Why does python behave this way with variables?

Question

I have been trying to understand why python behaves this way, in the block of code below. I have done my research but couldn\'t find a good answer so I came here to see if a

Answer 1

Here's a rule for Python Scope Resolution from this answer

LEGB Rule.

L, Local — Names assigned in any way within a function (def or lambda)), and not declared global in that function.

E, Enclosing-function locals — Name in the local scope of any and all statically enclosing functions (def or lambda), from inner to outer.

G, Global (module) — Names assigned at the top-level of a module file, or by executing a global statement in a def within the file.

B, Built-in (Python) — Names preassigned in the built-in names module : open,range,SyntaxError,...

So basically in your question, the scope resolution is "from inside out" and since you aren't using the global keyword, the interpreter doesn't know to look outside the local function scope to find that variable var. All the interpreter sees is that you are using a variable before declaring and defining it, thus throwing the error. Global variables are often dangerous, and so Python wants to make sure you know you want to use a global variable by forcing you to be explicit about it.

See this other answer for an explanation of the global keyword

Hope it helps.