. Here sys is a module that was never imported.

Python 3.10 started suggesting a similar name when you made a typo. Python 3.12 also recognises standard library modules, so this example now ends with "Did you forget to import 'sys'?".

Why it happens

• A missing import, as in the example.
• A typo or wrong case. Print, true, false and null are not Python: the builtins are print, True, False and None.
• A name used before the line that defines it, such as a function called above its def at module level.
• A variable defined only inside a function, or only in a branch that did not run, and used outside it.
• Python 2 names in old code: raw_input, unicode, xrange. In Python 3 they are input, str and range.

How to fix it

Import the module, spell the name as it was defined, and define it before it is used:

import sys

print(sys.version)

In a notebook, run the cell that defines the name first; cells share names only once they have run.

Example: a typo

message = "hello"
print(mesage)

Traceback (most recent call last):
  File "/home/me/project/main.py", line 2, in <module>
    print(mesage)
          ^^^^^^
NameError: name 'mesage' is not defined. Did you mean: 'message'?

Run on CPython 3.9.6, 3.10.22, 3.11.17, 3.12.15, 3.13.17, 3.14.9 and 3.15.1, on October 25, 2026.

How each Python version words it

Run on CPython 3.9.6, 3.10.22, 3.11.17, 3.12.15, 3.13.17, 3.14.9 and 3.15.1, on October 25, 2026.

3.9 says only that the name is not defined. From 3.10 a typo gets "Did you mean: …?", and from 3.12 a missing standard library import gets "Did you forget to import …?".