Local and Global Variable Lookup Weirdness in Python

Key Takeaways

hello and welcome i'm james murphy and today we're taking a look at a subtle rule that python has about variable scopes let's play a game what does the function print let's start with an easy one shout out the answers as we go tell me either what it's going to print or what error it's going to give let's start with f what do you think happens when we run it well of course when we run it we get an error specifically a name error okay the first one was really easy what about the second one g is it any different from f well of course we still get an error but it's actually a different error now technically unbound local error derives from name error but i think it might be surprising to many of you that something different happened at all somehow python knew that i was going to assign to x later in the function normally in python outside of a looping context we don't expect statements in a function that are later in the function to affect statements that are earlier in the function it turns out that whether or not a variable like x is determined to be a free variable or a local variable is actually determined at compile time not at runtime in the function f there are no assignments to the variable x so x is determined to be a free variable that means when we say print x it tries to look up x in the global scope and fails in g however the function does contain an assignment for x it says x equals 1. so that means that in the function g x is determined to be a local variable that means that when we try to print x it looks it up in the local scope and hence we get a local error not a global error keeping that in mind let's look at another two examples and see if you get them right what do you think happens when we call h we have defined the global variable y equals 0 before defining the function and then we try to print y in the function go on just shout it out okay this was another easy one it prints zero now what do you think happens for the j function take your time i'll wait okay have you locked your guess in here we go when i run the j function i get another unbound local error now you might look at this and say when i call j we have this global variable y that's defined to be zero so print y should be printing 0 just like in the previous case but that's not how the variable lookup rules work because somewhere in the function j assigns y a value everywhere in the function y is treated as a local variable that means at the beginning when we try to print y it doesn't even try to look it up in the global scope it only looks in the local scope this can be extra confusing because this assignment y equals 1 could theoretically be hundreds of lines later in the function this is yet another reason to avoid using global variables if you can i also want to point out that other languages like c and c plus do not do things this way when i run this function it first prints the global i and then prints the local i that's why we get 0 and then 1 down at the bottom here i even compile with wall and w extra to give me extra warnings and still no warnings to be seen if i want to get a warning for this kind of behavior i have to compile with w shadow the shadow here is referring to the fact that what i'm doing is called variable shadowing if i have a variable and an inner scope like inside a function where i define a variable that has the same name as another variable in an outer scope like the global scope then this is called variable shadowing in this case i would say that this variable i shadows the one in the global scope for obvious reasons you should really try to avoid this if at all possible of course the same goes for python shadowing variables puts you on the fast track to writing bugs in case you were ever wondering this is actually one of the reasons why i define a separate main function rather than just putting things down here any assignments that i do down here are actually in the global scope and that means there's a chance that they're being shadowed i might expect an error if i try to print y here but because the y down here is in the global scope i actually just print that y instead on the other hand if i define a main function then any assignments in here that i do are local to that function that means anything that i do in here i don't have to worry about affecting things in other functions like h now when i run the function i get the expected error in h that's all i've got on this topic now that you know the rules go back and watch the video again and let me know if you get them all right this time if you like the video please consider sharing with your friends subscribing and slapping that like button an odd number of times you