link_pyqt: Only link/copy files if they changed.

This reduces the output noise a bit and hopefully makes things a bit faster on
Windows.

scripts/link_pyqt.py

@@ -28,6 +28,7 @@ import sys
 import glob
 import subprocess
 import platform
+import filecmp
 
 
 class Error(Exception):
@@ -58,6 +59,22 @@ def get_ignored_files(directory, files):
     return filtered
 
 
+def needs_update(source, dest):
+    """Check if a file to be linked/copied needs to be updated."""
+    if os.path.islink(dest):
+        # No need to delete a link and relink -> skip this
+        return False
+    elif os.path.isdir(dest):
+        diffs = filecmp.dircmp(source, dest)
+        ignored = get_ignored_files(source, diffs.left_only)
+        has_new_files = set(ignored) != set(diffs.left_only)
+        return (has_new_files or diffs.right_only or
+                diffs.common_funny or diffs.diff_files or
+                diffs.funny_files)
+    else:
+        return not filecmp.cmp(source, dest)
+
+
 def link_pyqt(sys_path, venv_path):
     """Symlink the systemwide PyQt/sip into the venv.
 
@@ -75,16 +92,24 @@ def link_pyqt(sys_path, venv_path):
     for fn, required in files:
         source = os.path.join(sys_path, fn)
         dest = os.path.join(venv_path, fn)
+
         if not os.path.exists(source):
             if required:
                 raise FileNotFoundError(source)
             else:
                 continue
+
         if os.path.exists(dest):
-            if os.path.isdir(dest) and not os.path.islink(dest):
+            if needs_update(source, dest):
-                shutil.rmtree(dest)
+                remove(dest)
             else:
-                os.unlink(dest)
+                continue
+
+        copy_or_link(source, dest)
+
+
+def copy_or_link(source, dest):
+    """Copy or symlink source to dest."""
     if os.name == 'nt':
         if os.path.isdir(source):
             shutil.copytree(source, dest, ignore=get_ignored_files,
@@ -97,6 +122,14 @@ def link_pyqt(sys_path, venv_path):
         os.symlink(source, dest)
 
 
+def remove(filename):
+    """Remove a given filename, regardless of whether it's a file or dir."""
+    if os.path.isdir(filename):
+        shutil.rmtree(filename)
+    else:
+        os.unlink(filename)
+
+
 def get_python_lib(executable, venv=False):
     """Get the python site-packages directory for the given executable.