So I haven’t been doing a lot of Python recently, and I got tripped up by something that in retrospect should have been obvious.

You can write code that references an undefined thingy, and Python won’t complain until you actually run the code and try to access the thingy.

Eg:

>>> def f():
...     z()
... 
>>> f()
Traceback (most recent call last):
  File "", line 1, in 
  File "", line 2, in f
NameError: global name 'z' is not defined

Which kind of sucks when you forgot to write a test for that code, and you get a runtime exception.