Learning Python

Function Gotchas

Here are some of the more jagged edges of functions you might not expect. They’re all obscure, but most have been known to trip up a new user.

Local Names Are Detected Statically

As we’ve seen, Python classifies names assigned in a function as locals by default; they live in the function’s scope and exist only while the function is running. What we didn’t tell you is that Python detects locals statically, when it compiles the code, rather than by noticing assignments as they happen at runtime. Usually, we don’t care, but this leads to one of the most common oddities posted on the Python newsgroup by beginners.

Normally, a name that isn’t assigned in a function is looked up in the enclosing module:

>>> X = 99
>>> def selector():        # X used but not assigned
...     print X            # X found in global scope
...
>>> selector()
99

Here, the X in the function resolves to the X in the module outside. But watch what happens if you add an assignment to X after the reference:

>>> def selector():
...     print X              # does not yet exist!
...     X = 88               # X classified as a local name (everywhere)
...                          # can also happen if "import X", "def X",...
>>> selector()
Traceback (innermost last):
  File "<stdin>", line 1, in ?
  File "<stdin>", line 2, in selector
NameError: X

You get an undefined name error, but the reason is subtle. Python reads and compiles this code when it’s typed interactively or imported from a module. While compiling, Python sees the assignment to X and decides that X will be a local name ...