Common Errors

"Experience is the name every one gives to their mistakes."

    - Oscar Wilde

Students are sometimes amazed by how I can tell the cause of exception without looking at the code. This is the result of making many errors in Python myself and also from observing many students doing mistakes. Here's a short list I've compiled of most likely cause of errors:

NameError

• You forgot to import a module
• You made a typo

AttributeError: 'NoneType' object has no attribute ...

• You forgot a return in your function

AttributeError

• Typo on dot lookup (obj.foo)
• Object is different type from what you think (str vs int)
• Object does not implement a dunder method (e.g. __len__)

There's also the 3'rd party didyoumean module, which might be interesting for beginners. It changes the default stack trace to add a hint about what might be the problem.