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Python NameError: name is not defined

The code references a variable or function name that hasn't been defined.

Explanation

NameError occurs when Python encounters a name (variable, function, or class) that hasn't been defined in the current scope. Unlike many other languages, Python doesn't allow using variables before they are assigned. This commonly happens due to typos, scoping issues where a variable is defined in a different scope, misspelled function names, or trying to access a variable that was defined in a conditional branch that didn't execute.

Common Causes

  • Variable name typo
  • Variable used before assignment
  • Function called before definition
  • Variable in wrong scope
  • Shadowing built-in names

Solution

Check for typos in variable and function names. Ensure all variables are defined before use. Verify the variable is in the correct scope (not defined inside a function but used outside). Use an IDE with code completion to catch undefined names. Initialize variables with default values before conditional assignments. Check import statements for misspelled module names.

Code Example 

# Typo in variable name
user_name = "John"
print(user_nmae)  # NameError: name 'user_nmae' is not defined

# Fix
print(user_name)

# Variable defined inside if block not available outside
if True:
    x = 10
print(x)  # this works

if False:
    y = 20
print(y)  # NameError: name 'y' is not defined

# Fix: initialize before conditional
y = None
if False:
    y = 20

# Built-in function shadowed
list = [1, 2, 3]  # shadows built-in list
list()  # TypeError: 'list' object is not callable

# Fix: don't shadow built-ins
data_list = [1, 2, 3]

Error Information

Language

python

Difficulty

Beginner

Views

109

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