Python Language Commonwealth Exceptions NameError: name '???' is not defined


Is raised when you tried to use a variable, method or function that is not initialized (at least not before). In other words, it is raised when a requested local or global name is not found. It's possible that you misspelt the name of the object or forgot to import something. Also maybe it's in another scope. We'll cover those with separate examples.

It's simply not defined nowhere in the code

It's possible that you forgot to initialize it, specially if it is a constant

foo   # This variable is not defined
bar() # This function is not defined

Maybe it's defined later:


def baz():

Or it wasn't imported:

#needs import math

def sqrt():
    x = float(input("Value: "))
    return math.sqrt(x)

Python scopes and the LEGB Rule:

The so-called LEGB Rule talks about the Python scopes. It's name is based on the different scopes, ordered by the correspondent priorities:

Local → Enclosed → Global → Built-in.
  • Local: Variables not declared global or assigned in a function.
  • Enclosing: Variables defined in a function that is wrapped inside another function.
  • Global: Variables declared global, or assigned at the top-level of a file.
  • Built-in: Variables preassigned in the built-in names module.

As an example:

for i in range(4):
    d = i * 2

d is accesible because the for loop does not mark a new scope, but if it did, we would have an error and its behavior would be similar to:

def noaccess():
    for i in range(4):
        d = i * 2

Python says NameError: name 'd' is not defined