Python: Why is global needed only on assignment and not on reads?

With nested scopes, the variable lookups are easy. They occur in a chain starting with locals, through enclosing defs, to module globals, and then builtins. The rule is the first match found wins. Accordingly, you don't need a "global" declaration for lookups.

In contrast, with writes you need to specify which scope to write to. There is otherwise no way to determine whether "x = 10" in function would mean "write to a local namespace" or "write to a global namespace."

Executive summary, with write you have a choice of namespace, but with lookups the first-found rule suffices. Hope this helps :-)

Edit: Yes, it is this way "because the BDFL said so", but it isn't unusual in other languages without type declarations to have a first-found rule for lookups and to only require a modifier for nonlocal writes. When you think about it, those two rules lead to very clean code since the scope modifiers are only needed in the least common case (nonlocal writes).

Look at this code:

from module import functiondef foo(x):    return function(x)

The name function here is a global. It would get awfully tedious if I had to say global function to get this code to work.

Before you say that your X and my function are different (because one is a variable and the other is an imported function), remember that all names in Python are treated the same: when used, their value is looked up in the scope hierarchy. If you needed global X then you'd need global function. Ick.

Because explicit is better than implicit.

There's no ambiguity when you read a variable. You always get the first one found when searching scopes up from local until global.

When you assign, there's only two scopes the interpreter may unequivocally assume you are assigning to: local and global. Since assigning to local is the most common case and assigning to global is actually discouraged, it's the default. To assign to global you have to do it explicitly, telling the interpreter that wherever you use that variable in this scope, it should go straight to global scope and you know what you're doing. On Python 3 you can also assign to the nearest enclosing scope with 'nonlocal'.

Remember that when you assign to a name in Python, this new assignment has nothing to do with that name previously existing assigned to something else. Imagine if there was no default to local and Python searched up all scopes trying to find a variable with that name and assigning to it as it does when reading. Your functions' behavior could change based not only on your parameters, but on the enclosing scope. Life would be miserable.