You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
FIELD OF THE INVENTION 
   The invention relates to a padlock with a status indicator adopted for use on padlocks, locking devices or structural objects and particularly to a lock that combines two locking units that function independently to execute locking and unlocking operations. 
   BACKGROUND OF THE INVENTION 
   Locks are widely used to guard articles that have obvious or potential security concerns. For instance, a padlock may be used to lock the zipper of a piece of luggage to prevent unauthorized opening. 
   Due to security reasons, nowadays airports have very strict standards for luggage security inspection. With luggage that may be a security risk, mandatory opening of the luggage for security inspection could take place. For the luggage that has been locked, the lock has to be broken. After security inspection, the lock is damaged and becomes unusable. As a result, the damaged lock can no longer provide protection after the security inspection has been completed. 
   To remedy the problem of lock damage during security inspection, a lock equipped with two locking units has been developed. For instance, Travel Sentry™ Co. of U.S.A. introduced a number of locks for this purpose. They mainly have one locking unit to allow luggage owners to lock and unlock during use, and another locking unit for security inspection. Hence the security inspection people can perform routine security inspection through a normal unlocking procedure to open the lock, and close the lock after the security inspection is finished. Therefore damage of the lock may be prevented, and the luggage has proper protection after security inspection. 
   However, the lock mentioned above still has a drawback, namely that after the security inspection was done and the lock is relocked again, there is no indication to warn the luggage owners that the lock has been unlocked. 
   SUMMARY OF THE INVENTION 
   In view of the problem, the primary object of the invention is to provide a lock that combines two locking units that function independently so that the luggage owners can recognize if either of the locking unit has been unlocked. 
   In order to achieve the foregoing object, the invention provides a padlock with a status indicator that has a window on the surface of its body and an indicator in the lock underneath and corresponding to the window position. The indicator is switched to different indication positions corresponding to the window once different locking units have been unlocked so that luggage owners can see the indication position through the window and recognize the locking unit that has been unlocked. 
   The foregoing, as well as additional objects, features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
       FIG. 1  is a perspective view of the lock according to the invention. 
       FIG. 2  is a side view of the lock according to the invention. 
       FIGS. 3A and 3B  are schematic views of the structure of the lock of the invention. 
       FIG. 4  is a schematic view of another embodiment of the lock equipped with two locking units according to the invention. 
       FIG. 5  is a schematic view of yet another embodiment of the lock equipped with two locking units according to the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Refer to  FIGS. 1 and 2  for the perspective and side views of an embodiment of the invention. The lock according to the invention includes a body  10 , which contains two locking units  12  and  13  and a shackle  14  that is coupled on the body  10  externally, may be locked by the locking units  12  and  13 , and forms a closed space  15  jointly with the body  10 . The two locking units  12  and  13  are referred to as the first locking unit  12  and the second locking unit  13  hereinafter. In the following embodiments, the two locking units  12  and  13  adopt combination lock and key lock structures. However, in practice, this is not a limitation, since other types of locks may also be used. 
   Refer to  FIGS. 3A and 3B  for the structure of an embodiment of the invention. The body  10 , besides containing the first locking unit  12  and the second locking unit  13 , also houses an indicator  11 . The indicator  11  has recognition marks  111  and  112  that may be switched to a “first indication position” as shown in  FIG. 3A , and a “second indication position” as shown in  FIG. 3B , and may be seen through a window  100  (indicated by broken lines in the drawings) formed on the surface of the body  10  to recognize the position of the indicator  11 . 
   Hence when the indicator  11  is moved to different indication positions, the recognition marks  111  and  112  formed thereon also are moved to the corresponding window  100  to be recognized. The recognition marks  111  and  112  may be, but are not limit to, graphics, characters, color patches and combinations thereof, and may be formed on the indicator  11  by, but not limit to, printing, carving, painting, sticking paper, and the like. 
