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

Question

If a function needs to modify a variable declared in global scope, it need to use the global declaration. However, if the function just needs to read a global variable it can do so without using a global declaration:

X = 10 def foo():     global X     X = 20 # Needs global declaration def bar():     print( X ) # Does not need global 

My question is about the design of Python: why is Python designed to allow the read of global variables without using the global declaration? That is, why only force assignment to have global, why not force global upon reads too? (That would make it even and elegant.)

Note: I can see that there is no ambiguity while reading, but while assigning it is not clear if one intends to create a new local variable or assign to the global one. But, I am hoping there is a better reason or intention to this uneven design choice by the BDFL.

Answer 1

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).