Python

NameError: What It Means and How to Fix It

Error reference

NameError

Language

Python

Severity

Medium

When it happens

A NameError is raised in Python when you try to use a variable or function name that has not been defined in the current scope.

Potential fixes

Define the variable before using it. Double-check the spelling of every name. If a variable is defined inside a function, pass it as an argument or return it rather than accessing it from the outer scope. Use an IDE or linter to catch undefined names early.

Related errors

AttributeError ImportError

Deep dive

A NameError is raised in Python when you try to use a variable or function name that has not been defined in the current scope. Python looks up the name in local, enclosing, global, and built-in scopes and cannot find it.

Why it happens

•Using a variable before assigning a value to it.
•Typo in the variable or function name.
•Trying to use a name that is only defined inside a function from outside it.
•Forgetting to import a module before using its contents.

Potential fixes

Define the variable before using it. Double-check the spelling of every name. If a variable is defined inside a function, pass it as an argument or return it rather than accessing it from the outer scope. Use an IDE or linter to catch undefined names early.

Examples

Python

Code that triggers the error

def greet():
    print(message)

greet()

Error output

Traceback (most recent call last):
  File "app.py", line 4, in <module>
    greet()
  File "app.py", line 2, in greet
    print(message)
NameError: name 'message' is not defined

Fixed code

message = "Hello, world!"

def greet():
    print(message)

greet()  # Hello, world!

Practice in English

How would you explain a NameError to a fellow dev? Choose the right phrase:

"I hit an error...

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