Fix NameError: Python undefined name resolution at runtime

Sat, 13 Jun 2026 07:56:04 +0800

1. Symptoms

When a NameError occurs in Python, the interpreter halts execution and displays a traceback that clearly identifies the problem. The error message follows a consistent pattern: NameError: name 'identifier' is not defined, where identifier is the name that Python could not resolve.

Common manifestations:

Traceback (most recent call last):
  File "example.py", line 10, in <module>
    print(my_variable)
NameError: name 'my_variable' is not defined

Traceback (most recent call last):
  File "calculator.py", line 15, in <module>
    result = add_numbers(a, b)
NameError: name 'add_numbers' is not defined

The error always appears at the exact line where Python attempts to access the undefined name. The line number in the traceback pinpoints the location, making initial diagnosis straightforward. However, the root cause might be earlier in the code—perhaps a typo made hours ago that only manifests when execution reaches the problematic line.

Python Basics on ErrorVault — Developer Error Code Dictionary

Fix NameError: Python undefined name resolution at runtime

1. Symptoms