在PHP上运行python脚本会抛出错误“找不到默认python”

我试图在我的PHP页面中使用

<?php

$command = "py C:/wamp/www/py/python.py 2>&1";
$pid = popen( $command,"r");
while( !feof( $pid ) )
{
 echo fread($pid, 256);
 flush();
 ob_flush();
 usleep(100000);
}
pclose($pid);

?>

我的python代码是

hello = "hello"
world = "world"
print (hello + " " + world)