NameError: name 'a' is not defined

[rollbar]

View details in Rollbar: https://rollbar.com/ds19991999/Rollbar-2/items/2/

Traceback (most recent call last):
  File "<ipython-input-7-e3ba778928e1>", line 6, in <module>
    b = a + 1
NameError: name 'a' is not defined