Added interface to allow the printer device to be manually opened and closed.

This allows the printer object to retain the host side image cache and helps
the developer to open the printer only as needed to help avoid issues if the
device has been disconnected.

NOTE: This style of usage is optional and standard usage is otherwise unaffected.

escpos/printer.py

@@ -18,6 +18,8 @@ from exceptions import *
 class Usb(Escpos):
     """ Define USB printer """
 
+    is_open = False
+
     def __init__(self, idVendor, idProduct, interface=0, in_ep=0x82, out_ep=0x01):
         """
         @param idVendor  : Vendor ID
@@ -36,9 +38,14 @@ class Usb(Escpos):
 
     def open(self):
         """ Search device on USB tree and set is as escpos device """
+        if self.is_open:
+            return  # Already open; no need to reopen
+
         self.device = usb.core.find(idVendor=self.idVendor, idProduct=self.idProduct)
         if self.device is None:
             print "Cable isn't plugged in"
+        else:
+            self.is_open = True
 
         if self.device.is_kernel_driver_active(0):
             try:
@@ -50,9 +57,21 @@ class Usb(Escpos):
             self.device.set_configuration()
             self.device.reset()
         except usb.core.USBError as e:
+            # Seems fatal when it occurs. Should the device be closed as a result?
+            #self.close()
             print "Could not set configuration: %s" % str(e)
 
 
+    def close(self):
+        """ Manually release USB interface """
+        self.is_open = False
+
+        if self.device:
+            usb.util.dispose_resources(self.device)
+
+        self.device = None
+
+
     def _raw(self, msg):
         """ Print any command sent in raw format """
         self.device.write(self.out_ep, msg, self.interface)
@@ -60,15 +79,15 @@ class Usb(Escpos):
 
     def __del__(self):
         """ Release USB interface """
-        if self.device:
-            usb.util.dispose_resources(self.device)
-        self.device = None
+        self.close()
 
 
 
 class Serial(Escpos):
     """ Define Serial printer """
 
+    is_open = False
+
     def __init__(self, devfile="/dev/ttyS0", baudrate=9600, bytesize=8, timeout=1):
         """
         @param devfile  : Device file under dev filesystem
@@ -84,15 +103,29 @@ class Serial(Escpos):
 
 
     def open(self):
-        """ Setup serial port and set is as escpos device """
+        """ Setup serial port and set it as escpos device """
+        if self.is_open:
+            return  # Already open; no need to reopen
+
         self.device = serial.Serial(port=self.devfile, baudrate=self.baudrate, bytesize=self.bytesize, parity=serial.PARITY_NONE, stopbits=serial.STOPBITS_ONE, timeout=self.timeout, dsrdtr=True)
 
         if self.device is not None:
             print "Serial printer enabled"
+            self.is_open = True
         else:
             print "Unable to open serial printer on: %s" % self.devfile
 
 
+    def close(self):
+        """ Manually close Serial interface """
+        self.is_open = False
+
+        if self.device is not None:
+            self.device.close()
+
+        self.device = None
+
+
     def _raw(self, msg):
         """ Print any command sent in raw format """
         self.device.write(msg)
@@ -100,14 +133,14 @@ class Serial(Escpos):
 
     def __del__(self):
         """ Close Serial interface """
-        if self.device is not None:
-            self.device.close()
-
+        self.close()
 
 
 class Network(Escpos):
     """ Define Network printer """
 
+    is_open = False
+
     def __init__(self,host,port=9100):
         """
         @param host : Printer's hostname or IP address
@@ -120,11 +153,26 @@ class Network(Escpos):
 
     def open(self):
         """ Open TCP socket and set it as escpos device """
+        if self.is_open:
+            return  # Already open; no need to reopen
+
         self.device = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
         self.device.connect((self.host, self.port))
 
         if self.device is None:
             print "Could not open socket for %s" % self.host
+        else:
+            self.is_open = True
+
+
+    def close(self):
+        """ Manually close TCP connection """
+        self.is_open = False
+
+        if self.device is not None:
+            self.device.close()
+
+        self.device = None
 
 
     def _raw(self, msg):
@@ -134,13 +182,15 @@ class Network(Escpos):
 
     def __del__(self):
         """ Close TCP connection """
-        self.device.close()
+        self.close()
 
 
 
 class File(Escpos):
     """ Define Generic file printer """
 
+    is_open = False
+
     def __init__(self, devfile="/dev/usb/lp0"):
         """
         @param devfile : Device file under dev filesystem
@@ -151,10 +201,25 @@ class File(Escpos):
 
     def open(self):
         """ Open system file """
+        if self.is_open:
+            return  # Already open; no need to reopen
+
         self.device = open(self.devfile, "wb")
 
         if self.device is None:
             print "Could not open the specified file %s" % self.devfile
+        else:
+            self.is_open = True
+
+
+    def close(self):
+        """ Manually close system file """
+        self.is_open = False
+
+        if self.device is not None:
+            self.device.close()
+
+        self.device = None
 
 
     def _raw(self, msg):
@@ -164,4 +229,4 @@ class File(Escpos):
 
     def __del__(self):
         """ Close system file """
-        self.device.close()
+        self.close()