Python Principles

Python NameError

When you run Python code, you may get a NameError such as the following:

NameError: name 'x' is not defined

The x in the error will vary depending on your program.

The error means that Python looks for something named x but finds nothing defined by that name.

Common causes

Common causes include:

  • you misspelled a variable name
  • you got the casing of a variable wrong; Python is case-sensitive
  • you forgot to put quotes around a string, so Python thinks it's a variable
  • you misspelled a function name, such as pring instead of print
  • you used a variable without first having assigned a value to it
  • you started using a variable before its definition
  • you are trying to use a variable whose definition is only valid in a different scope
    • for example, a variable defined inside a function cannot be accessed outside of it
  • you called a function before its definition was reached