The error message:
NameError: name 'function_name' is not defined
occurs when Python cannot recognize a function because it has not been defined or is used incorrectly.
1. Causes and Solutions
Cause 1: Calling a Function Before Defining It
Python reads the script from top to bottom. If you try to use a function before defining it, you’ll get a NameError.
Incorrect Code:
greet() # Function is called before it's defined
def greet():
print("Hello!")
Error Output:
NameError: name 'greet' is not defined
Solution: Define the Function Before Calling It
def greet(): # Define function first
print("Hello!")
greet() # Then call it
Cause 2: Function Name is Misspelled
Python is case-sensitive, so even a small typo will cause this error.
Incorrect Code:
def greet():
print("Hello!")
greett() # Typo in function name
Error Output:
NameError: name 'greett' is not defined
Solution: Correct the Function Name
def greet():
print("Hello!")
greet() # Correct name
Cause 3: Function is Defined Inside a Scope but Called Outside
If a function is defined inside another function, it cannot be accessed globally.
Incorrect Code:
def outer():
def inner():
print("Inside inner function")
inner() # Error: 'inner' is not defined globally
Error Output:
NameError: name 'inner' is not defined
Solution: Call the Function Inside Its Scope
def outer():
def inner():
print("Inside inner function")
inner() # Call it inside
outer()
Cause 4: Importing a Function Incorrectly
If a function is inside another module but is not properly imported, Python will not recognize it.
Incorrect Code:
import math
math.squareroot(16) # 'squareroot' is incorrect
Error Output:
AttributeError: module 'math' has no attribute 'squareroot'
Solution: Use the Correct Function Name
import math
print(math.sqrt(16)) # Use 'sqrt' instead of 'squareroot'
2. Best Practices to Avoid the Error
- Always define functions before calling them.
- Check for spelling errors.
- Be careful with scopes.
- Import functions correctly if using external modules.