diff --git a/src/calibre/db/cache.py b/src/calibre/db/cache.py
index d0fa2403b9..ff4476bc6a 100644
--- a/src/calibre/db/cache.py
+++ b/src/calibre/db/cache.py
@@ -1528,6 +1528,7 @@ class Cache(object):
         :param run_hooks: If True, file type plugins are run on the format before and after being added.
         :param dbapi: Internal use only.
         '''
+        needs_close = False
         if run_hooks:
             # Run import plugins, the write lock is not held to cater for
             # broken plugins that might spin the event loop by popping up a
@@ -1535,6 +1536,7 @@ class Cache(object):
             npath = run_import_plugins(stream_or_path, fmt)
             fmt = os.path.splitext(npath)[-1].lower().replace('.', '').upper()
             stream_or_path = lopen(npath, 'rb')
+            needs_close = True
             fmt = check_ebook_format(stream_or_path, fmt)
 
         with self.write_lock:
@@ -1550,8 +1552,17 @@ class Cache(object):
             if name and not replace:
                 return False
 
-            stream = stream_or_path if hasattr(stream_or_path, 'read') else lopen(stream_or_path, 'rb')
-            size, fname = self._do_add_format(book_id, fmt, stream, name)
+            if hasattr(stream_or_path, 'read'):
+                stream = stream_or_path
+            else:
+                stream = lopen(stream_or_path, 'rb')
+                needs_close = True
+            try:
+                stream = stream_or_path if hasattr(stream_or_path, 'read') else lopen(stream_or_path, 'rb')
+                size, fname = self._do_add_format(book_id, fmt, stream, name)
+            finally:
+                if needs_close:
+                    stream.close()
             del stream
 
             max_size = self.fields['formats'].table.update_fmt(book_id, fmt, fname, size, self.backend)
diff --git a/src/calibre/devices/udisks.py b/src/calibre/devices/udisks.py
index 0847b002bd..d0ad9e1985 100644
--- a/src/calibre/devices/udisks.py
+++ b/src/calibre/devices/udisks.py
@@ -20,10 +20,11 @@ def node_mountpoint(node):
         return raw.replace(b'\\040', b' ').replace(b'\\011', b'\t').replace(b'\\012',
                 b'\n').replace(b'\\0134', b'\\').decode('utf-8')
 
-    for line in open('/proc/mounts', 'rb').readlines():
-        line = line.split()
-        if line[0] == node:
-            return de_mangle(line[1])
+    with open('/proc/mounts', 'rb') as src:
+        for line in src.readlines():
+            line = line.split()
+            if line[0] == node:
+                return de_mangle(line[1])
     return None
 
 
diff --git a/src/calibre/devices/usbms/device.py b/src/calibre/devices/usbms/device.py
index fae4edc465..41d908f07c 100644
--- a/src/calibre/devices/usbms/device.py
+++ b/src/calibre/devices/usbms/device.py
@@ -456,6 +456,12 @@ class Device(DeviceConfig, DevicePlugin):
             q = getattr(detected_device, attr)
             return q == val
 
+        def getnum(usb_dir):
+            def rc(q):
+                with open(j(usb_dir, q), 'rb') as f:
+                    return raw2num(f.read().decode('utf-8'))
+            return rc
+
         for x, isfile in walk('/sys/devices'):
             if isfile and x.endswith('idVendor'):
                 usb_dir = d(x)
@@ -465,8 +471,7 @@ class Device(DeviceConfig, DevicePlugin):
                         break
                 if usb_dir is None:
                     continue
-                e = lambda q : raw2num(open(j(usb_dir, q), 'rb').read().decode('utf-8'))
-                ven, prod, bcd = map(e, ('idVendor', 'idProduct', 'bcdDevice'))
+                ven, prod, bcd = map(getnum(usb_dir), ('idVendor', 'idProduct', 'bcdDevice'))
                 if not (test(ven, 'idVendor') and test(prod, 'idProduct') and
                         test(bcd, 'bcdDevice')):
                     usb_dir = None
@@ -487,7 +492,8 @@ class Device(DeviceConfig, DevicePlugin):
                     sz = j(x, 'size')
                     node = parts[idx+1]
                     try:
-                        exists = int(open(sz, 'rb').read().decode('utf-8')) > 0
+                        with open(sz, 'rb') as szf:
+                            exists = szf.read().decode('utf-8') > 0
                         if exists:
                             node = self.find_largest_partition(x)
                             ok[node] = True