imported>Duckmather: /* Python 2.4 */ update to add Python 3 behavior

2024-02-16T20:31:52Z

Python 2.4: update to add Python 3 behavior

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An '''undefined variable''' in the [[source code]] of a [[computer program]] is a [[variable (programming)|variable]] that is accessed in the code but has not been [[Declaration (computer science)|declared]] by that code.<ref>"undefined variable." YourDictionary, n.d. Web. 24 July 2013. <http://computer.yourdictionary.com/undefined-variable>.</ref>

In some [[programming languages]], an implicit declaration is provided the first time such a variable is encountered at [[compile time]]. In other languages such a usage is considered to be sufficiently serious that a diagnostic being issued and the compilation fails.

Some language definitions initially used the implicit declaration behavior and as they matured provided an option to disable it (e.g. [[Perl]]'s "<code>use warnings</code>" or [[Visual Basic]]'s "<code>Option Explicit</code>").

==Examples==
The following provides some examples of how various programming language implementations respond to undefined variables. Each code snippet is followed by an error message (if any).

===[[CLISP]] ===
<syntaxhighlight lang="lisp">
(setf y x)
</syntaxhighlight>
*** - EVAL: variable X has no value

===[[C (programming language)|C]]===
<syntaxhighlight lang="c">
int main() {
int y = x;
return 0;
}
</syntaxhighlight>
foo.c: In function `main':
foo.c:2: error: `x' undeclared (first use in this function)
foo.c:2: error: (Each undeclared identifier is reported only once
foo.c:2: error: for each function it appears in.)

===[[JavaScript]] ===

A ReferenceError only happens if the same piece of executed code has a {{code|let|lang=js}} or a {{code|const|lang=js}} (but not {{code|var|lang=js}}) declaration later on, or if the code is executed in strict mode. In all other cases, the variable will have the special value {{code|undefined|lang=js}}.

<syntaxhighlight lang="javascript">
"use strict";
let y = x;
</syntaxhighlight>
<syntaxhighlight lang="javascript">
let y = x;
let x; // causes error on line 1
</syntaxhighlight>
ReferenceError: x is not defined
Source File: file:///c:/temp/foo.js

===[[Lua (programming language)|Lua]]===

<syntaxhighlight lang="lua">
y = x
</syntaxhighlight>
(no error, continuing)
<syntaxhighlight lang="lua">
print(y)
</syntaxhighlight>
nil

===[[Standard ML|ML]] (Standard ML of New Jersey) ===
<syntaxhighlight lang="sml">
val y = x;
</syntaxhighlight>
stdIn:1.9 Error: unbound variable or constructor: x

===[[MUMPS]]===

Set Y=X

<UNDEF>

===[[OCaml]] ===
<syntaxhighlight lang="ocaml">
let y = x;;
</syntaxhighlight>
Unbound value x

===[[Perl]]===
<syntaxhighlight lang="perl">
my $y = ($x // 0) + 1; # defined-or operator
</syntaxhighlight>
(no error)

===[[PHP]] 5===
<syntaxhighlight lang="php">
$y = $x;
</syntaxhighlight>
(no error)

<syntaxhighlight lang="php">
$y="";
$x="";
error_reporting(E_ALL);
$y = $x;
</syntaxhighlight>
PHP Notice: Undefined variable: x in foo.php on line 3

===[[Python (programming language)|Python]]===

==== Python 3 ====
<syntaxhighlight lang="python3">
x = y
Traceback (most recent call last):
File "<pyshell#0>", line 1, in <module>
x = y
NameError: name 'y' is not defined
</syntaxhighlight>

==== Python 2.4 ====
<syntaxhighlight lang="pycon">
>>> x = y
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'y' is not defined
</syntaxhighlight>

===[[REXX]]===
<syntaxhighlight lang="rexx">
signal on novalue
y = x
</syntaxhighlight>
+++ Error 30 in line 2: Label not found

===[[Ruby (programming language)|Ruby]] ===
<syntaxhighlight lang="irb">
irb(main):001:0> y = x
NameError: undefined local variable or method `x' for main:Object
from (irb):1
</syntaxhighlight>

===[[Tcl]] ===
<syntaxhighlight lang="tcl">
% set y $x
can't read "x": no such variable
</syntaxhighlight>

===[[VBScript]] ===
<syntaxhighlight lang="vbscript">
Dim y
y = x
</syntaxhighlight>
(no error)

<syntaxhighlight lang="vbscript">
Option Explicit

Dim y
y = x
</syntaxhighlight>
(3, 1) Microsoft VBScript runtime error: Variable is undefined: 'x'

==References==
{{Reflist}}

{{DEFAULTSORT:Undefined Variable}}
[[Category:Variable (computer science)]]
[[Category:Software bugs]]

Undefined variable - Revision history

imported>Duckmather: /* Python 2.4 */ update to add Python 3 behavior