   In practice, the shackle  14  consists of two latch arms  141  and  142 . One latch arm  141  is lockable by the first locking unit  12 , while the other latch arm  142  is lockable by the second locking unit  13 . By means of such a construction, when the first locking unit  12  is actuated and changed from a locking condition to an unlocking condition, the latch arm  141  is moved by a linking or indirectly driving pushbutton  16  to form a gap in the closed space  15  formed with the body  10 . Consequently, the indicator  11  is moved to the first indication position. When the pushbutton  16  and the latch arm  141  are moved back to the position before the force is applied, the indicator  11  is not moved back. 
   In such a condition, only when the second locking unit  13  is moved from the original locking position to the unlocking position can the latch arm  142  be moved to generate the gap in the closed space  15 . The second locking unit  13  has an actuation strut  131  that moves the indicator  11  to the second indication position at the same time while being switched to the unlocking position. As the second locking unit  13  can move the indicator  11  in only one way direction as the first locking unit  12 , when it returns to the locking position the indicator  11  remains in the second indication position. 
   Therefore, through the unlocking operations of the two different locking units  12  and  13 , the indicator  11  is also switched to different indication positions that can be seen through the corresponding window  100 . Hence the luggage owner can tell which of the locking units  12  and  13  has been unlocked. 
   In this embodiment, the other latch arm  142  may be coupled with the second locking unit  13  and moved therein synchronously. In addition, the switching movement of the indicator  11  is not limited to sliding as shown in the drawings. For instance, the indicator  11  may also be switched by swiveling, which may be seen as another embodiment of the invention. 
   Refer to  FIG. 4  for another embodiment of the invention. It differs from the previous embodiments by having a key (not shown in the drawing) to drive a rotary core to execute locking and unlocking operations of the first locking unit  12   a  and the second locking unit  13   a . That is, the two locking units  12   a  and  13   a  are key locks. They have respectively actuation struts  121   a  and  131   a  to move the indicator  11   a  to the different indication positions while the locking units  12   a  and  13   a  are switched from the locking position to the unlocking position. 
   Refer to  FIG. 5  for still another embodiment of the invention. It is an alteration of the two locking units based on the previous embodiments. The first locking unit  12   b  and the second locking unit  13   b  of the body  10   b  are combination locks, and may be actuated and switched respectively to an unlocking condition and a locking condition. The body  10   b  further has a first pushbutton  161   b  and a second pushbutton  162   b.  The first pushbutton  161   b  is lockable by the first locking unit  12   b.  While the first locking unit  12   b  is in the unlocking condition, the indicator  11   b  is moved to the first indication position. The second pushbutton  162   b  is lockable by the second locking unit  13   b . While the second locking unit  13   b  is in the unlocking condition, the indicator  11   b  is moved to the second indication position 
   The shackle  14   b  consists of a first latch arm  141   b  and a second latch arm  142   b . In practice, the first latch arm  141   b  and the second latch arm  142   b  may be coupled respectively with the first pushed button  161   b  and second pushbutton  162   b , and are lockable by the first locking unit  12   b  and the second locking unit  13   b  at the same time. 
   By means of the constructions set forth above, the indicator, which is in the lock underneath and corresponding to the window location, is switched to different indication positions when different locking units have finished locking and unlocking operations. Hence luggage owners can recognize through the window on the lock which locking unit has been unlocked. 
   While the preferred embodiments of the invention have been set forth for the purpose of disclosure, modifications of the disclosed embodiments of the invention as well as other embodiments thereof may occur to those skilled in the art. Accordingly, the appended claims are intended to cover all embodiments which do not depart from the spirit and scope of the invention.

Summary:
A padlock with a status indicator includes two locking units. The two locking units operate independently to execute locking and unlocking operations. The lock has a body that contains an indicator in the lock underneath. After the execution of the unlocking operation has been done sequentially, the indicator switches from an indication position to the other indication position. Thus through the window, which is formed on the body surface and corresponding to the indicator location, the indication positions of the indicator may be recognized.