None is often used to undefine a variable. Technically speaking, this does not undefine the variable, as the variable remains defined with a value of None.

#!/usr/bin/python3
foo = "Hello"
bar = "World"

print(f"foo = {foo}")
print(f"bar = {bar}")

foo = None
bar = None

print(f"foo = {foo}")
print(f"bar = {bar}")

In this example, the following should be returned. 

foo = Hello
bar = World
foo = None
bar = None

del can be used if you really want to undefine the variable.

#!/usr/bin/python3

foo = "Hello"

print(f"foo = {foo}")

del foo

try:
  foo
except NameError:
  print("foo is undefined")
else:
  print(f"foo = {foo}")

In this example, the following should be returned. 

foo = Hello
foo is undefined