[Fix] Update libinfo.py (#1830)

* Update libinfo.py

fix `if search_path is list` error, and improve the code.

* change string format for compatibility with py35

change string format

* Update libinfo.py

* Update libinfo.py
Co-authored-by: Minjie Wang <wmjlyjemaine@gmail.com>
Co-authored-by: Jinjing Zhou <VoVAllen@users.noreply.github.com>
Co-authored-by: Quan (Andy) Gan <coin2028@hotmail.com>

python/dgl/_ffi/libinfo.py

@@ -2,6 +2,7 @@
 from __future__ import absolute_import
 import sys
 import os
+import pathlib
 
 
 def find_lib_path(name=None, search_path=None, optional=False):
@@ -47,30 +48,32 @@ def find_lib_path(name=None, search_path=None, optional=False):
 
     dll_path.append(install_lib_dir)
 
-    dll_path = [os.path.abspath(x) for x in dll_path]
     if search_path is not None:
-        if search_path is list:
-            dll_path = dll_path + search_path
-        else:
+        if isinstance(search_path, (list, tuple, set)):
+            dll_path = dll_path + list(search_path)
+        elif isinstance(search_path, str):
             dll_path.append(search_path)
-    if name is not None:
-        if isinstance(name, list):
-            lib_dll_path = []
-            for n in name:
-                lib_dll_path += [os.path.join(p, n) for p in dll_path]
-        else:
-            lib_dll_path = [os.path.join(p, name) for p in dll_path]
         else:
+            raise ValueError("type(search_path)={} is invalid".format(type(search_path)))
+    dll_path = [str(x.absolute()) if isinstance(x, pathlib.Path)
+                else os.path.abspath(x) for x in dll_path]
+
+    if name is None:
         if sys.platform.startswith('win32'):
-            lib_dll_path = [os.path.join(p, 'libdgl.dll') for p in dll_path] +\
-                           [os.path.join(p, 'dgl.dll') for p in dll_path]
+            name = ['libdgl.dll', 'dgl.dll']
         elif sys.platform.startswith('darwin'):
-            lib_dll_path = [os.path.join(p, 'libdgl.dylib') for p in dll_path]
+            name = 'libdgl.dylib'
         else:
-            lib_dll_path = [os.path.join(p, 'libdgl.so') for p in dll_path]
+            name = 'libdgl.so'
+
+    if isinstance(name, str):
+        name = [name]
+
+    lib_dll_path = []
+    for n in name:
+        lib_dll_path += [os.path.join(p, n) for p in dll_path]
 
-    # try to find lib_dll_path
-    lib_found = [p for p in lib_dll_path if os.path.exists(p) and os.path.isfile(p)]
+    lib_found = [p for p in lib_dll_path if os.path.isfile(p)]
 
     if not lib_found:
         message = ('Cannot find the files.\n' +