How variable scope works in Python

Someone asked me to take a look at a piece of code recently and tell him why it wasn’t working. The problem was that he didn’t really understand Python variable scoping. That’s what I’m going to talk about today. It is quite basic, but you really need to have it down cold, and there are a few surprises in there too.

What you need to know

A variable in Python is defined when you assign something to it. You don’t declare it beforehand, like you can in C. You just start using it.

Any variable you declare at the top level of a file or module is in global scope. You can access it inside functions.

Before I go on I need to add a disclaimer: global variable are almost always a bad idea. Yes, sometimes you need them, but you almost always don’t. A good rule of thumb is that a variable should have the narrowest scope it needs to do its job. There’s a good discussion of global variables and the associated issues here.

Modifying the value of a global variable is less simple. Take a look at this example.

What happened? Why is the value of x 123 for the second print statement? It turns out that when we assigned the value 321 to x inside foo we actually declared a new variable called x in the local scope of that function. That x has absolutely no relation to the x in global scope. When the function ends, that variable with the value of 321 does not exist anymore.

To get the desired effect, we have to use the global keyword.

That’s more like it.

There is one more scope we have to worry about: the enclosing scope created by declaring one function inside another one. Watch.

What if you want to modify the value of x declared in the outer function? You’ll run into the same problem that made us use global. But we don’t want to use global here. x is not a global variable. It is in the local scope of a function.

Python 3 introduced the nonlocal keyword for this exact situation. I wrote a post about it on this page, but I’ll show you a quick example now.

A simple way to remember Python scoping rules

In the book Learning Python by Mark Lutz, he suggests the following mnenomic for remember how Python scoping works: LEGB

Going from the narrowest scope to the widest scope:

  • L stands for “Local”. It refers to variables that are defined in the local scope of functions.
  • E stands for “Enclosing”. It refers to variables defined in the local scope of functions wrapping other functions.
  • G stands for “Global”. These are the variables defined at the top level of files and modules.
  • B stands for “Built in”. These are the names that are loaded into scope when the interpreter starts up. You can look at them here: https://docs.python.org/3.5/library/functions.html

And that is everything you need to learn about this topic for the vast majority of Python programming tasks.

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