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Programming-Idioms

  • JS
  • Python

Idiom #108 Determine if variable name is defined

Print the value of variable x, but only if x has been declared in this program.
This makes sense in some languages, not all of them. (Null values are not the point, rather the very existence of the variable.)

if 'x' in locals():
	print(x)

variable name must be quoted.
try:
    x
except NameError:
    print("does not exist")
try {
	console.log(x);
} catch (e) {
	if (!e instanceof ReferenceError) {
		throw e;
	}
}
if (typeof x !== 'undefined') {
    console.log(x);
}

However if x has previously been declared and set to undefined, this will not print x even though x has been declared.
import std.stdio;
static if (is(typeof(x = x.init)))
    writeln(x);

We test, at compile-time, if an assignExpression with x as lhs would compile, if so we use it.

This technic is sometimes used in template constraints, but rather to test if an operation would be accepted on an argument.
New implementation...
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programming-idioms.org 
if 'x' in locals():
	print(x)
try:
    x
except NameError:
    print("does not exist")
try {
	console.log(x);
} catch (e) {
	if (!e instanceof ReferenceError) {
		throw e;
	}
}
if (typeof x !== 'undefined') {
    console.log(x);
}
static if (is(typeof(x = x.init)))
    writeln(x);
int x = 42;

void printIfDefined(alias name)()
{
    import std.stdio: writeln;
    static if( __traits(compiles, writeln(mixin(name))))
        writeln(mixin(name));
}

void main(string[] args)
{
    printIfDefined!"x";
    printIfDefined!"Foo.bar";
}
implicit none

print *,x
try {
    Class<?> c = getClass();
    Field f = c.getDeclaredField("x");
    out.println(f.get(this));
} catch (NoSuchFieldException e) {

} catch (IllegalAccessException e) {

}
if x then print(x) end
$x = 'foo';

if (isset($x)) {
    echo $x;
}
if(@$foo) print($foo);
{$if DECLARED(x)}
writeln(x);
{$endif}
print $x;  # compile error

print $x if $main::{x}; # prints $x if it was declared


use strict 'vars';
# or
use v5.11; # or higher

print $x if $main::{x};  # unavoidable compile error


puts x if defined?(x)