Name/variable binding errors

When you try to read from a variable that Python hasn’t heard of, you get a NameError. Here’s an example, where x isn’t bound:

Observe a few things. The name error doesn’t stop our program from running—we see the hello output. But the NameError in missing_x means we don’t see output from either of the later print calls.

It’s possible for a program to have hidden or latent NameErrors. Here’s a slight modification of our example:

Our program ran correctly to completion. That’s good! But if someone ever calls missing_x, they’re in a for a nasty NameError surprise. That’s bad! In some programming languages—Java, for example— NameError’s will prevent you from running your program at all. Python is more relaxed, which means you get to run your program earlier… but also means that your program could have bugs hidden in it.

Like syntax errors, NameError’s aren’t really “handled” in the program. The way to avoid NameError’s is to make sure your variable names are correct in your program, just like the way to avoid syntax errors is to make sure your program is syntactically valid.