Python NameError exception, how do I debug it?

I have a Python NameError exception, for example:

>>> def test():
...   var = 123
...
>>> test()
>>> print(var)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'var' is not defined

What are the steps should I take to debug it? How do I fix it?

Solution

Step 1 - Find out where was the variable used:

Read the NameError exception:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'var' is not defined

The last line contains the name of the variable, in this case var. Make sure the spelling is correct and you made no mistake.

Right above the NameError, you can see the file name and the line number. This is the location where you tried to use the variable.

Step 2 - Figure out where was the variable defined:

Look for a var = or any function which contains var as the argument:

def func(var):
    ...

Where is the variable you're trying to use? Where does it exist?

Try finding that name in all of your Python files using ctrl+f in Windows or cmd+f in Mac.

Step 3 - Try to understand the scoping rules:

If the variable was defined in a function:

Am I in the same function?

def test():
    var = 1

print(var)  # This will not work.

def test2():
    print(var)  # This will not work either.

If you're not in the same function, try fixing by returning the variable:

def test():
    var = 1
    return var

var = test()

print(var)  # This will work

or by accepting the variable as an argument:

var = test()

def test2(x):  # var is now named 'x'
    print(x)  

test2(var)

You can also return multiple variables:

def test():
     var1 = "hello"
     var2 = "world" 
     return var1, var2
var1, var2 = test()
print(var1, var2)  # Prints hello world.

If you are in the same function or using a global, did you use if statements and never entered them? Maybe an exception was thrown and you suppressed?

def test():
     if False:  # Never true
         var = 123
     print(var)  # This will not work

def test2():
     try:
         2/0  # Throws an error
         var = 123
     except:
         pass
     print (var)  # This will not work

If the variable was defined in the class:

class Test:
    var = 123
    def func(self):
        print(var)  # This will not work

Make sure you use self to access it:

class Test:
    var = 123
    def func(self):
        print(self.var)  # This will work

pass it using a default parameter:

class Test:
    var = 123
    def func(self, param=var):
        print(param)

or access it using the instance:

class Test:
    var = 123
    def func(self, param=var):
        print(param)
inst = Test()
inst.var  # This will work
inst.func()  # This will work

If the variable is the name of a function or class:

Make sure you define before using it:
```
func()
def func():  # This will not work
    pass
```
If you can't find it or it's not in the same file:

Are you trying to import it and forgot?
```
import operator
itemgetter # This will not work
operator.itemgetter  # This will work
```
Are you sure you spelled it correctly? Remember, Python is case-sensitive!

For more information about scoping, see this answer.

If everything failed and you still haven't found your answer, you're always welcome to open a question and ask, or write in the comments of your existing stackoverflow question.

Read the question checklist an provide a minimal example - make sure your example contains enough code to see the location where the variable is defined, and where it is used, but not too much code as it will be hard to understand.