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Global and nonlocal statements#

Code Example

Runnable Example in Jac and JacLib

# Global and Nonlocal Statements
glob x = "global x";
glob a = 1 , b = 2;

def test_global {
    # Global with single name
    global x;
    x = "modified global x";
    print(x);
}

def test_multiple_globals {
    # Global with multiple names
    global a,b;
    a = 10;
    b = 20;
    print(f"{a} {b}");
}

def outer {
    y = "outer y";

    def inner {
        # Nonlocal with single name
        nonlocal y ;
        y = "modified by inner";
        print(y);
    }

    print(y);
    inner();
    print(y);
}

def outer_multi {
    p = 1;
    q = 2;
    r = 3;

    def inner_multi {
        # Nonlocal with multiple names
        nonlocal p , q , r ;
        p = 10;
        q = 20;
        r = 30;
    }

    print(f"{p} {q} {r}");
    inner_multi();
    print(f"{p} {q} {r}");
}

with entry {
    print(x);
    test_global();
    print(x);

    print(f"{a} {b}");
    test_multiple_globals();
    print(f"{a} {b}");

    outer();
    outer_multi();
}

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# Global and Nonlocal Statements
glob x = "global x";
glob a = 1 , b = 2;

def test_global {
    # Global with single name
    global x;
    x = "modified global x";
    print(x);
}

def test_multiple_globals {
    # Global with multiple names
    global a,b;
    a = 10;
    b = 20;
    print(f"{a} {b}");
}

def outer {
    y = "outer y";

    def inner {
        # Nonlocal with single name
        nonlocal y ;
        y = "modified by inner";
        print(y);
    }

    print(y);
    inner();
    print(y);
}

def outer_multi {
    p = 1;
    q = 2;
    r = 3;

    def inner_multi {
        # Nonlocal with multiple names
        nonlocal p , q , r ;
        p = 10;
        q = 20;
        r = 30;
    }

    print(f"{p} {q} {r}");
    inner_multi();
    print(f"{p} {q} {r}");
}

with entry {
    print(x);
    test_global();
    print(x);

    print(f"{a} {b}");
    test_multiple_globals();
    print(f"{a} {b}");

    outer();
    outer_multi();
}

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from __future__ import annotations

x = "global x"
a = 1
b = 2


def test_global() -> None:
    global x
    x = "modified global x"
    print(x)


def test_multiple_globals() -> None:
    global a
    global b
    a = 10
    b = 20
    print(f"{a} {b}")


def outer() -> None:
    y = "outer y"

    def inner() -> None:
        nonlocal y
        y = "modified by inner"
        print(y)

    print(y)
    inner()
    print(y)


def outer_multi() -> None:
    p = 1
    q = 2
    r = 3

    def inner_multi() -> None:
        nonlocal p
        nonlocal q
        nonlocal r
        p = 10
        q = 20
        r = 30

    print(f"{p} {q} {r}")
    inner_multi()
    print(f"{p} {q} {r}")


print(x)
test_global()
print(x)
print(f"{a} {b}")
test_multiple_globals()
print(f"{a} {b}")
outer()
outer_multi()
Jac Grammar Snippet 
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global_ref: GLOBAL_OP name_list
nonlocal_ref: NONLOCAL_OP name_list
name_list: (named_ref COMMA)* named_ref

Description

Global and Nonlocal Statements

The global and nonlocal statements control variable scope, allowing functions to modify variables from outer scopes rather than creating local shadows.

Module-Level Global Variables

Lines 3-4 declare module-level global variables. These variables exist at module scope and are accessible throughout the module.

Global Statement - Single Variable

Lines 6-11 demonstrate the global statement. Line 8: global x declares that x refers to the module-level global variable (line 3). Without this statement, line 9's assignment would create a new local variable instead of modifying the global.

Global Statement - Multiple Variables

Lines 13-19 show declaring multiple global variables. Line 15: global a, b is more concise than separate global statements for each variable.

Nonlocal Statement - Single Variable

Lines 21-34 demonstrate the nonlocal statement for nested functions. Line 26: nonlocal y declares that y refers to the enclosing function's variable (line 22), not a global or new local. Line 27's assignment modifies the outer function's y.

Nonlocal Statement - Multiple Variables

Lines 36-52 show declaring multiple nonlocal variables. Line 43: nonlocal p, q, r modifies all three variables from the enclosing scope.

Scope Resolution Rules

Without global or nonlocal, assignments create new local variables. These keywords alter this behavior:

Keyword Accesses Scope Level Use Case
global Module-level variables Global Modify module state from functions
nonlocal Enclosing function variables Enclosing Closures, nested function state

Execution Flow

Lines 54-65 demonstrate the execution and effects:

Variable Modification Flow

flowchart TD
    Start([Assignment in Function]) --> Check{global or<br/>nonlocal<br/>declared?}
    Check -->|global| ModGlobal[Modify module-level<br/>global variable]
    Check -->|nonlocal| ModEnclosing[Modify enclosing<br/>function variable]
    Check -->|Neither| CreateLocal[Create new<br/>local variable]
    ModGlobal --> Done([Assignment Complete])
    ModEnclosing --> Done
    CreateLocal --> Done

Scope Levels Example

flowchart TD
    Global[Global Scope<br/>x, a, b] --> Outer1[outer function<br/>y variable]
    Outer1 --> Inner1[inner function<br/>accesses y with nonlocal]
    Global --> Outer2[outer_multi function<br/>p, q, r variables]
    Outer2 --> Inner2[inner_multi function<br/>accesses p,q,r with nonlocal]
    Global --> TestGlobal[test_global function<br/>accesses x with global]

Common Patterns

Modifying module state:

Closure with state:

Multiple scope levels:

Key Differences

Global vs Nonlocal:

  • global reaches to module level (skips all intermediate scopes)
  • nonlocal reaches to the nearest enclosing function scope (not global)
  • global can declare variables that don't exist yet
  • nonlocal requires the variable to exist in an enclosing scope

Important Rules

  1. Without declaration: Assignments create new local variables
  2. With global: Assignments modify module-level variables
  3. With nonlocal: Assignments modify enclosing function's variables
  4. Reading vs Writing: You can read outer scope variables without declarations; declarations are needed for assignment
  5. Multiple names: Both statements support comma-separated variable lists