ctypes: don't fail to load when embedded as a "frozen" package (pearu#187)

* ctypes: don't fail to load when embedded as a "frozen" package

When the module is embedded in a executable (aka "frozen"), the __file__
path is virtual, and might not actually exists on the computer.
This path is used at init to look for the DLL, by changing the current
working directory to the containing folder. As it doesn't exist, it
fails, and libtiff_ctypes cannot be imported.

Instead of completely failing, now handle such case, and just don't use
that folder as an extra location to look for the DLL.

* lsm: drop useless "global" usage

As noted by the latest flake8:
F824 `global tiff_module_dict` is unused: name is never assigned in scope

As the tiff_module_dict variable is only read, there is no need to use
a "global".

Author: pieleric

libtiff/libtiff_ctypes.py

@@ -23,7 +23,15 @@
 
 cwd = os.getcwd()
 try:
-    os.chdir(os.path.dirname(__file__))
+    try:
+        # Typically, on Windows, the CWD is among the folders searched to locate
+        # a DLL. So change it to the directory containing this module, in case
+        # the libtiff DLL was installed aside it (although that's not typically the case).
+        os.chdir(os.path.dirname(__file__))
+    except FileNotFoundError:
+        # If "frozen" (ie, embedded in an executable), the directory is not real, and chdir fails
+        # => just ignore (and look for the DLL in all the other standard locations)
+        pass
     if os.name == 'nt':
         # assume that the directory of the libtiff DLL is in PATH.
         for lib in ('tiff', 'libtiff', 'libtiff3'):

libtiff/lsm.py

@@ -34,7 +34,6 @@ def IFDEntry_lsm_str_hook(entry):
 def IFDEntry_lsm_init_hook(ifdentry):
     """Make tiff.IFDENTRYEntry CZ_LSM aware.
     """
-    global tiff_module_dict
     if ifdentry.tag == CZ_LSMInfo_tag:
         # replace type,count=(BYTE,500) with (CZ_LSMInfo, 1)
         reserved_bytes = (ifdentry.count - CZ_LSMInfo_dtype_fields_size)