Scope determines which name binding Python uses at a particular point in a program. A namespace maps names to objects. A module, script, or interactive session has a namespace for its top-level names; each function call gets a local namespace of its own. section 10.7
For an ordinary name lookup, keep these rules in mind:
- Python checks the local namespace first, then the global namespace, then built-ins. If the name is absent from all three, it raises NameError. section 10.7
- Parameters and names assigned inside a function are local to that call. A function’s global namespace belongs to its containing module, not to the code that called it. section 9.4 section 10.7
- Use global to assign to a top-level binding. In a nested function, use nonlocal to assign to a previously bound name in the closest enclosing scope. section 9.4
A local assignment does not change a top-level name
Start with `a = "one"` and `b = "two"`. A function containing `global a; a = 1; b = 2` changes top-level `a` to `1`, but leaves top-level `b` as `"two"`. Its assignment to `b` creates a local binding. section 9.4
Listing 9.1 File nonlocal.py
g_var = 0 #1
nl_var = 0 #2
print("top level-> g_var: {0} nl_var: {1}".format(g_var, nl_var))
def test():
nl_var = 2 #3
print("in test-> g_var: {0} nl_var: {1}".format(g_var, nl_var))
def inner_test():
global g_var #4
nonlocal nl_var #5
g_var = 1
nl_var = 4
print("in inner_test-> g_var: {0} nl_var: {1}".format(g_var,
nl_var))
inner_test()
print("in test-> g_var: {0} nl_var: {1}".format(g_var, nl_var))
test()
print("top level-> g_var: {0} nl_var: {1}".format(g_var, nl_var))
Run the nested-function example alongside its printed output: `global g_var` changes the top-level binding, while `nonlocal nl_var` changes the binding in `test()`, leaving the top-level `nl_var` unchanged. section 9.4
Scope also explains two practical surprises. Assigning `list = [1, 3, 5, 7]` hides the built-in `list` at that level; `del list` removes that binding and restores access. And a name defined in an imported module is normally accessed through that module, rather than becoming a top-level name in the importing script. When a name seems wrong or missing, inspect `locals()`, `globals()`, or `dir()`. section 10.7.1 section 10.9
